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  • 1.
    Alasdair, Skelton
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Fredrik, Arghe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Pitcairn, Iain
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Spatial coupling between spilitization and carbonation ofbasaltic sills in SW Scottish Highlands: evidence of amineralogical control of metamorphic fluid flow2011In: Geofluids, ISSN 1468-8115, E-ISSN 1468-8123, Vol. 11, no 3, p. 245-259Article in journal (Refereed)
    Abstract [en]

    In a geochemical and petrological analysis of overprinting episodes of fluid–rock interaction in a well-studied metabasaltic sill in the SW Scottish Highlands, we show that syn-deformational access of metamorphic fluids and consequent fluid–rock interaction is at least in part controlled by preexisting mineralogical variations. Lithological and structural channelling of metamorphic fluids along the axis of the Ardrishaig Anticline, SW Scottish Highlands, caused carbonation of metabasaltic sills hosted by metasedimentary rocks of the Argyll Group in the Dalradian Supergroup. Analysis of chemical and mineralogical variability across a metabasaltic sill at Port Cill Maluaig shows that carbonation at greenschist to epidote–amphibolites facies conditions caused by infiltration of H2O-CO2 fluids was controlled by mineralogical variations, which were present before carbonation occurred. This variability probably reflects chemical and mineralogical changes imparted on the sill during premetamorphic spilitization. Calculation of precarbonation mineral modes reveals heterogeneous spatial distributions of epidote, amphibole, chlorite and epidote. This reflects both premetamorphic spilitization and prograde greenschist facies metamorphism prior to fluid flow. Spilitization caused albitization of primary plagioclase and spatially heterogeneous growth of epidote ± calcic amphibole ± chlorite ± quartz ± calcite. Greenschist facies metamorphism caused breakdown of primary pyroxene and continued, but spatially more homogeneous, growth of amphibole + chlorite ± quartz. These processes formed diffuse epidote-rich patches or semi-continuous layers. These might represent precursors of epidote segregations, which are better developed elsewhere in the SW Scottish Highlands. Chemical and field analyses of epidote reveal the evidence of local volume fluctuations associated with these concentrations of epidote. Transient permeability enhancement associated with these changes may have permitted higher fluid fluxes and therefore more extensive carbonation. This deflected metamorphic fluid such that its flow direction became more layer parallel, limiting propagation of the reaction front into the sill interior.

  • 2.
    Andersson, Tommy
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Hermelin, Otto
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Bottom characterization of Lagoa das Furnas on Sao Miguel, Azores archipelago2016In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Vol. 321, p. 196-207Article in journal (Refereed)
    Abstract [en]

    Lagoa das Furnas is a crater lake located in an area exposed to geohazards from earthquakes and volcanic activity on the island of sao Miguel in the Azores Archipelago. Geophysical mapping of Lagoa das Furnas reveals a previously undiscovered volcanic dome. This dome is comprised largely of subaquatic pyroclastic debris of trachytic composition. Sedimentological, petrological, geochemical and geochronological studies of pyroclastic deposits from the dome link it to the historically documented Furnas 1630 eruption. The chemistry of glass and crystal fragments sampled from the dome suggests that it is comprised of more evolved magma than that of the main Furnas 1630 dome located 1400 m away. This suggests that the dome was formed during a final phase of the 1630 eruption in the Lagoa das Furnas area.

  • 3.
    Andrén, Margareta
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Stockmann, Gabrielle
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Sturkell, Erik
    Mörth, Carl-Magnus
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Guðrúnardóttir, Helga Rakel
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Keller, Nicole Simone
    Odling, Nic
    Dahrén, Börje
    Broman, Curt
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Balic-Zunic, Tonci
    Hjartarson, Hreinn
    Siegmund, Heike
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Freund, Friedemann
    Kockum, Ingrid
    Coupling between mineral reactions, chemical changes in groundwater, and earthquakes in Iceland2016In: Journal of Geophysical Research - Solid Earth, ISSN 2169-9313, E-ISSN 2169-9356, Vol. 121, no 4, p. 2315-2337Article in journal (Refereed)
    Abstract [en]

    Chemical analysis of groundwater samples collected from a borehole at Hafralaekur, northern Iceland, from October 2008 to June 2015 revealed (1) a long-term decrease in concentration of Si and Na and (2) an abrupt increase in concentration of Na before each of two consecutive M 5 earthquakes which occurred in 2012 and 2013, both 76km from Hafralaekur. Based on a geochemical (major elements and stable isotopes), petrological, and mineralogical study of drill cuttings taken from an adjacent borehole, we are able to show that (1) the long-term decrease in concentration of Si and Na was caused by constant volume replacement of labradorite by analcime coupled with precipitation of zeolites in vesicles and along fractures and (2) the abrupt increase of Na concentration before the first earthquake records a switchover to nonstoichiometric dissolution of analcime with preferential release of Na into groundwater. We attribute decay of the Na peaks, which followed and coincided with each earthquake to uptake of Na along fractured or porous boundaries between labradorite and analcime crystals. Possible causes of these Na peaks are an increase of reactive surface area caused by fracturing or a shift from chemical equilibrium caused by mixing between groundwater components. Both could have been triggered by preseismic dilation, which was also inferred in a previous study by Skelton et al. (2014). The mechanism behind preseismic dilation so far from the focus of an earthquake remains unknown.

  • 4. Barbieri, Maurizio
    et al.
    Franchini, Stefania
    Barberio, Marino Domenico
    Billi, Andrea
    Boschetti, Tiziano
    Giansante, Livio
    Gori, Francesca
    Jónsson, Sigurjón
    Petitta, Marco
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Stockmann, Gabrielle
    Changes in groundwater trace element concentrations before seismic and volcanic activities in Iceland during 2010-20182021In: Science of the Total Environment, ISSN 0048-9697, E-ISSN 1879-1026, Vol. 793, article id 148635Article in journal (Refereed)
    Abstract [en]

    We analysed temporal variations of trace element concentrations in groundwater from a 101 m-deep borehole (HA01) in northern Iceland during 2010–2018 and compared them with seismic and volcanic events that occurred in the same period to identify potential hydrogeochemical precursors. An increase of B, Al, V, Li and Mo concentrations started from eight months to one month before the 2014 Bárðarbunga eruption (~115 km from HA01), a major rifting event in central Iceland, while Ga and V concentrations began to increase one day and one month after the onset of the event, respectively. We also found that concentrations of some trace elements (Li, B, Ga, Mo, Sr, Rb and Fe) significantly increased before an Mw 5.0 earthquake that occurred ~80 km from the borehole in 2018. However, other notable hydrogeochemical changes were detected during the monitoring period without apparent correlation with the seismic and volcanic events in the region. This study shows that the systematic long-term hydrogeochemical monitoring in seismic and volcanic areas is critical to advance the science of seismic and eruptive precursors. Furthermore, the use of statistical tools, such as Principal Component Analysis (PCA) and Change Point (CP) detection can help identify the most useful chemical elements and validate the trend variability of those elements in the time series, reducing arbitrary choices of pre-seismic and pre-volcanic hydrogeochemical anomalies as potential precursors.

  • 5. Batki, Aniko
    et al.
    Pal-Molnar, Elemer
    Dobosi, Gabor
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Petrogenetic significance of ocellar camptonite dykes in the Ditrau Alkaline Massif, Romania2014In: Lithos, ISSN 0024-4937, E-ISSN 1872-6143, Vol. 200, p. 181-196Article in journal (Refereed)
    Abstract [en]

    Camptonite dykes intrude the rift-related Mesozoic igneous body of the Ditrau Alkaline Massif, Eastern Carpathians, Romania. We present and discuss mineral chemical data, major and trace elements, and the Nd isotopic compositions of the dykes in order to define their nature and origin. The dykes are classified as the clinopyroxene-bearing (camptonite-I) and clinopyroxene-free (camptonite-II) varieties. Camptonite-I consists of aluminian-ferroan diopside phenocrysts accompanied by kaersutite, subordinate Ti-rich annite, albite to oligoclase and abundant calcite-albite ocelli. Camptonite-II comprises K-rich hastingsite to magnesiohastingsite, Ti-rich annite, albite to andesine, abundant accessory titanite and apatite, and silicate ocelli filled mainly with plagioclase (An(4-34)). Age-corrected Nd-143/Nd-144 ratios vary from 0.51258 to 0.51269. The high epsilon(Nd) values of +4.0 to +6.1 which are consistent with intra-plate composition, together with light rare earth element (LREE), large ion lithophile element (LIE) and high field strength element (HFSE) enrichment in the camptonites is ascribed to the formation of small melt batches of a metasomatised sub-lithospheric mantle source. The presence of an asthenospheric 'high mu' ocean island basalt (HIMU-OIB)-type mantle component in the source region has also been revealed. A 1-4% degree of partial melting of an enriched garnet Iherzolite mantle source containing pargasitic amphibole followed by fractionation is inferred to have been involved in the generation of the camptonites. They are deduced to be parental melts to the Ditrau Alkaline Massif.

  • 6. Boschetti, Tiziano
    et al.
    Barbieri, Maurizio
    Domenico Barberio, Marino
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Stockmann, Gabrielle
    Toscani, Lorenzo
    Geothermometry and water-rock interaction modelling at Hafralækur: Possible implications of temperature and CO2 on hydrogeochemical changes previously linked to earthquakes in northern Iceland2022In: Geothermics, ISSN 0375-6505, E-ISSN 1879-3576, Vol. 105, article id 102535Article in journal (Refereed)
    Abstract [en]

    The low enthalpy (T < 150 °C) groundwater in the HA01 borehole at Hafralækur has a long time series (2008–2018) of chemical and isotopic data. In the previous studies, the variations in chemical and isotope parameters were statistically related to seismic activity. However, the possible effect of temperature has not yet been evaluated. To fill this gap, the results obtained from the classical geothermometric equations (silica solid phases, Na/K, Na-K-Ca) were compared. However, considering that the use of classical geothermometry using the Na/K ratio or silica solid phases solubility is limited by the presence of clay minerals and alkaline conditions (i.e., the presence of pH-dependant silicate anions), new equilibria reactions between labradorite, zeolites (analcime, stilbite) and the activity of the dissolved species in the fluid are presented to overcome this problem. In addition, kinetic reaction path models are presented to trace the possible role of both temperature and CO2 during the most evident chemical variations during earthquakes.

  • 7.
    Claesson, Lillemor
    et al.
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry. Geologi.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry. Geologi.
    Graham, Colin
    Mörth, Carl-Magnus
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry. Geokemi.
    The timescale and mechanisms of fault sealing and water-rock interaction after an earthquake2007In: Geofluids, Vol. 7, p. 427-440Article in journal (Other (popular science, discussion, etc.))
    Abstract [en]

    Hydrogeochemical monitoring of a basalt-hosted aquifer, which contains Ice Age meteoric water and is situated

    at 1220 m below sea level in the Tjornes Fracture Zone, northern Iceland, has been ongoing since July 2002.

