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Preservation of blueschist-facies minerals along a shear zone by coupled metasomatism and fast-flowing CO2-bearing fluids
Stockholm University, Faculty of Science, Department of Geological Sciences.ORCID iD: 0000-0001-6435-2732
Stockholm University, Faculty of Science, Department of Geological Sciences.
Stockholm University, Faculty of Science, Department of Geological Sciences.
2014 (English)In: Journal of Petrology, ISSN 0022-3530, E-ISSN 1460-2415, Vol. 55, no 10, 1905-1939 p.Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
2014. Vol. 55, no 10, 1905-1939 p.
Keyword [en]
blueschist preservation, fluid flux calculation, fluid-rock interaction, Si-Na metasomatism, thermodynamic modeling
National Category
Geology
Research subject
Geology
Identifiers
URN: urn:nbn:se:su:diva-108429DOI: 10.1093/petrology/egu045ISI: 000343696800001OAI: oai:DiVA.org:su-108429DiVA: diva2:757888
Available from: 2014-10-23 Created: 2014-10-23 Last updated: 2017-12-05Bibliographically approved
In thesis
1. How do metamorphic fluids move through rocks?: An investigation of timescales, infiltration mechanisms and mineralogical controls
Open this publication in new window or tab >>How do metamorphic fluids move through rocks?: An investigation of timescales, infiltration mechanisms and mineralogical controls
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis aims to provide a better understanding of the role of mountain building in the carbon cycle. The amount of CO2 released into the atmosphere due to metamorphic processes is largely unknown. To constrain the quantity of CO2 released, fluid-driven reactions in metamorphic rocks can be studied by tracking fluid-rock interactions along ancient fluid flow pathways. The thesis is divided into two parts: 1) modeling of fluid flow rates and durations within shear zones and fractures during greenschist- and blueschist-facies metamorphism and 2) the assessment of possible mechanisms of fluid infiltration into rocks during greenschist- to epidote-amphibolite-facies metamorphism and controlling chemical and mineralogical factors of reaction front propagation.

On the island Syros, Greece, fluid-rock interaction was examined along a shear zone and within brittle fractures to calculate fluid flux rates, flow velocities and durations. Petrological, geochemical and thermodynamic evidence show that the flux of CO2-bearing fluids along the shear zone was 100-2000 times larger than the fluid flux in the surrounding rocks. The time-averaged fluid flow velocity and flow duration along brittle fractures was calculated by using a governing equation for one-dimensional transport (advection and diffusion) and field-based parameterization. This study shows that fluid flow along fractures on Syros was rapid and short lived.

Mechanisms and controlling factors of fluid infiltration were studied in greenschist- to epidote-amphibolite-facies metabasalts in SW Scotland. Fluid infiltration into metabasaltic sills was unassisted by deformation and occurred along grain boundaries of hydrous minerals (e.g. amphibole) while other minerals (e.g. quartz) prevent fluid infiltration. Petrological, mineralogical and chemical studies of the sills show that the availability of reactant minerals and mechanical factors, e.g. volume change in epidote, are primary controls of reaction front propagation.

Place, publisher, year, edition, pages
Stockholm: Department of Geological Sciences, Stockholm University, 2015. 35 p.
Series
Meddelanden från Stockholms universitets institution för geologiska vetenskaper, 356
Keyword
Metamorphic fluid flow, fluid-rock interaction, fluid infiltration mechanisms, fluid flux rates, thermodynamic modeling, reaction front propagation, fluid flux calculation
National Category
Geology
Research subject
Geology
Identifiers
urn:nbn:se:su:diva-115172 (URN)978-91-7649-120-1 (ISBN)
Public defence
2015-04-28, De Geersalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Manuscript. Paper 3: Manuscript. Paper 4: Manuscript.

 

Available from: 2015-03-31 Created: 2015-03-17 Last updated: 2015-06-18Bibliographically approved

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