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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. 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.

  • 5.
    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

  • 6.
    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.

  • 7.
    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.

  • 8.
    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

  • 9.
    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.

  • 10. 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, ISSN 2045-2322, 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.

  • 11.
    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.

  • 12.
    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.

  • 13.
    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.

  • 14.
    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.

  • 15.
    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.

  • 16.
    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.

  • 17.
    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.

  • 18.
    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)
  • 19.
    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.

  • 20.
    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)
  • 21. 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.

  • 22. 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)
  • 23.
    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.

  • 24.
    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
    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.

  • 25.
    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.

  • 26.
    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.

  • 27.
    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.

  • 28.
    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.

  • 29.
    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).

  • 30.
    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.

  • 31.
    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.

  • 32.
    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.

  • 33.
    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.

  • 34.
    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.

  • 35.
    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.

  • 36.
    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.

  • 37.
    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.

  • 38.
    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.

  • 39.
    Skelton, Alasdair
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Lewerentz, Alexander
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Kleine, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Webster, David
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Pitcairn, Iain
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Structural Channelling of Metamorphic Fluids on Islay, Scotland: Implications for Paleoclimatic Reconstruction2015In: Journal of Petrology, ISSN 0022-3530, E-ISSN 1460-2415, Vol. 56, no 11, p. 2145-2171Article in journal (Refereed)
    Abstract [en]

    Analysis of the delta O-18 and delta C-13 values of carbonate rocks from Islay, Scotland reveals structural channelling of metamorphic fluids through the axial region of a major en echelon anticlinal fold system. Metamorphic fluid flow produced axial planar veins with higher vein density in the axial region of the fold. Fluid: rock ratios were more than 30: 1 within this axial region, at least four times greater than the regional mean ratio of 7.6 +/- 1.5:1 for carbonate rocks on Islay. This supports the interpretation that metamorphic fluids were channelled through the axial region of the Islay Anticline. Fluid: rock ratios were calculated using a model for coupled delta O-18 and delta C-13 exchange with a metamorphic fluid. The metamorphic fluid was calculated to have delta O-18 and delta C-13 values of 15.3 parts per thousand and -6.1 parts per thousand, respectively and X-CO2 of 0.2. This is in isotopic and chemical equilibrium with chlorite- and graphite-bearing metamudstones that are structurally below the folded metacarbonate rocks on Islay. Devolatilization of these metamudstones is therefore a likely source mechanism for this metamorphic fluid. Removal of the effects of metamorphic fluid flow on delta C-13 values recorded by metacarbonate rocks on Islay allows us to re-evaluate evidence used to reconstruct Neoproterozoic climate. This evidence includes a large negative delta C-13 excursion reported from the Lossit Limestone Formation. This unit underlies the Port Askaig Formation, which is dominated by diamictites that have been interpreted as glacial tillites. This 'Islay anomaly' has been correlated with other such anomalies worldwide and together with overlying tillites has been cited as evidence of major (worldwide) glaciation events. In this study, we show that the magnitude of this negative delta C-13 anomaly can partly be explained by exchange with metamorphic fluids. However, we also show that extremely negative delta C-13 values in the Bonahaven Dolomite Formation, which overlies the Port Askaig Formation and has been interpreted as a 'cap carbonate', cannot be attributed to metamorphic fluid flow.

  • 40.
    Skelton, Alasdair
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Peillod, Alexandre
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Ring, Uwe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Glodny, Johannes
    Coupled replacement and preservation of high pressure rocks caused by infiltration of mixed H2O-CO2 fluidsManuscript (preprint) (Other academic)
    Abstract [en]

    Eclogites, blueschists and greenschists are found in close proximity to one another along a 1-km coastal section where the Cyclades Blueschist Unit is exposed on SE Syros, Greece. Here we show that the eclogites and blueschists experienced similar metamorphic histories: prograde lawsonite blueschist facies metamorphism at 1.1–1.4 GPa and 380–460 °C followed, at 38–43 Ma, by peak blueschist/eclogite facies metamorphism at 1.5–1.8 GPa and 470–500 °C. We explain co-existence of eclogites and blueschists by compositional variation probably reflecting original (sedimentary) layering. We also show that the greenschists record retrogression at 0.34 ± 0.21 GPa and T = 456 ± 68 °C associated with fluid flow along a shear zone, bearing eclogite facies ultramafic knockers, that cuts across the section. Finally, we show that greenschist facies retrogression ended at (or slightly after) 27 Ma. This implies a period of metamorphic quiescence after eclogite/blueschist facies metamorphism and before greenschist facies retrogression that may have lasted 13–16 million years. We suggest that this reflects an absence of metamorphic fluid flow at that time. Referring to local preservation of blueschists and eclogites along the coastal section, we infer that retrogression only occurred when and where metamorphic fluid flow occurred. Finally, from a tectonic perspective, our findings are consistent with studies showing that 1) the Cyclades Blueschist Unit is a high-pressure nappe stack consisting of belts in which high pressure metamorphism occurred at different times and 2) these rocks were affected by regionally extensive greenschist facies metamorphism during the Oligocene.