    Based on hydrogeochemical changes following an earthquake of magnitude (Mw) 5.8 on 16 September 2002, we

    constrained the timescales of post-seismic fault sealing and water–rock interaction. We interpret that the earthquake

    ruptured a hydrological barrier, permitting a rapid influx of chemically and isotopically distinct Ice Age

    meteoric water from a second aquifer. During the two subsequent years, we monitored a chemical and isotopic

    recovery towards pre-earthquake aquifer compositions, which we interpret to have been mainly facilitated by

    fault-sealing processes. This recovery was interrupted in November 2004 by a second rupturing event, which was

    probably induced by two minor earthquakes and which reopened the pathway to the second aquifer. We conclude

    that the timescale of fault sealing was approximately 2 years and that the approach to isotopic equilibrium

    (from global meteoric water line) was approximately 18% after >10^4 years. Key words: earthquake, fault sealing, hydrogeochemistry, Iceland, Tjornes Fracture Zone, water–rock interaction

  • 8.
    Engström, Anna
    et al.
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Grassineau, N.
    Isotopic and petrological evidence of fluid-rock interaction at a Tethyan ocean-continent transition in the Alps: implications for tectonic processes and carbon transferduring early ocean formation2007In: Geofluids, ISSN 1468-8115, E-ISSN 1468-8123, Vol. 7, no 4, p. 401-414Article in journal (Refereed)
    Abstract [en]

    We report overprinting stable isotope evidence of fluid–rock interaction below two detachment faults along which mantle rocks were exhumed to the seafloor, between the respective landward and seaward limits of oceanic and continental crust, at a Tethyan ocean–continent transition (OCT). This OCT, which is presently exposed in the Tasna nappe (south-eastern Switzerland) is considered an on-land analogue of the well-studied Iberian OCT. We compare our results with the fault architecture (fault core–damage zone–protolith) described by Caine et al. [Geology (1996) Vol. 24, pp. 1025–1028]. We confirm the existence of a sharp boundary between the fault core and damage zone based on isotopic data, but the boundary between the damage zone and protolith is gradational. We identify evidence for: (1) pervasive isotopic modification to 8.4 ± 0.1‰ which accompanied or post-dated serpentinization of these mantle rocks at an estimated temperature of 67–109°C, (2) either (i) partial isolation of some highly strained regions [fault core(s) and mylonite] from this pervasive isotopic modification, because of permeability reduction (Caine et al.) or (ii) subsequent isotopic modification caused by structurally channelled flow of warm fluids within these highly strained regions, because of permeability enhancement, and (3) isotopic modification, which is associated with extensive calcification at T = 54–100°C, primarily beneath the younger of the two detachment faults and post-dating initial serpentinization. By comparing the volumetric extent of calcification with an experimentally verified model for calcite precipitation in veins, we conclude that calcification could have occurred in response to seawater infiltration, with a calculated flux rate of 0.1–0.2 m year−1 and a minimum duration of 0.2–4.0 × 104 years. The associated time-averaged uptake flux of carbon during this period was 8–120 mol m−2 year−1. By comparison with the estimated area of exhumed mantle rocks at the Iberian OCT, we calculate a maximum annual uptake flux for carbon of 2–30 Tg year−1. This is an order of magnitude greater than that for carbon exchange at the mid-ocean ridges and 0.1–1.4% of the global oceanic uptake flux for carbon.

  • 9. Fairchild, Ian J.
    et al.
    Spencer, Anthony M.
    Ali, Dilshad O.
    Anderson, Ross P.
    Anderton, Roger
    Boomer, Ian
    Dove, Dayton
    Evans, Jonathan D.
    Hambrey, Michael J.
    Howe, John
    Sawaki, Yusuke
    Shields, Graham A.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Tucker, Maurice E.
    Wang, Zhengrong
    Zhou, Ying
    Tonian-Cryogenian boundary sections of Argyll, Scotland2018In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 319, p. 37-64Article in journal (Refereed)
    Abstract [en]

    The Tonian-Cryogenian System boundary is to be defined at a GSSP (Global Boundary Stratigraphic Section and Point) beneath the first evidence of widespread glaciation. A candidate lies within the Dalradian Supergroup of Scotland and Ireland, which is least deformed and metamorphosed in Argyll, western Scotland. We present new stratigraphic profiles and interpretations from the Isle of Islay and the Garvellach Islands, update the chemostratigraphy of the Appin Group Tonian carbonates underlying the thick (ca. 1 km) glacigenic Port Askaig Formation (PAF) and demonstrate an environmental transition at the contact. The Appin Group forms a regionally extensive, > 4 km-thick, succession of limestones, shales and sandstones deposited on a marine shelf. On Islay, the upper part of the lithostratigraphy has been clarified by measuring and correlating two sections containing distinctive stratigraphic levels including molar tooth structure, oolite, stromatolitic dolomite and intraclastic microbial mounds. Significantly deeper erosion at the unconformity at the base of the overlying PAF is demonstrated in the southern section. Carbonate facies show a gradual decline in delta C-13(VPDB) from +5 to + 2 parts per thousand upwards. In NE Garbh Eileach (Garvellach Islands), a continuously exposed section of Appin Group carbonates, 70 m thick, here designated the Garbh Eileach Formation (GEF), lies conformably beneath the PAF. The GEF and the GEF-PAF boundary relationships are re -described with new sedimentological logs, petrological and stable isotope data. Interstratified limestone and dolomicrosparite with delta C-13 of -4 to -7 parts per thousand (a feature named the Garvellach anomaly, replacing the term Islay anomaly) are overlain by dolomite in which the isotope signature becomes weakly positive (up to +1 parts per thousand) upwards. Shallow subtidal conditions become peritidal upwards, with evidence of wave and storm activity. Gypsum pseudomorphs and subaerial exposure surfaces are common near the top of the GEF. The basal diamictite (D1) of the PAF is rich in carbonate clasts similar to slightly deeper water parts of the underlying succession. D1 is typically several metres thick with interstratified sandstone and conglomerate, but dies out laterally. Scattered siliciclastic coarse sandstone to pebble conglomerate with dropstones associated with soft -sediment deformation is interbedded with carbonate below and above D1. Dolomite beds with derived intraclasts and gypsum pseudomorphs are found above D1 (or equivalent position, where Dl is absent). Published and new Sr isotope studies, including successive leach data, demonstrate primary Tonian Sr-87/Sr-86 values of 0.7066-0.7069 on Islay, decreasing to 0.7064-0.7066 in the younger GEF limestones on the Garvellachs, with 1700-2700 ppm Sr. Other typically Tonian characteristics of the carbonates are the Sr-rich nature of limestones, molar tooth structure, and dolomitized peritidal facies with evidence of aridity. Seabed surveys suggesting uniformly-dipping strata and shallow borehole core material illustrate the potential for extending the Tonian record offshore of the Garvellachs. A candidate Tonian-Cryogenian GSSP is proposed on Garbh Eileach within the smooth delta C-13 profile at the cross-over to positive delta C-13 signatures, 4 m below the first occurrence of ice-rafted sediment and 9 m below the first diamictite. Although lacking radiometric constraints or stratigraphically significant biotas or biomarkers, the Scottish succession has a thick and relatively complete sedimentary record of glaciation, coherent carbon and strontium chemostratigraphy, lateral continuity of outcrops and 100% exposure at the proposed boundary interval.

  • 10.
    Fritzon, Ruben
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Goodfellow, Bradley
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Stroeven, Arjen
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Estholm, Madelene
    Caffee, Marc
    Evaluating geochemical evidence of earthquake periodicity, Sparta Fault, Southern GreeceManuscript (preprint) (Other academic)
    Abstract [en]

    Determining prehistoric earthquake periodicity and magnitudes is important for risk assessments in seismically active areas. We evaluate a geochemical method, which has previously been used to identify prehistoric slips on normal fault scarps through an analysis of variations in the concentration of rare earth elements and Y (REE-Y) along vertical transects. Our study object is the Sparta Fault, a normal fault in southern Greece, developed in limestone and previously documented, and dated using 36Cl, to have been last active 464 BC. From geochemical analyses of 39 fault rock samples, we conclude that REE-Y concentrations correlate strongly with the abundance of quartz and possibly other heterogeneities in the fault scarp. Because the sampled fault rock is a protocataclasite, formed at depth, variations in the abundance of quartz are not associated with prehistoric movements along the fault. We therefore conclude that geochemical evidence does not provide a reliable paleoseismic proxy for fault movement. We also present data indicating a co-variation between quartz and 36Cl concentrations, which we suggest requires a re-examination of this widely used application of the cosmogenic nuclide surface exposure dating method.

  • 11. Fru, Ernest Chi
    et al.
    Al Bahri, Jalila
    Brosson, Christophe
    Bankole, Olabode
    Aubineau, Jeremie
    El Albani, Abderrazzak
    Nederbragt, Alexandra
    Oldroyd, Anthony
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Löwhagen, Linda
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Webster, David
    Stockholm University.
    Fantong, Wilson Y.
    Mills, Benjamin J. W.
    Alcott, Lewis J.
    Konhauser, Kurt O.
    Lyons, Timothy W.
    Transient fertilization of a post-Sturtian Snowball ocean margin with dissolved phosphate by clay minerals2023In: Nature Communications, E-ISSN 2041-1723, Vol. 14, no 1, article id 8418Article in journal (Refereed)
    Abstract [en]

    Marine sedimentary rocks deposited across the Neoproterozoic Cryogenian Snowball interval, similar to 720-635 million years ago, suggest that post-Snowball fertilization of shallow continental margin seawater with phosphorus accelerated marine primary productivity, ocean-atmosphere oxygenation, and ultimately the rise of animals. However, the mechanisms that sourced and delivered bioavailable phosphate from land to the ocean are not fully understood. Here we demonstrate a causal relationship between clay mineral production by the melting Sturtian Snowball ice sheets and a short-lived increase in seawater phosphate bioavailability by at least 20-fold and oxygenation of an immediate post-Sturtian Snowball ocean margin. Bulk primary sediment inputs and inferred dissolved seawater phosphate dynamics point to a relatively low marine phosphate inventory that limited marine primary productivity and seawater oxygenation before the Sturtian glaciation, and again in the later stages of the succeeding interglacial greenhouse interval.

  • 12.
    Goodfellow, Bradley W.
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology. Stanford University, USA.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Martel, Stephen J.
    Stroeven, Arjen P.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Jansson, Krister N.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Hättestrand, Clas
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Controls of tor formation, Cairngorm Mountains, Scotland2014In: Journal Of Geophysical Research: Earth Surface, ISSN 2169-9003, Vol. 119, no 2, p. 225-246Article in journal (Refereed)
    Abstract [en]

    Tors occur in many granitic landscapes and provide opportunities to better understand differential weathering. We assess tor formation in the Cairngorm Mountains, Scotland, by examining correlation of tor location and size with grain size and the spacing of steeply dipping joints. We infer a control on these relationships and explore its potential broader significance for differential weathering and tor formation. We also assess the relationship between the formation of subhorizontal joints in many tors and local topographic shape by evaluating principle surface curvatures from a digital elevation model of the Cairngorms. We then explore the implications of these joints for tor formation. We conclude that the Cairngorm tors have formed in kernels of relatively coarse grained granite. Tor volumes increase with grain size and the spacing of steeply dipping joints. We infer that the steeply dipping joints largely formed during pluton cooling and are more widely spaced in tor kernels because of slower cooling rates. Preferential tor formation in coarser granite with a wider joint spacing that is more easily grusified indicates that joint spacing is a dominant control on differential weathering. Sheet jointing is well developed in tors located on relatively high convex surfaces. This jointing formed after the gross topography of the Cairngorms was established and before tor emergence. The presence of closely spaced (tens of centimeters), subhorizontal sheeting joints in tors indicates that these tors, and similarly sheeted tors elsewhere, formed either after subaerial exposure of bedrock or have progressively emerged from a regolith only a few meters thick. Key Points <list list-type=bulleted id=jgrf20195-list-0001> <list-item id=jgrf20195-li-0001>Tors form in kernels of coarse-grained granite among finer-grained granite <list-item id=jgrf20195-li-0002>Wide joint spacing in tors attributable to a slow cooling rate of the granite <list-item id=jgrf20195-li-0003>Sheet jointing discounts tor formation within a thick regolith

  • 13.
    Jakobsson, Martin
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Björck, Svante
    O'Regan, Matt
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Flodén, Tom
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Greenwood, Sarah L.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Swärd, Henrik
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Lif, Arne
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Ampel, Linda
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Koyi, Hemin
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Major earthquake at the Pleistocene-Holocene transition in Lake Vattern, southern Sweden2014In: Geology, ISSN 0091-7613, E-ISSN 1943-2682, Vol. 42, no 5, p. 379-382Article in journal (Refereed)
    Abstract [en]

    Lake Vattern, Sweden, is within a graben that formed through rifting along the boundary between two Precambrian terrains. Geophysical mapping and geological coring show that substantial tectonic movements along the Lake Vattern graben occurred at the very onset of the Holocene. This is evident from deformation structures in the soft sediment accumulated on the lake floor. Our interpretation of these structures suggests as much as 13 m of vertical tectonic displacements along sections of a >80-km-long fault system. If these large displacements are from one tectonic event, Lake Vattern must have had an earthquake with seismic moment magnitudes to 7.5. In addition, our geophysical mapping shows large landslides along sections of the steep lake shores. Pollen analysis of sediment infillings of some of the most prominent sediment deformation structures places this major seismic event at the Younger Dryas-Preboreal transition, ca. 11.5 ka. We suggest that this event is mainly related to the rapid release of ice-sheet load following the deglaciation. This paleoseismic event in Lake Vattern ranks among the larger known intraplate tectonic events in Scandinavia and attests to the significance of glacio-isostatic unloading.

  • 14. Jonsson, Erik
    et al.
    Troll, Valentin R.
    Hogdahl, Karin
    Harris, Chris
    Weis, Franz
    Nilsson, Katarina P.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Magmatic origin of giant 'Kiruna-type' apatite-iron-oxide ores in Central Sweden2013In: Scientific Reports, E-ISSN 2045-2322, Vol. 3, p. 1644-Article in journal (Refereed)
    Abstract [en]

    Iron is the most important metal for modern industry and Sweden is by far the largest iron-producer in Europe, yet the genesis of Sweden's main iron-source, the 'Kiruna-type' apatite-iron-oxide ores, remains enigmatic. We show that magnetites from the largest central Swedish 'Kiruna-type' deposit at Grangesberg have delta O-18 values between -0.4 and +3.7%, while the 1.90-1.88 Ga meta-volcanic host rocks have d18O values between +4.9 and +9%. Over 90% of the magnetite data are consistent with direct precipitation from intermediate to felsic magmas or magmatic fluids at high-temperature (delta O-18(mgt). > +0.9 parts per thousand, i.e. ortho-magmatic). A smaller group of magnetites (delta O-18(mgt) <= +0.9 parts per thousand), in turn, equilibrated with high-delta O-18, likely meteoric, hydrothermal fluids at low temperatures. The central Swedish 'Kiruna-type' ores thus formed dominantly through magmatic iron-oxide precipitation within a larger volcanic superstructure, while local hydrothermal activity resulted from low-temperature fluid circulation in the shallower parts of this system.

  • 15.
    Katrantsiotis, Christos
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Norström, Elin
    Stockholm University, Faculty of Science, Department of Physical Geography. Stockholm University, Faculty of Science, Department of Geological Sciences.
    Holmgren, Karin
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    High-resolution environmental reconstruction in SW Peloponnese, Greece, covering the last c. 6000years: Evidence from Agios Floros fen, Messenian plain2016In: The Holocene, ISSN 0959-6836, E-ISSN 1477-0911, Vol. 26, no 2, p. 188-204Article in journal (Refereed)
    Abstract [en]

    A paleolimnological record from the central Messenian plain (southwestern Peloponnese, southern Greece) indicates rapid changes in the water level and chemistry of a transient lake on the flanks of the Taygetos Mountains during the last c. 6000years. The analyses are based on diatoms as well as carbon and nitrogen isotopes from bulk sediments in a 7.5-m-long sediment core retrieved from the drained fen of Agios Floros, at the northern banks of the ancient River Pamisos. The sequence consists of fen peat in the uppermost section underlain by lacustrine sediments, which are punctuated by two layers of clay with diatomaceous silt bands. High accumulation rate is recorded in the oldest part of the section (up to 23mm/yr), particularly during two decadal-long periods centered at c. 5700 and c. 5300 cal. BP. The diatom record reveals pronounced peaks in the planktonic taxon Cyclotella distinguenda, which correspond to the laminated sequences, reflecting the rapid development of a deep lake with an open water environment during these two time periods. Another two events with intermediate water levels are inferred at c. 5200 and c. 4600 cal. BP. These short-lived phases were probably, to a large extent, caused by local tectonic processes and the consequent hydrological anomalies of the nearby karst springs, although abrupt climatic changes with enhanced precipitation might have also played a role. At c. 4500 cal. BP, our data suggest the development of terrestrial conditions in this area, which can be attributed to the decreasing activity/dry up of springs, probably associated with more arid climate. After c. 2500 cal. BP, the diatom record infers a return to wetter conditions, probably as a response to more humid climate with marked seasonality and human activities, developing the present-day environment with cultivated and seasonally semi-flooded fields.

  • 16.
    Kleine, Barbara I.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. University of Iceland, Iceland.
    Pitcairn, Iain K.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair D. L.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Mineralogical controls on metamorphic fluid flow in metabasaltic sills from Islay, Scotland2016In: Lithos, ISSN 0024-4937, E-ISSN 1872-6143, Vol. 248, p. 22-39Article in journal (Refereed)
    Abstract [en]

    In this study we show that mineralogy was the primary control of metamorphic fluid flow in the well-studied metabasaltic sills in the SW Scottish Highlands. Here, basaltic sills have been partially carbonated by H2O-CO2 fluids at greenschist facies conditions. This has led to mineral zonation with carbonate-poor sill interiors separated from carbonate-rich sill margins by reaction fronts. Although deformation set the stage for metamorphic fluid flow in the SW Scottish Highlands by causing the preferred alignment of mineral grains, metamorphic fluid flow was not coupled with active deformation but occurred later utilizing the pre-existing mineral alignment as a means of accessing the sill interiors. The sills which were studied were partially carbonated with well-preserved reaction fronts. They were selected because (atypically for the SW Scottish Highlands) they are mineralogically heterogeneous making them ideal for a study of mineralogical controls of metamorphic fluid flow. Their mineralogical heterogeneity reflects chemical heterogeneity arising from magmatic flow differentiation and spilitization, which occurred before greenschist facies metamorphism. Magmatic flow differentiation resulted in parts of the sill containing large crystals with no preferred alignment. Large (up to 3 cm) plagioclase phenocrysts were concentrated in the sill interior whereas large (up to 1 cm) amphibole (after pyroxene) grains formed cumulate layers close to the sill margins. These large randomly oriented crystals were replaced by an interface-coupled dissolution-precipitation mechanism. Replacement is constant volume and with hydration and carbonation affecting the cores of these minerals while the rims are remained intact and unaltered. This finding points to intro-granular metamorphic fluid flow. In contrast inter-granular metamorphic fluid flow was facilitated by mineral alignment on different scales. Pre-metamorphic spilitization, produced layers of epidote called segregations, whereas regional deformation caused preferred alignment mainly of amphibole and chlorite. Epidote undergoes a series of volume changes during greenschist facies metamorphism. This created porosity which produced preferred pathways for metamorphic fluids affecting the advancement of fluid-driven reaction fronts. Preferred alignment of amphibole and chlorite also affected the advancement of reaction fronts. In this case, fluid flow was preferentially parallel to the foliation. In both cases, inter-granular metamorphic fluid flow utilized a pre-existing fabric albeit on different scales. These results show intra-granular metamorphic fluid flow in unfoliated rock and inter-granular metamorphic fluid flow in foliated rock. In both cases metamorphic fluid flow occurred after deformation controlled by pre-existing mineralogical heterogeneities, such as grain composition and shape anisotropy as well as preferred alignment of mineral grains.

  • 17.
    Kleine, Barbara I.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Pitcairn, Iain K.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair D. L.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    The mechanism of infiltration of metamorphic fluids recorded by hydration and carbonation of epidote-amphibolite facies metabasaltic sills in the SW Scottish Highlands2015In: American Mineralogist, ISSN 0003-004X, E-ISSN 1945-3027, Vol. 100, no 11-12, p. 2702-2717Article in journal (Refereed)
    Abstract [en]

    In this study we investigate a group of metabasaltic sills from the SW Scottish Highlands metamorphosed at epidote-amphibolite facies conditions that provide useful insight into the mechanisms and characteristics of fluid infiltration during metamorphism. The sills are amphibole and garnet bearing and exhibit a strong foliation in the sill margins that developed pre- to syn- peak metamorphism. Fluid infiltration caused hydration and carbonation in the sills, expressed as 1) replacement of garnet and amphibole by chlorite and calcite and 2) replacement of amphibole and epidote to form chlorite and calcite. Using garnet-amphibole and garnet-chlorite geothermometers we show that these reactions occurred after peak metamorphism at T = 290 to 400°C. Reaction textures show that the fluid infiltration into the sill that caused hydration and carbonation occurred in the absence of deformation. The fluid infiltration was mineralogically controlled with greater fluid access in areas of abundant fine-grained elongate minerals such as amphibole and chlorite. The replacement of garnet by chlorite most likely occurred by an interface-coupled dissolution-precipitation mechanism as evidenced by perfect pseudomorphic textures of garnet, porosity generation behind the reactive interface and fracturing ahead of this interface. Porosity generated in the product chlorite enhanced fluid access to the replacement front. The study shows that deformation was not required for extensive fluid infiltration and alteration during metamorphism. Fluid flow uses a pre-existing foliation to gain access to the rock, taking advantage of the anisotropic shape of the aligned minerals.

  • 18.
    Kleine, Barbara I.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair D. L.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Huet, Benjamin
    Pitcairn, Iain K.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Preservation of blueschist-facies minerals along a shear zone by coupled metasomatism and fast-flowing CO2-bearing fluids2014In: Journal of Petrology, ISSN 0022-3530, E-ISSN 1460-2415, Vol. 55, no 10, p. 1905-1939Article in journal (Refereed)
    Abstract [en]

    Two types of blue halo (types I and II) composed of blueschist-facies minerals are centered around a brittle, normal shear zone in greenschist-facies rocks on the island of Syros, Aegean Sea, Greece. The shear zone is steeply dipping and cuts a near-horizontal layer of greenschist-facies rocks (albite + epidote + actinolite + chlorite + quartz). Type I and II blue haloes are 0.3 m and c. 1m wide respectively, and are seen on both sides of the shear zone. The inner type I haloes are composed of nearly pure glaucophane schist and were formed by metasomatic addition of Na2O and SiO2, and to a lesser extent of K2O and large ion lithophile elements (LILE), coupled with loss of CaO, Al2O3 and MnO. The outer type II haloes consist of a carbonated blueschist-facies assemblage (glaucophane + calcite + phengite + epidote + garnet + quartz).These experienced only slight metasomatic changes (i.e. addition of K2O and LILE), which cannot alone explain halo formation.We present  petrological, geochemical and thermodynamic evidence that this assemblage was preserved at greenschist-facies conditions because XCO2 was elevated by flow of a CO2-bearing fluid along the shear zone, which was approximately contemporaneous with greenschist-facies hydration in the surrounding rocks. We further note that the flux of CO2-bearing fluid along the shear zone was rapid with respect to the fluid flux in the surrounding rocks. Mass-balance calculations reveal that the fluid flux within the shear zone was at least 100-2000 times greater than the fluid flux within the surrounding rocks. Mineral textures show greenschist-facies minerals replacing blueschist minerals in the type II haloes, supporting our interpretation that the blueschist-facies minerals were preserved during greenschist-facies retrogression. A simplified P-T vs XCO2 pseudosection confirms that preservation of carbonated blueschist can occur at greenschist-facies conditions in the presence of a CO2-bearing fluid.