  • 41.
    Skelton, Alasdair
    et al.
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry. Geologi.
    Sanden, Michael
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry. Marine geovetenskap.
    Asavanant, Jack
    Eyewitness reports of the impact of the 2004 tsunami in the Khao Lak area, Thailand2008In: International Geological Congress, 33, 2008Conference paper (Other (popular science, discussion, etc.))
    Abstract [en]

    On December 26, 2004, an MW 9.1 earthquake which occurred off the western coast of Sumatra triggered tsunami waves which propagated across the Indian Ocean. We present data collected from 20 eyewitness reports , who experienced the tsunami on December 26, 2004 in the class IV impact region, near Khao Lak, Thailand. These data include (1) paths by which eyewitnesses were carried by the tsunami wave(s), (2) number of waves experienced, (3) relative strength and height of the wave(s), (4) the geometry of the first impacting wave front and (5) the time interval between the arrivals of subsequent waves. These data are broadly consistent with (1) tidal gauge measurements, (2) measured runup heights (Tsuji et al., 2006; Choi et al., 2006) and (3) numerical simulations (Ioulalen et al., 2007). Based on these data we make the following tentative interpretations:

    1. The tsunami impacted in an east-northeasterly to northeasterly direction.

    2. The sea began retreating rapidly at approximately 10.00 a.m. local time.

    3. The first wave front impacted at 10.30 a.m. local time.

    4. The wave period was 35-50 minutes. Shorter wave periods (21-27 and 16-30 minutes) were estimated from eyewitness data. These probably reflect the complex geometry of the wave maxima with multiple (2-4) crests, which may have been experienced as separate waves.

    6. The wavelength was between 15-20 and 6-8 km. Given the wave period (35-50 minutes), we note that this could reflect attenuation of the incoming wave from a velocity of 18-34 km/hour to a velocity of 7-14 km/hour.

    7. Eye witness reports suggest that the wave height was 5-12 m. These estimates are broadly consistent with tidal gauge measurements (7-8 m) and runup heights (8-11 m).

    8. Wave refraction off the peninsula, west of Taph Lamu, might have been responsible for a smaller and weaker “pre-wave” which was experienced by survivors in the central part of the Khao Lak area and evident both from tidal gauge measurements and the model simulations.

    Based on the broad consistency between eyewitness reports, tidal gauge and runup measurements and model simulations, we conclude that eyewitness reports can provide a robust source of both qualitative and quantitative data, which can be used to constrain numerical models of tsunami propagation.

  • 42.
    Skelton, Alasdair
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Sturkell, Erik
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Einarsson, Draupnir
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Tollefsen, Elin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Orr, Tim
    Dimmuborgir: a rootless shield complex in northern Iceland2016In: Bulletin of Volcanology, ISSN 0258-8900, E-ISSN 1432-0819, Vol. 78, no 5, article id 40Article in journal (Refereed)
    Abstract [en]

    The origin of Dimmuborgir, a shield-like volcanic structure within the Younger Laxa lava flow field near Lake Myvatn, in northern Iceland, has long been questioned. New airborne laser mapping (light detection and ranging (LiDAR)), combined with ground-penetrating radar results and a detailed field study, suggests that Dimmuborgir is a complex of at least two overlapping rootless shields fed by lava erupting from the nearby Ludentarborgir crater row. This model builds upon previous explanations for the formation of Dimmuborgir and is consistent with observations of rootless shield development at Kilauea Volcano, Hawaii. The larger rootless shields at Dimmuborgir, 1-1.5 km in diameter, elliptical in plan view, similar to 30 m in height, and each with a 500-m-wide summit depression, were capable of storing as much as 2-3x10(6) m(3) of lava. They were fed by lava which descended 30-60 min lava tubes along a distance of 3 km from the crater row. The height difference generated pressure sufficient to build rootless shields at Dimmuborgir in a timescale of weeks. The main summit depressions, inferred to be drained lava ponds, could have emptied via a 30-m-wide x 5-m-deep channel, with estimated effusion rates of 0.7-7 m(3) s(-1) and minimum flow durations of 5-50 days. We argue that the pillars for which Dimmuborgir is famed are remnants of lava pond rims, at various stages of disintegration that formed during pond drainage.