  • 19.
    Kleine, Barbara I.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. University of Iceland, Iceland.
    Zhao, Zhihong
    Skelton, Alasdair D. L.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    RAPID FLUID FLOW ALONG FRACTURES AT GREENSCHIST FACIES CONDITIONS ON SYROS, GREECE2016In: American Journal of Science, ISSN 0002-9599, E-ISSN 1945-452X, Vol. 316, no 2, p. 169-201Article in journal (Refereed)
    Abstract [en]

    Brittle fractures cut through greenschist facies metavolcanic rocks at Delfini on Syros, Greece. An equation for one-dimensional transport by advection along a single fracture and transverse diffusion outwards from this fracture was used to calculate time-averaged fluid velocities and the duration of fluid flow along the brittle fractures. These quartz and carbonate filled fractures are surrounded by symmetrical dark reaction halos. These reaction halos were formed by diffusion of CO2 outwards from the fracture in a hydrous fluid which caused carbonation of the country rock. Changes in concentration of relatively mobile elements (for example K, Na, Cs, Ba, Pb and Sr) occurred. However, little to no changes in most of the major elements and less mobile trace elements were observed. This implies that carbonation was largely isochemical with respect to most non-volatile components. The Sr/Ca ratio was used to model time-averaged fluid velocities and the duration of fluid flow along the fractures. Fluid flow along narrower fractures with discernibly tapering haloes was found to be rapid (10(-6) - 10(-5) ms(-1)) and short lived (0.1-400 years). These are time-averaged values and can therefore alternatively record a series of even shorter and faster pulses, perhaps associated with fracture propagation and associated seismicity. Within the widest fractures with constant halo widths (ca. 60 cm) fluid flow was slower (10(-8) to 10(-6) ms(-1)) and longer lived (100-15000 years). We suspect that the constant width of these haloes reflects a steady state having been reached at which halo width was controlled by the relative rates of fluid flow along the fracture and in the surrounding rock.

  • 20.
    Kleine, Barbara Irene
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Pitcairn, Iain K.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair D. L.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Pre-metamorphic controls on the propagation of fluid-driven reaction fronts at greenschist-facies metamorphic conditionsManuscript (preprint) (Other academic)
    Abstract [en]

    In this study we show that pre-metamorphic mineralogical and chemical heterogeneities control fluid flow and fluid-driven propagation of reaction fronts during regional metamorphism. The study was undertaken at Port Ellen, Islay, in SW Scottish Highlands. Here, basaltic sills have been partially carbonated by H2O-CO2 fluids at greenschist-facies conditions. This has led to mineral zonation with carbonate poor sill interiors separated from carbonate-rich sill margins by reaction fronts. The sills which were studied are partially carbonated and reaction fronts are well-preserved. These sills are unique for the Scottish Highlands in that they show excellent evidence of 1) extensive magmatic flow differentiation and 2) spilitization having occurred before greenschist-facies metamorphism. Magmatic flow differentiation concentrated large (up to 3 cm) phenocrysts of plagioclase in the sill interior and spilitic alteration produced layers of segregated epidote and caused albitization of these plagioclase phenocrysts resulting in their preservation throughout greenschist-facies metamorphism. Coupled with this pre-metamorphic mineralogical zonation, sill margins where enriched in Ti, Fe, P, HFSEs and REEs whereas the sill interiors were enriched in Al, Na and LILEs. In this study, we show spatial correlation of metamorphic reaction fronts with pre-metamorphic mineralogical zonation produced by magmatic flow differentiation (plagioclase phenocrysts size distributions) and epidote segregations produced by spilitization. We infer a pre-metamorphic mineralogical and chemical control on the positions of fluid-driven metamorphic reaction fronts. Based on mineralogical and chemical profiles across these sills and reaction textures preserved within them, we conclude that availability of reactant minerals and mechanical factors, such as volume change in epidote and foliation development due to chlorite formation are primary controls of fluid-driven reaction front propagation during metamorphism. We further suggest that elevated oxygen fugacity in the sill margins may have further promoted carbonation.

  • 21.
    Kleine, Barbara Irene
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Zhao, Zhihong
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair D. L.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Rapid fluid flow along fractures at greenschist-facies conditions on Syros, GreeceManuscript (preprint) (Other academic)
    Abstract [en]

    Using an equation for one-dimensional transport by advection along a single fracture and transverse diffusion outwards from this fracture to model field, petrological and geochemical data we calculated time-averaged fluid velocities and constrain the duration of fluid flow along brittle fractures cutting through greenschist-facies metamorphosed quartz-mica schists at Delfini on Syros, Greece. These quartz and carbonate filled fractures are surrounded by symmetrical dark reaction halos. These reaction halos were formed by diffusion of CO2 outwards from the fracture in a hydrous fluid which caused carbonation of the country rock. Changes in concentration of relatively mobile elements (e.g. K, Na, Cs, Ba, Pb and Sr) occurred. However, little to no changes in most of the major elements and less mobile trace elements were observed. This implies that carbonation was largely isochemical with respect to most non-volatile components. The Sr/Ca ratio was used to model time-averaged fluid velocities and the duration of fluid flow along the fractures. Fluid flow along narrower fractures with discernibly tapering haloes was found to be rapid (10-6 to 10-5 ms-1) and short lived (0.1 to 400 years). These are time-averaged values and can therefore alternatively record a series of even shorter and faster pulses, perhaps associated with fracture propagation and associated seismicity. Within the widest fractures with constant halo widths (ca. 60 cm) fluid flow was slower (10-8 to 10-6 ms-1) and longer lived (100 to 15000 years). We suspect that the constant width of these haloes reflects a steady state having been reached at which halo width was controlled by the relative rates of fluid flow along the fracture and in the surrounding rock.

  • 22.
    Kleman, Johan
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography. Navarino Environmental Observatory (NEO), Greece.
    Borgström, Ingmar
    Stockholm University, Faculty of Science, Department of Physical Geography. Navarino Environmental Observatory (NEO), Greece.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences. Navarino Environmental Observatory (NEO), Greece.
    Hall, Adrian
    Stockholm University, Faculty of Science, Department of Physical Geography. Navarino Environmental Observatory (NEO), Greece.
    Landscape evolution and landform inheritance in tectonically active regions: The case of the Southwestern Peloponnese, Greece2016In: Zeitschrift für Geomorphologie, ISSN 0372-8854, Vol. 60, no 2, p. 171-193Article in journal (Refereed)
    Abstract [en]

    The Peloponnese in southwestern Greece fronts the Hellenic Arc at the boundary between the European and African plates. The relief is developed across deeply eroded nappes and folds that represent the roots of Alpine mountains developed during Early Miocene collision. During the Plio-Pleistocene, the geomorphological development of the region was affected by both large-amplitude climatic swings and neotectonic uplift/subsidence of individual blocks. Combined tectonic and climatic forcing acting on existing mountain, piedmont and basin terrains developed across diverse low grade metamorphic and sedimentary rocks, including thick carbonate units, led to a geomorphological evolution that was both area-specific and highly variable over time. We here identify and analyse landforms of the southwestern Peloponnese and the climatic and tectonic events that have been driving the geomorphic evolution during the Plio-Pleistocene. The observational database consists of studies of river profiles, spatial landform distribution and genetic classification of landforms, some of them not previously described from the area. We observe that some important landforms and landscape elements can only be understood in the context of a long Plio-Pleistocene time perspective and reflect particular tectonic trends and events. We examine a partly dissected and southward-tilted pediment surface along the west side of the Mani Peninsula. The seaward truncation of this surface is interpreted to reflect Late Pliocene rifting and uplifting of the edge of Taygetos horst. Analysis of the spatial relations between landforms suggests that before the onset of rifting and the late-Pliocene-Pleistocene phase of uplift, a mature mountain-piedmont morphology already existed in the Taygetos-Mani block. The along-crest elevation differences were less pronounced than they are today, and the central part of the massif was fluvially dissected to a lesser depth than today. The pre-rifting elevation of the highest Taygetos summits is inferred to have been 1800-2000 m. Despite active neotectonics, the Peloponnese retains major landforms that have persisted through 1-3 Myr of slow erosion, due to partial exhumation, karstification and remoteness from drainage lines. A coherent inherited or relict surface comprising the highest summit of the Taygetos Mountains and a disjunct high-elevation, low-gradient valley is identified. We infer that this older morphology formed at considerably lower elevation and has since been uplifted to its present position. It is indicative of locally low summit erosion rates throughout Plio-Pleistocene uplift of the Taygetos horst. The impact of climate changes is most obvious at the lowest and highest elevations (<500 m and >2000 m). At the lowest elevations, eustatic sea level changes influenced the spatial location of erosion and sedimentation, and ravine systems developed in uplifted marine sediments. At the highest elevations, glaciation has during the last few glacial cycles left a diagnostic imprint. At intermediate elevations, the landscape can be described as a continuously evolving fluvial landscape in which climatic changes have left few or no diagnostic landforms. Our results have implications for interpretation of other mountainous carbonate areas, particularly in the climatically distinctive Mediterranean region. Tectonics appears to have been the first-order driver for geomorphic evolution, the effects of which must be clarified before study of climate impact on landform development can be possible or meaningful. The Taygetos Mountains and Mani Peninsula provide a case study that illuminates how a multiple age landscape can result from spatially extremely uneven erosional impacts, where tectonic isolation, remoteness from drainage lines, and karstification are important processes for creating inherited and only slowly changing landscape elements.

  • 23.
    Kylander, Malin E.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Söderlindh, Jenny
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Schenk, Frederik
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Gyllencreutz, Richard
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Rydberg, Johan
    Bindler, Richard
    Martínez Cortizas, Antonio
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    It's in your glass: a history of sea level and storminess from the Laphroaig bog, Islay (southwestern Scotland)2020In: Boreas, ISSN 0300-9483, E-ISSN 1502-3885, Vol. 49, no 1, p. 152-167Article in journal (Refereed)
    Abstract [en]

    Severe winter windstorms have become an increasingly common occurrence over recent decades in northwestern Europe. Although there exists considerable uncertainty, storminess is projected to increase in the future. On centennial to millennial time scales in particular, the mechanisms forcing storminess remain unsettled. We contribute to available palaeostorm records by reconstructing changes over the last 6670 years using a coastal peat sequence retrieved from the ombrotrophic Laphroaig bog on Islay, southwestern Scotland. We use a combination of ash content, grain size and elemental chemistry to identify periods of greater storminess, which are dated to 6605, 6290-6225, 5315-5085, 4505, 3900-3635, 3310-3130, 2920-2380, 2275-2190, 2005-1860, 1305-1090, 805-435 and 275 cal. a BP. Storm signals in the first half of the record up to similar to 3000 cal. a BP are mainly apparent in the grain-size changes. Samples from this time period also have a different elemental signature than those later in the record. We speculate that this is due to receding sea levels and the consequent establishment of a new sand source in the form of dunes, which are still present today. The most significant events and strongest winds are found during the Iron Ages Cold Epoch (2645 cal. a BP), the transition into, and in the middle of, the Roman Ages Warm Period (2235 and 1965 cal. a BP) and early in the Little Ice Age (545 cal. a BP). The Laphroaig record generally agrees with regionally relevant peat palaeostorm records from Wales and the Outer Hebrides, although the relative importance of the different storm periods is not the same. In general, stormier periods are coeval with cold periods in the region as evidenced by parallels with increased ice-rafted debris in the North Atlantic, highlighting that sea-ice conditions could impact future storminess and storm track position.

  • 24.
    Lewerentz, Alexander
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Alasdair, Skelton
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Fluid and carbon flux estimation of regional metamorphic fluid flow in Glen Esk, SE Scottish Highlands: the role of hydrodynamic dispersion for broadening of an isotopic frontIn: American Journal of Science, ISSN 0002-9599, E-ISSN 1945-452XArticle in journal (Refereed)
  • 25.
    Lewerentz, Alexander
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair D. L.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Fluid and carbon flux estimation of regional metamorphic fluid flow in Glen Esk, SE Scottish Highlands: The role of hydrodynamic dispersion for broadening of an isotopic front2018In: American Journal of Science, ISSN 0002-9599, E-ISSN 1945-452X, Vol. 318, no 4, p. 435-457Article in journal (Refereed)
    Abstract [en]

    During metamorphism, large quantities of fluids are mobilized and transported through the crust. Such fluids may interact with surrounding rock and serve as a medium for chemical transport. In this study, we use coupled carbonation and oxygen isotope fronts to estimate fluid and carbon fluxes for metamorphic fluid flow in vein pathways in the Dalradian metasediments that crop out in Glen Esk, SE Scottish Highlands. Comparison of wall rock and vein oxygen Isotope ratios indicate that the Barrovian sequence in Glen Esk was infiltrated by at least two separate fluid flow events, where fluid flow from the North Esk Fault overprints an earlier fluid flow event, for which the syn-metamorphic magmatic intrusions north of Glen Esk are a probable fluid source. Advection is the dominating mode of fluid transportation in veins. By advection-dispersion and advection-diffusion modeling, we estimate a time-averaged fluid flux of 0.0005 to 0.0135 m(3).m(-2).yr(-1) and a carbon flux of 0.04 to 0.71 mol C.m(-2).yr(-1) for fluid flow from the North Esk fault into the Dalradian metasediments. The duration of this fluid flow event is estimated to between 11 and 230 kyr. Our results also indicate that hydrodynamic dispersion was the main reason for broadening of the oxygen isotope front.

  • 26.
    Lewerentz, Alexander
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair D. L.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Linde, Josefin K.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Nilsson, Jonas
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Möller, Charlotte
    Crill, Patrick M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Spicuzza, Michael J.
    On the Association between Veining and Index Mineral Distributions in Barrow’s Metamorphic Zones, Glen Esk, Scotland2017In: Journal of Petrology, ISSN 0022-3530, E-ISSN 1460-2415, Vol. 58, no 5, p. 885-908Article in journal (Refereed)
    Abstract [en]

    The concept of index mineral based metamorphic zones was first introduced by George Barrow in 1912 and the Barrovian metamorphic zones continue to be used as a framework for describing regional metamorphism. Pressure, temperature, and protolith composition are widely recognized as primary controls on index mineral distribution. Today, metamorphic fluid flow is also recognized as an important driver of metamorphic reactions. The aim of this study is to establish if and how metamorphic fluids control index mineral distribution during Barrovian metamorphism. We use samples from Barrow’s type locality in Glen Esk, SE Scottish Highlands, to study possible relationships between veining and index mineral distribution. In addition to petrographic and textural observations, we use whole-rock compositions, mineral compositions and oxygen isotope analyses. At low grade, in the chlorite zone and most of the biotite zone, no correlation between veining and index mineral distribution is seen. At higher grade, in the garnet and staurolite zones, index mineral abundance is shown to be higher adjacent to veins. These trends coincide with other mineralogical, chemical, and isotopic changes in the vein-proximal rock, indicative of fluid–rock interaction. Kyanite distribution is homogeneous in the kyanite zone. However, we show that this too relates to extensive fluid–rock interaction. Garnet-, staurolite-, and kyanite-bearing selvedges are common in the sillimanite zone. However, sillimanite distribution is unrelated to these selvedges, which supports models arguing that sillimanite formed during a separate metamorphic event. We infer fluid flow from high grade to low grade because the fluid was out of isotopic equilibrium with the lower grade rocks, but in equilibrium with the higher grade rocks. We conclude that fluid flow played a major role in the stabilization and distribution of Barrovian index minerals in Glen Esk, and that the importance of fluid flow was greater at higher metamorphic grades.

  • 27.
    Lewerentz, Alexander
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Graham, Colin
    Broman, Curt
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Däcker, Elisabeth
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Post-peak metamorphic kyanite stabilisation in greenschist facies metasedimentary rocks on the Isle of Islay, SW Scottish HighlandsManuscript (preprint) (Other academic)
  • 28. Lupi, Matteo
    et al.
    Frehner, Marcel
    Weis, Philipp
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Saenger, Erik H.
    Tisato, Nicola
    Geiger, Sebastian
    Chiodini, Giovanni
    Driesner, Thomas
    Regional earthquakes followed by delayed ground uplifts at Campi Flegrei Caldera, Italy: Arguments for a causal link2017In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 474, p. 436-446Article in journal (Refereed)
    Abstract [en]

    Earthquake-triggered volcanic activity promoted by dynamic and static stresses are considered rare and difficult-to-capture geological processes. Calderas are ideal natural laboratories to investigate earthquake volcano interactions due to their sensitivity to incoming seismic energy. The Campi Flegrei caldera, Italy, is one of the most monitored volcanic systems worldwide. We compare ground elevation time series at Campi Flegrei with earthquake catalogues showing that uplift events at Campi Flegrei are associated with large regional earthquakes. Such association is supported by (yet non-definitive) binomial tests. Over a 70-year time window we identify 14 uplift events, 12 of them were preceded by an earthquake, and for 8 of them the earthquake-to-uplift timespan ranges from immediate responses to 1.2 yr. Such variability in the response delay may be due to the preparedness of the system with faster responses probably occurring in periods during which the Campi Flegrei system was already in a critical state. To investigate the process that may be responsible for the proposed association we simulate the propagation of elastic waves and show that passing body waves impose high dynamic strains at the roof of the magmatic reservoir of the Campi Flegrei at about 7 km depth. This may promote a short-lived embrittlement of the magma reservoir's carapace otherwise marked by a ductile behaviour. Such failure allows magma and exsolved volatiles to be released from the magmatic reservoir. The fluids, namely exsolved volatiles and/or melts, ascend through a nominally plastic zone above the magmatic reservoir. This mechanism and the associated inherent uncertainties require further investigations but the new concept already implies that geological processes triggered by passing seismic waves may become apparent several months after passage of the seismic waves. (C) 2017 Elsevier B.V. All rights reserved.

  • 29. Niemi, Maria
    et al.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Noone, Kevin
    Stockholm University, Faculty of Science, Department of Environmental Science.
    Olsson, Mats J.
    Lockdown Measures Which Reduced Greenhouse Gas Emissions With Little Negative Impact on Quality of Life2021In: Earth's Future, E-ISSN 2328-4277, Vol. 9, no 5, article id e2020EF001909Article in journal (Refereed)
    Abstract [en]

    Lockdown measures in response to the new Covid-19 virus have caused the largest ever fall of annual greenhouse gas emissions. A key question that we attempt to answer in this study is which, if any, of these measures can be productively encouraged post-lockdown in efforts to sustain at least part of this reduction in emissions. Sweden is uniquely suited for our study because the voluntary nature of lockdown in Sweden allowed us to assess the level of compliance to recommendations and its effects on greenhouse gas emissions. First, we assessed the change of perceived quality of life (QOL) among 746 individuals from Stockholm region due to adhering to lockdown measures. Second, we calculated the associated change of annual per capita greenhouse emissions. We found that avoiding travel for work, avoiding purchasing, and avoiding restaurants had the least negative effect on QOL, and at the same time the largest positive effect on carbon dioxide equivalent (CO(2)e) emission reductions. We conclude that these are potential leverage points for stimulating behavioral change that has a positive climatic impact.

  • 30. O'Regan, M.
    et al.
    Moran, K.
    Backman, Jan
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Sangiorgi, F.
    Brinkhuis, H.
    Pockalny, R.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Stickley, C.
    Koc, N.
    Brumsack, H.
    Willard, D.
    Mid-Cenozoic tectonic and paleoenvironmental setting of the central Arctic Ocean2008In: Paleoceanography, Vol. 23, no PA1S20, p. PA1S20-Article in journal (Refereed)
  • 31.
    Peillod, Alexandre
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Ring, Uwe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Glodny, Johannes
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    An Eocene/Oligocene blueschist-/greenschist facies P-T loop from the Cycladic Blueschist Unit on Naxos Island, Greece: Deformation-related re-equilibration vs. thermal relaxation2017In: Journal of Metamorphic Geology, ISSN 0263-4929, E-ISSN 1525-1314, Vol. 35, no 7, p. 805-830Article in journal (Refereed)
    Abstract [en]

    Geothermobarometric and geochronological work indicates a complete Eocene/early Oligocene blueschist/greenschist facies metamorphic cycle of the Cycladic Blueschist Unit on Naxos Island in the Aegean Sea region. Using the average pressure-temperature (P-T) method of thermocalc coupled with detailed textural work, we separate an early blueschist facies event at 576 +/- 16 to 619 +/- 32 degrees C and 15.5 +/- 0.5 to 16.3 +/- 0.9kbar from a subsequent greenschist facies overprint at 384 +/- 30 degrees C and 3.8 +/- 1.1kbar. Multi-mineral Rb-Sr isochron dating yields crystallization ages for near peak-pressure blueschist facies assemblages between 40.5 +/- 1.0 and 38.3 +/- 0.5Ma. The greenschist facies overprint commonly did not result in complete resetting of age signatures. Maximum ages for the end of greenschist facies reworking, obtained from disequilibrium patterns, cluster near c. 32Ma, with one sample showing rejuvenation at c. 27Ma. We conclude that the high-P rocks from south Naxos were exhumed to upper mid-crustal levels in the late Eocene and early Oligocene at rates of 7.4 +/- 4.6km/Ma, completing a full blueschist-/greenschist facies metamorphic cycle soon after subduction within c. 8Ma. The greenschist facies overprint of the blueschist facies rocks from south Naxos resulted from rapid exhumation and associated deformation/fluid-controlled metamorphic re-equilibration, and is unrelated to the strong high-T metamorphism associated with the Miocene formation of the Naxos migmatite dome. It follows that the Miocene thermal overprint had no impact on rock textures or Sr isotopic signatures, and that the rocks of south Naxos underwent three metamorphic events, one more than hitherto envisaged.

  • 32.
    Peillod, Alexandre
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Ring, Uwe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Linnros, Henrik
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Hansman, Reuben J.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    The role of ductile flow of the lower crust in controlling heat advection in the footwall of the Naxos extensional fault system (Aegean Sea, Greece)In: Article in journal (Refereed)
    Abstract [en]

    Geothermobarometric and structural work indicates that metamorphism in the footwall of the migmatitic Naxos core complex is strongly coupled to heat advection in the lower crust. Using the average pressure-temperature (P-T) method of THERMOCALC for geothermobarometric calculations coupled with detailed textural work, we constructed P-T paths for nine samples that have different spatial relationships to the Naxos extensional fault system and a migmatite dome in the center of the Naxos core complex. All nine samples show early near isothermal decompression. The late, cooling segment of the P-T paths shows systematic spatial differences in temperature gradients relative to the distance of the samples below the Naxos extensional detachment (representing the top of the Naxos extensional fault system). The differences in late thermal gradients correlate with finite strain, a proxy for the intensity of ductile flow. High finite strain correlates with high thermal gradients that do not change with distance below the Naxos detachment, whereas low finite strain correlates with downward decreasing thermal gradients. The difference in late thermal gradients and finite strain define a thermal asymmetry controlled by ductile flow, which we interpret to show that ductile flow controlled advection of heat and thus high-grade metamorphism on Naxos. We conclude that advective heating is an important driver of metamorphism in extensional settings.

  • 33.
    Pitcairn, Iain K.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair D. L.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Broman, Curt
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Arghe, Fredrik
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Boyce, Adrian
    Structurally focused fluid flow during orogenesis: the Islay Anticline, SW Highlands, Scotland2010In: Journal of the Geological Society, ISSN 0016-7649, E-ISSN 2041-479X, Vol. 167, no 4, p. 659-674Article in journal (Refereed)
    Abstract [en]

    Displacement of isotopic compositions at boundary layers across strata of contrasting composition is commonly used to investigate hydrothermal fluid flow during orogeny. This study investigates whether hydrothermal fluid flow was focused along the Islay Anticline, Islay, SW Highlands of Scotland, as shown in the axial zone of the neighbouring Ardrishaig Anticline. Four localities from the limb to the axial plane of the Islay Anticline were investigated for isotopic homogenization of metacarbonate units to silicate values. At Mull of Oa on the limb of the anticline, metacarbonate samples show limited isotopic resetting and the fluid flux is estimated to be <1 m(3) m(-2). Within the axial zone of the Islay Anticline, metacarbonate units from Port a' Chotain and Bagh an Da Dhoruis show complete isotopic homogenization to silicate values indicating higher fluid fluxes. Fluid flow was enhanced along localized parasitic folds such as at Port an t-Sruthain, where metacarbonates have been isotopically reset, and there are abundant quartz-carbonate veins that precipitated during D-1-D-2 deformation. Metamorphic fluid flow was higher in the axial zone of the Islay Anticline and in localized antiformal structures. Fluid fluxes are estimated to be considerably lower than at the neighbouring Ardrishaig Anticline.