  • 43.
    Skelton, Alasdair
    et al.
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Whitmarsh, R.
    Edinburgh Univ.
    Arghe, F.
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Crill, Patrick
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Koyi, H.
    Uppsala Univ.
    Constraining the rate and extent of mantle serpentization from seismic and petrological data: Implications for chemosynthesis and tectonic processes.2005In: Geofluids, ISSN 1468-8115, E-ISSN 1468-8123, Vol. 5, no 3, p. 153-164Article in journal (Refereed)
    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 kinetically limited transport of the serpentinization front yields a front displacement of 2197 ± 89 m, a time-integrated fluid flux of 1098 ± 45 m<sup>3</sup> m<sup>−2</sup> and a Damköhler number of 6.0 ± 0.2. Whether either surface reaction or chemical transport limit the rate of reaction, we calculate timescales for serpentinization of approximately 10<sup>5</sup>–10<sup>6</sup> years. This yields time-average fluid flux rates for H<sub>2</sub>O, entering and reacting with the mantle, of 60–600 mol m<sup>−2</sup> a<sup>−1</sup> and for CH<sub>4</sub>, produced as a by-product of oxidation of Fe<sup>++</sup> to magnetite and exiting the mantle, of 0.55–5.5 mol m<sup>−2</sup> a<sup>−1</sup>. This equates to a CH<sub>4</sub>-flux of 0.18–1.8 Tg a<sup>−1</sup> for coeval serpentinization of the mantle that was exhumed west of Iberia. This represents 0.03–0.3% of the present-day annual CH<sub>4</sub>-flux from all sources and a higher fraction of pre-anthropogenic (lower) CH<sub>4</sub> levels. CH<sub>4</sub> released by serpentinization at or beneath the seafloor could provide substrate for biological chemosynthesis and/or promote gas-hydrate formation. Finally, noting its volumetric extent and rapidity (<10<sup>6</sup> years), we interpret serpentinization to be a reckonable component of tectonic processes, contributing both diapiric and expansional forces and helping to ‘lubricate’ extensional processes. Given its anisotropic permeability, actively deforming serpentinite might impede melt migration which may be of interest, given the apparent lack of melt in some rifted margins.

  • 44.
    Stockmann, Gabrielle
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Tollefsen, Elin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Brüchert, Volker
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Balic-Zunic, Tonci
    Langhof, Jörgen
    Skogby, Henrik
    Karlsson, Andreas
    Control of a calcite inhibitor (phosphate) and temperature on ikaite precipitation in Ikka Fjord, southwest Greenland2018In: Applied Geochemistry, ISSN 0883-2927, E-ISSN 1872-9134, Vol. 89, p. 11-22Article in journal (Refereed)
    Abstract [en]

    Ikaite (CaCO3 center dot 6H(2)O) forms submarine tufa columns in Ikka Fjord, SW Greenland. This unique occurrence is thought to relate to aqueous phosphate concentration and low water temperatures (< 6 degrees C). Phosphate ions are well-known inhibitors of calcite precipitation and Ikka Fjord has a naturally high-phosphate groundwater system that when mixing with seawater leads to the precipitation of ikaite. In the study presented here, experiments simulating conditions of Ikka Fjord show that a) the formation of ikaite is unrelated to the aqueous phosphate concentration (0-263 mu mol/ kg PO43-) in 0.1 M NaHCO3/0.1 M Na2CO3 solutions mixing with seawater at 5 degrees C and pH 9.6-10.6, and b) ikaite forms at temperatures up to 15 degrees C without phosphate and in open beakers exposed to air. Instead, supersaturation of ikaite and the seawater composition are the likely factors causing ikaite to precipitate in Ikka Fjord. This study shows that adding Mg2+ to a NaHCO3/Na2CO3 - CaCl2 mixed solution leads to the formation of ikaite along with hydrated Mg carbonates, which points to the high Mg2+ concentration of seawater, another known inhibitor of calcite, as a key factor promoting ikaite formation. In experiments at 10 and 15 degrees C, increasing amounts of either nesquehonite (Mg(HCO3)(OH)center dot 2H(2)O) or an amorphous phase co-precipitate with ikaite. At 20 degrees C, only the amorphous phase is formed. In warming Arctic seawater, this suggests Mg carbonate precipitation could become dominant over ikaite in the future.