  • 34.
    Pitcairn, Iain K.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair D. L.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Wohlgemuth-Ueberwasser, Cora C.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Mobility of gold during metamorphism of the Dalradian in Scotland2015In: Lithos, ISSN 0024-4937, E-ISSN 1872-6143, Vol. 233, p. 69-88Article in journal (Refereed)
    Abstract [en]

    Mobility of Au and related metals during metamorphism has been suggested to be the source of metals enriched in orogenic Au deposits. This study investigates the mobility of Au, As, and Sb during metamorphism of the Dalradian metasedimentary rocks of Scotland. The metamorphic processes in the Dalradian of Scotland are extremely well studied, and the terrane is an ideal area to investigate mobility of these metals. Our results show that of the 25 major and trace elements analysed, only Au, As, Sb, S and volatile contents as shown by loss on ignition (LOI) values show systematic variation with the metamorphic grade of the samples. Average Au concentrations decrease from 1.1 +/- 0.55 ppb and 0.72 +/- 0.34 ppb in chlorite and biotite zone rocks down to 0.4 +/- 0.22 ppb and 034 +/- 0.13 ppb in kyanite and sillimanite zone rocks. Average As concentrations decrease from 4.8 ppm (range 0.5 to 17.8 ppm) and 1.96 +/- 1.9 ppm in chlorite and biotite zone rocks down to 0.24 +/- 0.15 ppm and 0.2 +/- 0.12 ppm in kyanite and sillimanite zone rocks. Average Sb concentrations decrease from 0.18 +/- 0.15 ppm and 0.11 +/- 0.10 ppm in chlorite and biotite zone rocks down to 0.04 +/- 0.02 ppm in both kyanite and sillimanite zone rocks. Sulphur and LOI concentrations also show significant decreases. Mass balance calculations indicate that compared to chlorite and biotite zone samples, sillimanite zone samples have an average mass loss of 62 +/- 14%, 94 +/- 4% and 74 +/- 14% for Au, As, and Sb respectively. Every 1 km(3) of chlorite-biotite zone mixed psammitic-pelitic protolith rock that is metamorphosed to sillimanite zone conditions would release 1.5 t Au, 8613 t As, 270 t Sb, and 1.02 Mt S. The mobility of these elements is strongly controlled by the paragenesis of sulphide minerals. Pyrite, sphalerite, galena and cobaltite (as well as gersdorffite) decrease in abundance with increasing metamorphic grade in the Dalradian metasedimentary rocks. A critical aspect of the sulphide paragenesis is the transition of pyrite to pyrrhotite. This transition is complete by mid greenschist facies in the Loch Lomond samples but is more gradual at Glen Esk occurring between biotite and sillimanite zones. The Au, As, and Sb content of the sulphide assemblage also decreases with increasing metamorphic grade, and we suggest that this is a controlling factor on the mobility of these metals from the Dalradian metasedimentary rocks during metamorphism. Chlorite may be an important host mineral for As in the greenschist fades rocks. Breakdown of chlorite indirectly drives the mobility of Au, As, and Sb, as this produces the bulk of metamorphic fluid that drives transition between pyrite and pyrrhotite. We suggest that there is potential for significant undiscovered mineralisation in the Central and SW Highlands of Scotland. However, as the total mass of gold mobilised is lower than observed in other metasedimentary terranes such as the Otago and Alpine Schist's, New Zealand, very efficient fluid focussing and trapping mechanisms would be required to form large deposits in the Dalradian of Scotland.

  • 35. Ranta, E.
    et al.
    Stockmann, G.
    Wagner, T.
    Fusswinkel, T.
    Sturkell, E.
    Tollefsen, Elin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Fluid-rock reactions in the 1.3Ga siderite carbonatite of the GrOnnedal-ika alkaline complex, Southwest Greenland2018In: Contributions to Mineralogy and Petrology, ISSN 0010-7999, E-ISSN 1432-0967, Vol. 173, no 10, article id 78Article in journal (Refereed)
    Abstract [en]

    Petrogenetic studies of carbonatites are challenging, because carbonatite mineral assemblages and mineral chemistry typically reflect both variable pressure-temperature conditions during crystallization and fluid-rock interaction caused by magmatic-hydrothermal fluids. However, this complexity results in recognizable alteration textures and trace-element signatures in the mineral archive that can be used to reconstruct the magmatic evolution and fluid-rock interaction history of carbonatites. We present new LA-ICP-MS trace-element data for magnetite, calcite, siderite, and ankerite-dolomite-kutnohorite from the iron-rich carbonatites of the 1.3Ga GrOnnedal-ika alkaline complex, Southwest Greenland. We use these data, in combination with detailed cathodoluminescence imaging, to identify magmatic and secondary geochemical fingerprints preserved in these minerals. The chemical and textural gradients show that a 55m-thick basaltic dike that crosscuts the carbonatite intrusion has acted as the pathway for hydrothermal fluids enriched in F and CO2, which have caused mobilization of the LREEs, Nb, Ta, Ba, Sr, Mn, and P. These fluids reacted with and altered the composition of the surrounding carbonatites up to a distance of 40m from the dike contact and caused formation of magnetite through oxidation of siderite. Our results can be used for discrimination between primary magmatic minerals and later alteration-related assemblages in carbonatites in general, which can lead to a better understanding of how these rare rocks are formed. Our data provide evidence that siderite-bearing ferrocarbonatites can form during late stages of calciocarbonatitic magma evolution.

  • 36.
    Ring, Uwe
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Glodny, Johannes
    Peillod, Alexandre
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    The timing of high-temperature conditions an d ductile shearing in the footwall of the Naxos metamorphic core complex, Aegean Sea, GreeceIn: Article in journal (Refereed)
    Abstract [en]

    We present eight Rb-Sr multi-mineral isochron ages showing that high-temperature metamorphic conditions and partial melting during top-to-the-NNE extensional shearing in the footwall of the Naxos metamorphic core complex lasted until about 14-12 Ma. One migmatite sample yielded an age of 14.34 ± 0.20 Ma (2σ uncertainties) for crystallization of migmatization related melt pockets. Four pegmatite samples, which are in part associated with partial melting of their host rocks, provided overlapping ages ranging from 13.81 to 12.23 Ma (age range includes 2σ uncertainties). Additional three samples of amphibolite-facies schist supplied Rb-Sr ages of around 14 Ma. Samples showing fluid- and/or deformation-assisted white mica and biotite reworking gave Rb-Sr mineral apparent ages of 11.1 ± 2.7, 10.16 ± 0.24, 9.7 ± 0.7 and 9.6 ± 0.15 Ma. These ages are interpreted to be associated with late stages of extensional shearing under greenschist-facies metamorphic conditions. Together with published U-Pb zircon ages of migmatite, and S- and I-type granite crystallization, the data indicate that presence of melt in the Naxos migmatite dome lasted for at least 7 Ma (from ~18 to ~11 Ma). This demonstrates that the thermal anomaly resulting from and aiding extensional deformation was a long-lived and not a transient event. We conclude that melt-assisted deformation facilitated large-scale displacement on the Naxos extensional fault system by drastically weakening the extending crust for long periods of time.

  • 37.
    Ring, Uwe
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Glodny, Johannes
    Peillod, Alexandre
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    The timing of high-temperature conditions and ductile shearing in the footwall of the Naxos extensional fault system, Aegean Sea, Greece2018In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 745, p. 366-381Article in journal (Refereed)
    Abstract [en]

    We present eight Rb-Sr multi-mineral isochron ages showing that high-temperature metamorphic conditions and partial melting during top-to-the-NNE extensional shearing in the footwall of the Naxos extensional fault system (i.e. Naxos metamorphic core complex) lasted until about 14-12 Ma. One migmatite sample yielded an age of 14.34 +/- 0.2 Ma (2 sigma uncertainty) for crystallization of migmatization-related melt pockets. Four pegmatite samples, which are in part associated with partial melting of their host rocks, provided overlapping ages ranging from 13.81 to 12.23 Ma (age range includes 2 sigma uncertainty). Additional three samples of amphibolite-facies schist supplied Rb-Sr ages of around 14 Ma. Samples showing fluid- and/or deformation-assisted white mica and biotite reworking gave Rb-Sr mineral apparent ages of 11.1 +/- 2.7, 10.16 +/- 0.24, 9.7 +/- 0.7 and 9.6 +/- 0.15 Ma. These ages are interpreted to be associated with late stages of extensional shearing under greenschist-facies metamorphic conditions. Together with published U-Pb zircon ages of migmatite, and S- and I-type granite crystallization, the data indicate that the presence of melt in the footwall of the Naxos extensional fault system lasted for at least 7 Ma (from similar to 18 to similar to 11 Ma). This demonstrates that high temperatures and crustal melting resulting from and aiding extensional deformation was a long-lived and not a transient event. We conclude that melt-assisted deformation facilitated large-scale displacement on the Naxos extensional fault system by drastically weakening the extending crust for long periods of time.

  • 38.
    Ring, Uwe
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Pantazides, Hermes
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Glodny, Johannes
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Forced Return Flow Deep in the Subduction Channel, Syros, Greece2020In: Tectonics, ISSN 0278-7407, E-ISSN 1944-9194, Vol. 39, no 1, article id e2019TC005768Article in journal (Refereed)
    Abstract [en]

    We present the results of a detailed structural study in the Cycladic Blueschist Unit at Fabrika on Syros Island, Greece, and discuss their significance for tectonic processes at the subduction interface. Some samples record top-to-the-west shear reflecting prograde (burial), peak high-pressure (HP) and initial decompression (exhumation) conditions. Other nearby samples record top-to-the-east shear during HP metamorphism and exhumation. Some rocks re-equilibrated at greenschist-facies conditions and record top-to-the-west shear. Greenschist-facies top-to-the-west shear is also found at the base of non-HP upper units above the Fabrika HP sequence. We interpret the HP structures to reflect forced return flow and incipient formation of an extrusion wedge in the subduction channel. The HP top-to-the-west structures resulted from thrusting along the base of the wedge and started to form during burial before the rocks reached their deepest point. The HP top-to-the-east structures reflect deformation near the top of the developing extrusion wedge. After considerable exhumation during ongoing subduction, out-of-sequence, top-to-the-west thrusts emplaced the non-HP upper units above the exhuming extrusion wedge 10 Myr after the wedge initially formed. Our work suggests that the HP rocks were considerably exhumed during sustained lithospheric shortening in the subduction channel by forced return flow. Because return flow is controlled by the velocity of the subducting slab, it may explain why HP rocks can be exhumed at subduction rates. On the regional scale we find that four distinct HP belts were sequentially accreted and exhumed between 50 and 20 Ma suggesting continuous subduction-channel return flow in the Hellenic subduction zone.

  • 39.
    Sjöberg, Susanne
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Allard, Bert
    Rattray, Jayne E.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Callac, Nolwenn
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Grawunder, Anja
    Ivarsson, Magnus
    Sjöberg, Viktor
    Karlsson, Stefan
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Dupraz, Christophe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Rare earth element enriched birnessite in water-bearing fractures, the Ytterby mine, Sweden2017In: Applied Geochemistry, ISSN 0883-2927, E-ISSN 1872-9134, Vol. 78, p. 158-171Article in journal (Refereed)
    Abstract [en]

    Characterization of a black substance exuding from fractured bedrock in a subterranean tunnel revealed a secondary manganese oxide mineralisation exceptionally enriched in rare earth elements (REE). Concentrations are among the highest observed in secondary ferromanganese precipitates in nature. The tunnel is located in the unsaturated zone at shallow depth in the former Ytterby mine, known for the discovery of yttrium, scandium, tantalum and five rare earth elements.