  • 45.
    Wästeby, Niklas
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Tollefsen, Elin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Andrén, Margareta
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Stockmann, Gabrielle
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Liljedahl, Lillemor Claesson
    Sturkell, Erik
    Mörth, Magnus
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Hydrochemical monitoring, petrological observation, and geochemical modeling of fault healing after an earthquake2014In: Journal of Geophysical Research - Solid Earth, ISSN 2169-9313, E-ISSN 2169-9356, Vol. 119, no 7, p. 5727-5740Article in journal (Refereed)
    Abstract [en]

    Based on hydrochemical monitoring, petrological observations, and geochemical modeling, we identify a mechanism and estimate a time scale for fault healing after an earthquake. Hydrochemical monitoring of groundwater samples from an aquifer, which is at an approximate depth of 1200 m, was conducted over a period of 10 years. Groundwater samples have been taken from a borehole (HU-01) that crosses the Husavik-Flatey Fault (HFF) near Husavik town, northern Iceland. After 10 weeks of sampling, on 16 September 2002, an M 5.8 earthquake occurred on the Grimsey Lineament, which is approximately parallel to the HFF. This earthquake caused rupturing of a hydrological barrier resulting in an influx of groundwater from a second aquifer, which was recorded by 15-20% concentration increases for some cations and anions. This was followed by hydrochemical recovery. Based on petrological observations of tectonically exhumed fault rocks, we conclude that hydrochemical recovery recorded fault healing by precipitation of secondary minerals along fractures. Because hydrochemical recovery accelerated with time, we conclude that the growth rate of these minerals was controlled by reaction rates at mineral-water interfaces. Geochemical modeling confirmed that the secondary minerals which formed along fractures were saturated in the sampled groundwater. Fault healing and therefore hydrochemical recovery was periodically interrupted by refracturing events. Supported by field and petrographic evidence, we conclude that these events were caused by changes of fluid pressure probably coupled with earthquakes. These events became successively smaller as groundwater flux decreased with time. Despite refracturing, hydrochemical recovery reached completion 8-10 years after the earthquake.

  • 46.
    Zhao, Zhihong
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    An assessment of the role of nonlinear reaction kinetics in parameterization of metamorphic fluid flow2014In: Journal of Geophysical Research - Solid Earth, ISSN 2169-9313, E-ISSN 2169-9356, Vol. 119, no 8, p. 6249-6262Article in journal (Refereed)
    Abstract [en]

    Based on inverse modeling of reaction progress data using a numerical framework that considers coupled advection and diffusion, linear and nonlinear reaction kinetics, and with effective diffusivity given by Archie's law, we show that (1) choice of reaction order has little effect (<0.3 orders of magnitude) on estimates of time-integrated and time-averaged metamorphic fluid fluxes and metamorphic fluid flow durations based on reaction progress data, (2) reaction order must be known for robust determination of time-averaged net reaction rates based on reaction progress data and that underestimation of this term by more than 3 orders of magnitude can arise from assuming linear reaction kinetics, (3) differing reaction orders between laboratory experiments and natural metamorphic systems and/or a nonlinear dependence of effective diffusivity on porosity can explain order-of-magnitude discrepancies between field-based and laboratory-based estimates of time-averaged net reaction rates, and (4) parameterization of metamorphic fluid flow is limited to time-averaged values which fail to account for the possibility that metamorphism occurs in short-lived pulses during longer time periods of metamorphic quiescence.

  • 47.
    Zhao, Zhihong
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Simultaneous calculation of metamorphic fluid fluxes, reaction rates and fluid-rock interaction timescales using a novel inverse modeling framework2013In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 373, p. 217-227Article in journal (Refereed)
    Abstract [en]

    To study metamorphic carbonation at greenschist facies conditions in the SW Scottish Highlands, a novel inverse modeling framework, which combines solutions of the transport equation with a global optimization method of differential evolution, was developed. Using this framework, we calculated simultaneously time-integrated and time-averaged metamorphic fluid fluxes of 83.4 +/- 35.4 m(3) m(-2) and 10(-10.1) (+/-) (0.5) m(3) m(-2) s(-1), respectively, a time-averaged reaction rate constant of 10(-12.7--10.2) m s(-1) and comparable timescales for fluid flow and fluid-driven reaction of 10(4.3 +/- 0.5) yr and 10(2.7-5.2) yr, respectively. These parameters were calculated using an empirical estimate of the coefficient of molecular diffusion and a calculated value for metamorphic porosity. Our estimates are (1) consistent with single pass flow of fluid released by metamorphic devolatilization, (2) within the range where heat is transported by conduction and matter is transported by advection, (3) in agreement with an emerging consensus that metamorphic events are relatively short-lived, and (4) supportive of applying laboratory-based estimates of kinetic parameters to metamorphic systems. Based on a sensitivity analysis, we show that (1) selecting the diffusion coefficient (rather than fluid velocity, reaction rate or flow duration) as an input parameter yields more robust estimates of metamorphic fluid flow parameters, and (2) ignoring reaction-dependent porosity and reaction rates can result in an order-of-magnitude uncertainty in best-fit flow parameters, evaluated from concentration profiles. Finally, similarity between our calculated time-averaged metamorphic fluid fluxes which were obtained numerically and those which were obtained analytically confirms the validity of using the 'quasi-stationary state' assumption to quantify metamorphic fluid flow parameters.

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