    Elemental analysis and X-ray diffraction of the black substance establish that the main component is a manganese oxide of the birnessite type. Minor fractions of calcite, other manganese oxides, feldspars, quartz and about 1% organic matter were also found, but no iron oxides were identified. The Ytterby birnessite contains REE, as well as calcium, magnesium and traces of other metals. The REE, which constitute 1% of the dry mass and 2% of the metal content, are firmly included in the mineral structure and are not released by leaching at pH 1.5 or higher. A strong preference for the trivalent REE over divalent and monovalent metals is indicated by concentration ratios of the substance to fracture water. The REE-enriched birnessite has the general formula Mx(Mn3+,Mn4+)(2)O-4 center dot(H2O)(n) with M = (0.37-0.41) Ca + 0.02 (REE + Y), 0.04 Mg and (0.02-0.03) other metals, and with [Mn3+]/[Mn4+] = 0.86-1.00.

    The influence of microorganisms on the accumulation of this REE enriched substance is demonstrated by electron paramagnetic resonance spectroscopy. Results show that it is composed of two or more manganese phases, one of which has a biogenic signature. In addition, the occurrence of C-31 to C-35 extended side chain hopanoids among the identified lipid biomarkers combined with the absence of ergosterol, a fungal lipid biomarker, indicate that the in-situ microbial community is bacterial rather than fungal.

  • 40.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Can metamorphic reactions influence atmospheric greenhouse gas concentrations?2008In: International Geological Congress, 33, 2008, p. CD-ROMConference paper (Other academic)
    Abstract [en]

    Metamorphism is an ongoing process which occurs to accommodate changes of pressure and temperature at active plate boundaries. Metamorphic reactions commonly involve the uptake, storage and/or release of a fluid which typically contains H2O, CO2 and/or CH4. These metamorphic fluids are free to migrate along fracture and/or grain boundary pathways providing a mechanism for chemical transfer both within the lithosphere and between the lithosphere and the surface systems. In this respect metamorphic fluids represent a (poorly understood) part of global C-O-H cycles and are thus a potential influence on atmospheric greenhouse gas concentrations. To assess whether metamorphic fluids can therefore significantly impact climate systems requires knowledge of fluid compositions, volumes, flux rates, flow pathways. The present day flow of metamorphic fluids can not be measured directly because this occurs at depths which exceed those attainable by scientific drilling. However, evidence of metamorphic fluid flow is often “fossilised” in ancient metamorphic terrains where rocks from the middle and lower parts of the crust or lithospheric mantle are exposed at the surface. Thus ancient metamorphic terrains provide a natural laboratory in which to study the role of the lithosphere in global C-O-H cycles. The metamorphic terrains of the SW Scottish Highlands and New England are excellent examples. These terrains host well-preserved, sometimes overprinting, yet quantifiable evidence of metamorphic fluid flow events which occurred during both extensional and collisional tectonics and in both middle and lower parts of the Earth’s crust. In these studies, time-integrated fluid fluxes are obtained by chromatographic modelling of the propagation of volatilization fronts from lithological boundaries. Only time-integrated fluid fluxes can be obtained in these studies because the rock being studies preserves a time-integrated record of metamorphic fluid flow. However, chromatographic modelling permits conversion of time-integrated fluid fluxes to time-averaged fluid fluxes by comparison with other processes (e.g. diffusion) for which rates can be determined experimentally. These time-averaged fluid fluxes can be used to obtain time-averaged carbon fluxes where fluid compositions can be constrained based on mineralogy. Time-averaged carbon (C) flux rates obtained in these studies are 0.001-10 mol-C.m-2.yr-1. This estimate can be compared with the average global carbon flux rate to the atmosphere which is 10 mol-C.m-2.yr-1 (Houghton & Hackler, 2001).

  • 41.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Flux rate for water and carbon during greenschist facies metamorphism2011In: Geology, ISSN 0091-7613, E-ISSN 1943-2682, Vol. 39, no 1, p. 43-46Article in journal (Refereed)
    Abstract [en]

    The time-averaged flux rate for a CO2-bearing hydrous fluid during greenschist facies regional metamorphism was estimated to be 10–10.2 ± 0.4 m3 m−2 s−1 by combining (1) Peclet numbers obtained by chromatographic analysis of the propagation of reaction fronts in 33 metamorphosed basaltic sills in the southwest Scottish Highlands (UK), (2) empirical diffusion rates for CO2 in water, and (3) calculated time-averaged metamorphic porosities. The latter were calculated using an expression obtained by combining estimated Peclet numbers with empirical porosity-permeability relationships and Darcy's law. This approach yielded a time-averaged metamorphic porosity of 10–2.6 ± 0.2 for greenschist facies conditions. The corresponding time scale for metamorphic fluid flow was 103.6 ± 0.1 yr. By using mineral assemblages to constrain fluid compositions, a time-averaged annual flux rate for carbon of 0.5–7 mol C m−2 yr−1 was calculated. This matches measured emission rates for metamorphic CO2 from orogenic hot springs and exceeds estimated rates of CO2 drawdown by orogenic silicate weathering, suggesting that orogenesis is a source rather than a sink of atmospheric CO2.

  • 42.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Flux rates for water and carbon during greenschist facies metamorphism2011In: Geology, ISSN 0091-7613, E-ISSN 1943-2682, Vol. 39, no 1, p. 43-46Article in journal (Refereed)
    Abstract [en]

    The time-averaged flux rate for a CO2-bearing hydrous fluid during greenschist facies regional metamorphism was estimated to be 10–10.2 ± 0.4 m3 m−2 s−1 by combining (1) Peclet numbers obtained by chromatographic analysis of the propagation of reaction fronts in 33 metamorphosed basaltic sills in the southwest Scottish Highlands (UK), (2) empirical diffusion rates for CO2 in water, and (3) calculated time-averaged metamorphic porosities. The latter were calculated using an expression obtained by combining estimated Peclet numbers with empirical porosity-permeability relationships and Darcy's law. This approach yielded a time-averaged metamorphic porosity of 10–2.6 ± 0.2 for greenschist facies conditions. The corresponding time scale for metamorphic fluid flow was 103.6 ± 0.1 yr. By using mineral assemblages to constrain fluid compositions, a time-averaged annual flux rate for carbon of 0.5–7 mol C m−2 yr−1 was calculated. This matches measured emission rates for metamorphic CO2 from orogenic hot springs and exceeds estimated rates of CO2 drawdown by orogenic silicate weathering, suggesting that orogenesis is a source rather than a sink of atmospheric CO2.

  • 43.
    Skelton, Alasdair
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Andrén, Margareta
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Kristmannsdottir, Hrefna
    Stockmann, Gabrielle
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Mörth, Carl-Magnus
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Sveinbjoernsdottir, Arny
    Jonsson, Sigurjon
    Sturkell, Erik
    Gudorunardottir, Helga Rakel
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Hjartarson, Hreinn
    Siegmund, Heike
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Kockum, Ingrid
    Changes in groundwater chemistry before two consecutive earthquakes in Iceland2014In: Nature Geoscience, ISSN 1752-0894, E-ISSN 1752-0908, Vol. 7, no 10, p. 752-756Article in journal (Refereed)
    Abstract [en]

    Groundwater chemistry has been observed to change before earthquakes and is proposed as a precursor signal. Such changes include variations in radon count rates(1,2), concentrations of dissolved elements(3-5) and stable isotope ratios(4,5). Changes in seismicwave velocities(6), water levels in boreholes(7), micro-seismicity(8) and shear wave splitting(9) are also thought to precede earthquakes. Precursor activity has been attributed to expansion of rock volume(7,10,11). However, most studies of precursory phenomena lack sufficient data to rule out other explanations unrelated to earthquakes(12). For example, reproducibility of a precursor signal has seldom been shown and few precursors have been evaluated statistically. Here we analyse the stable isotope ratios and dissolved element concentrations of groundwater taken from a borehole in northern Iceland between 2008 and 2013. We find that the chemistry of the groundwater changed four to six months before two greater than magnitude 5 earthquakes that occurred in October 2012 and April 2013. Statistical analyses indicate that the changes in groundwater chemistry were associated with the earthquakes. We suggest that the changes were caused by crustal dilation associated with stress build-up before each earthquake, which caused different groundwater components to mix. Although the changes we detect are specific for the site in Iceland, we infer that similar processes may be active elsewhere, and that groundwater chemistry is a promising target for future studies on the predictability of earthquakes.

  • 44.
    Skelton, Alasdair
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Arghe, Fredrik
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Pitcairn, Iain
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Regional mapping of pre-metamorphic spilitization and associated chemical mobility in greenschist-facies metabasalts of the SW Scottish Highlands2010In: Journal of the Geological Society, ISSN 0016-7649, E-ISSN 2041-479X, Vol. 167, no 5, p. 1049-1061Article in journal (Refereed)
    Abstract [en]

    Both spilitic and non-spilitic metabasaltic sills are hosted by greenschist-facies metasediments in the SW Scottish Highlands. Spilitization is mainly characterized by enrichment in Na2O, elevated modal plagioclase and epidote segregations. Mapping of the spatial distribution of spilitic metabasalts reveals an ancient sub-sea-floor fluid cell centred on the extrusive Tayvallich Volcanics. Fluid circulation was most extensive at shallow levels where most sills were spilitized. We attribute this to pervasive flow of saline fluid, which was thermally driven by the cooling suite of lava flows and sills. Spilitization below this lithostratigraphic depth was restricted to only a few sills. Their spilitization is largely unrelated to specific properties of these sills (e. g. width, chemistry or host lithology). We conclude that fluid channelling was an intrinsic property of sub-sea-floor fluid flow either at deeper levels or earlier during fluid circulation. By profiling of the size distributions of relic phenocrysts in a partly spilitized sill, we conclude that spilitization proceeds with the symmetric propagation of a spilitization front from the sill margins towards the sill interior. Based on chemical profiling across the margin of an epidote segregation, we conclude that spilitization is associated with chemical transport on scales ranging from 0.1 to 10 m.

  • 45.
    Skelton, Alasdair
    et al.
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry. Geologi.
    Claesson, Lillemor
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry. Geologi.
    Chakrapani, Govinda
    Mahanta, Chandan
    Routh, Joyanto
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry. Geokemi.
    Mörth, Carl-Magnus
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry. Geokemi.
    Khanna, Param
    Seismic-hydrogeochemical coupling in north-eastern India2008In: International Geological Congress, 33, 2008, p. CD-ROMConference paper (Other academic)
    Abstract [en]

    We report the result of a hydrogeochemical monitoring program, which has been operational north of the Shillong Plateau, Assam, India from December 2003. The aim of this ongoing study is to test for coupling between the groundwater chemistry collected from a granite-hosted aquifer, located at a depth of 110m, and seismic activity. Based on molar Na+/Ca2+ and molar HCO3-/SiO2 ratios after Garrels (1967), we interpret that groundwater chemistry is normally buffered by the alteration of feldspar (plagioclase) to kaolinite.

    During the study, we monitored transient chemical changes which coincided temporally with a period of increased seismic activity. This included (1) MW = 5.3 and MW = 5.0 earthquakes which occurred on December 9, 2004 and February 15, 2005, south of the Shillong Plateau and 206 and 213 km from the sampling station, respectively, and (2) the Great Sumatra – Andaman Islands Earthquake of December 26, 2004. These are the only three MW > 5 earthquakes which have occurred during our study and for which our monitoring site is within their respective strain radii as given by Dobrovolsky et al. (1979).

    The most dramatic chemical change was a coincident and approximately 2-fold increase of the ratios [Na+K]/Si, Na/K and [Na+K]/Ca. This was accompanied by significant increases of conductivity, alkalinity and chloride concentration. The onset of this chemical shift occurred 3-5 weeks before the first (MW = 5.3) earthquake. We interpret a transient switchover between source aquifers, which induced an influx of groundwater from a second and probably deeper aquifer, where groundwater chemistry was dominantly buffered by the alteration of feldspar to smectite. This could have occurred in response to fracturing of a hydrological barrier. We also recorded a rapid drop in the ratio Ba/Sr, which occurred 3-6 days before the final (MW = 5.0) earthquake. We interpret a transient switchover to anorthite dissolution caused by exposure of fresh plagioclase to groundwater interaction. This could have been induced by microfracturing, locally within the main aquifer. Both of these changes were transient and “recovery” occurred over periods of 2-4 weeks. By comparison with experimental studies of feldspar dissolution, we suggest that hydrogeochemical recovery was facilitated by groundwater interaction and clay mineralization, which could have been coupled with fracture sealing.

    The main argument in support of seismic-hydrogeochemical coupling is the coincidence in timing of two hydrogeochemical events with two MW  5 earthquakes. Reasons for ambiguity include the lack of similar hydrogeochemical anomalies temporally coupled with smaller seismic events which occurred much closer to the monitoring station, the >200 km length scale of inferred seismic-hydrogeochemical coupling, and the potential for far-field effects related to the Great Sumatra – Andaman Islands Earthquake of December 26, 2004. The hydrogeochemical anomalies reported in this study meet some of the validation criteria of the IASPEI (International Association of Seismology and Physics of the Earth’s Interior) sub-commission on earthquake prediction (Wyss, 1991; 1997) in that a relation to pre-seismic stress and that some dependence on distance from the earthquake foci is inferred. However, hydrogeochemical data was collected from only one site, and even although the hydrogeochemical anomalies are recorded using several instrumental methods the reported anomalies are not truly independent of one another.

  • 46.
    Skelton, Alasdair
    et al.
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry. Geologi.
    Claesson, Lillemor
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry. Geologi.
    Chakrapani, Govinda
    Mahanta, Chandan
    Routh, Joyanto
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry. Geokemi.
    Mörth, Magnus
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry. Geokemi.
    Khanna, Param
    Coupling Between Seismic Activity and Hydrogeochemistry at the Shillong Plateau, Northeastern India2008In: Pure and Applied GeophysicsArticle in journal (Other (popular science, discussion, etc.))
    Abstract [en]

    Transient hydrogeochemical anomalies were detected in a granite-hosted aquifer, which is

    located at a depth of 110 m, north of the Shillong Plateau, Assam, India, where groundwater chemistry is mainly

    buffered by feldspar alteration to kaolinite. Their onsets preceded moderate earthquakes on December 9, 2004

    (MW = 5.3) and February 15, 2005 (MW = 5.0), respectively, 206 and 213 km from the aquifer. The

    ratios [Na+K]/Si, Na/K and [Na+K]/Ca, conductivity, alkalinity and chloride concentration began increasing

    3–5 weeks before the MW = 5.3 earthquake. By comparison with field, experimental and theoretical studies, we

    interpret a transient switchover between source aquifers, which induced an influx of groundwater from a second

    aquifer, where groundwater chemistry was dominantly buffered by the alteration of feldspar to smectite.

    This could have occurred in response to fracturing of a hydrological barrier. The ratio Ba/Sr began decreasing

    3–6 days before the MW = 5.0 earthquake. We interpret a transient switchover to anorthite dissolution caused

    by exposure of fresh plagioclase to groundwater interaction. This could have been induced by microfracturing,

    locally within the main aquifer. By comparison with experimental studies of feldspar dissolution, we interpret

    that hydrogeochemical recovery was facilitated by groundwater interaction and clay mineralization, which could

    have been coupled with fracture sealing. The coincidence in timing of these two hydrogeochemical events with

    the only two MW C 5 earthquakes in the study area argues in favor of cause-and-effect seismichydrogeochemical

    coupling. However, reasons for ambiguity include the lack of similar hydrogeochemical

    anomalies coupled with smaller seismic events near the monitoring station, the >200 km length scale of inferred

    seismic-hydrogeochemical coupling, and the potential for far-field effects related to the Great Sumatra–

    Andaman Islands Earthquake of December 26, 2004.

  • 47.
    Skelton, Alasdair
    et al.
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry. Geologi.
    Crill, Patrick
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry. Geokemi.
    Arghe, Fredrik
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry. Geologi.
    Whitmarsh, Bob
    Koyi, Hemin
    Quantification of the rate of methane production by serpentinization2008In: International Geological Congress, 33, 2008, p. CD-ROMConference paper (Other academic)
    Abstract [en]

    We used seismic velocity as a proxy for serpentinization of the mantle, which occurred beneath thinned but laterally continuous continental crust during continental break up, prior to opening of the Atlantic Ocean. The serpentinized sub-continental mantle is now exhumed, beneath the Iberia Abyssal Plain and was accessed by scientific drilling on Ocean Drilling Program legs 149 and 173. Chromatographic modelling of the serpentinization front yields a front displacement (z) of 2197 ± 89 m. We ignored diffusive broadening of this front in the direction of fluid flow and used the shape of the front to constrain a Damköhler number (ND). This was scaled such that ND = t, where  is a rate constant for serpentinization in s-1 and t is time in s. We thus obtained ND = 6.0 ± 0.2. We then estimated values of  for (1) surface reaction as rate-limiting and (2) chemical transport as rate-limiting. Combining these values with our estimate of the Damköhler number yielded comparable timescales of 10,000 – 1,000,000 years for serpentinization. Combining this timescale with the front displacement yielded a time-averaged volumetric rate for serpentinization of 0.002-0.2 m3-serpentinite.m-2.yr-1.

    We then referred to the experimental results of Janecky and Seyfried (1986), which predicted that serpentinization by reaction with seawater probably occurs by a coupled set of reactions, with olivine, orthopyroxene and clinopyroxene dissolving independently and at different rates. Of this set of reactions, olivine dissolution is predicted to occur by the reaction:

    2(Mg,Fe)2SiO4 + 2H+ + H2O = (Mg,Fe)3Si2O5(OH)4 + (Mg,Fe)2+ (1)

    Because lizardite, which is the most abundant of the serpentine minerals typically contains no more than 5 wt. % FeO (Wicks & O’ Hanley 1988), excess Fe2+ will preferentially (with respect to Mg2+) enter the solution. Oxidation of Fe2+ by H2O and CO2 can then produce magnetite and CH4 by the reaction:

    12Fe2+ + 14H2O + CO2 = 4Fe3O4 + 24H+ + CH4 (2)

    Fe3O4 (magnetite) can be used as a proxy for CH4 produced by reactions (1) and (2) and exiting the mantle. The average mode of magnetite in 16 samples of >95% serpentinized peridotite recovered by scientific drilling within the region of exhumed mantle is 4.5 vol. %. This is equivalent to 1000 mol-Fe3O4.m-3. Combining this value with the time-averaged volumetric rate for serpentinization, we obtain a time-averaged annual production rate for magnetite of 2-200 mol.m-2.yr-1. Finally, based on the stoichiometry of reaction (2), we thus obtain an annual flux rate for CH4 production of 0.5-50 mol.m-2.yr-1.

  • 48.
    Skelton, Alasdair
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Greiser, Caroline
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Fopp, David
    Stockholm University, Faculty of Social Sciences, Department of Child and Youth Studies.
    Lagerlund, Henrik
    Stockholm University, Faculty of Humanities, Department of Philosophy.
    Björk, Mats
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Glantz, Paul
    Stockholm University, Faculty of Science, Department of Environmental Science.
    Carton, Wim
    10 myths about net zero targets and carbon offsetting, busted2020In: Climate Home NewsArticle in journal (Other (popular science, discussion, etc.))
  • 49.
    Skelton, Alasdair
    et al.
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry. Geologi.
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry. Marin geovetenskap.
    Could peridotite hydration reactions have provided a contributory driving force for Cenozoic uplift and accelerated subsidence along the margins of the North Atlantic and Labrador Sea?2007In: Norwegian Journal of Geology, ISSN 029-196X, Vol. 87, p. 241-248Article in journal (Refereed)
    Abstract [en]

    This study evaluates the hypothesis that peridotite hydration reactions (e.g. serpentinisation) at the landward termination of transform fracture zones provide a contributory driving force for coupled uplift and accelerated subsidence along the margins of the North Atlantic and Labrador Sea in the Cenozoic. This evaluation is partly based on the extent and rate of serpentinisation, calculated by Skelton et al. (2005) by using seismic velocity as a proxy for progress of the serpentinisation reactions. The hypothesis is supported by 1) spatial coincidence between most of the uplifted segments of the margin with the landward termination of transform fracture zones, 2) the theoretical capacity of serpentinisation to generate 102-103 m of uplift at a rate of mm.a-1 to cm.a-1 which is consistent with observations from the margin, and 3) the potential for landward material flow of a hydrated peridotite inclusion, providing a mechanism for sustaining uplift and its pairing with accelerated subsidence. Also, serpentinisation is more effective than other metamorphic reactions (e.g. granulite to amphibolite, eclogite to amphibolite) as a driving force for uplift. Shortfalls of this model are that 1) extensive peridotite hydration is unlikely at depths exceeding 10-20km and 2) the timing of uplift requires that pulses of extensive peridotite hydration occurred along inactive segments of transform fracture zones. We conclude that the volume expansion caused by peridotite hydration was probably insufficient to account for widespread uplift during the Cenozoic. However, we suggest that the following processes could occur at or near the landward terminations of transform fracture zones: 1) volume expansion caused by extensive peridotite hydration beneath thinned crust at or near the ocean-continent transition and 2) mechanical weakening caused by limited peridotite hydration beneath thicker continental crust. These processes may have important implications for models aimed at explaining Cenozoic uplift and accelerated subsidence.

  • 50.
    Skelton, Alasdair
    et al.
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    The role of peridotite hydration in sustaining elevation of passive margins2008In: International Geological Congress, 33, 2008, p. CD-ROMConference paper (Other (popular science, discussion, etc.))
    Abstract [en]

    Hydration of mantle peridotite, occurs in active tectonic settings, such as mid-oceanic ridges, magma-poor continental rifts, subduction zones, and transform faults and fracture zones. Hydration of mantle peridotite can produce talc and/or anthophyllite above 400-600C and the serpentine minerals: antigorite, lizardite and chrysotile at temperatures below 400-600C. These reactions are coupled with volume gains of 1% and >15%, respectively. Both reactions are also accompanied by considerable mechanical weakening. This is seen from the coefficients of internal fraction of peridotite, lizardite and talc, which are 0.75, 0.35 and 0.2, respectively.

    This study evaluates the hypothesis that, because of the accompanying volume gain and mechanical weakening, peridotite hydration reactions, occurring at the landward termination of transform fracture zones, provide a contributory driving force for elevating, or sustaining the elevation of, passive continental margins of the North Atlantic region during the Cenozoic.

    Based on a time-averaged propagation velocity of the serpentinization front of 0.2-20 cm/yr., which was estimated by Skelton et al. (2005) for serpentinization of exhumed mantle at the Iberia margin, we calculate a maximum time-averaged uplift rate of 0.03 – 3 cm/yr. or 1 km of uplift in 0.03 – 3 million yrs. This result (which assumes a volume expansion of 15.5% and that this volume expansion is entirely translated into vertical motion) is broadly consistent with observations from the passive continental margins of the North Atlantic region. Serpentinisation might thus be more effective than other metamorphic reactions (e.g. granulite to amphibolite, eclogite to amphibolite) as a driving force for elevation of passive continental margins. This hypothesis gains further support from the spatial coincidence between most of the uplifted segments of the margin with the landward termination of transform fracture zones. However, critical shortfalls of this model are that (1) extensive peridotite hydration is unlikely at depths exceeding 10-20km because temperatures exceeding 400-600C will result in the production of talc and/or anthophyllite, not the serpentine minerals, and therefore the accompanying volume expansion is unlikely to exceed 1% and (2) the timing of uplift requires that pulses of extensive peridotite hydration occurred along inactive segments of transform fracture zones. We must therefore conclude that the volume expansion caused by peridotite hydration was probably insufficient to account for widespread elevation of the passive continental margins of the North Atlantic region. However, we suggest that mechanical weakening, which accompanies peridotite hydration, even at depths exceeding 10-20 km, might promote and/or focus tectonic motion. This could, for example, enable landward “flow” of partly hydrated peridotite, which would sustain elevation of a passive continental margin and explain the common observation of pairing with offshore subsidence.

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