EGU2020-20622
https://doi.org/10.5194/egusphere-egu2020-20622
EGU General Assembly 2020
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Oxidized slab fluids revealed in metasomatized eclogites: A case study from Syros, Greece

Jesse Walters1,2, Horst Marschall2, Pierre Lanari3, and Alicia Cruz-Uribe1
Jesse Walters et al.
  • 1University of Maine, School of Earth and Climate Science, United States of America (jesse.walters@maine.edu)
  • 2Goethe-Universität Frankfurt, Institut für Geowissenschaften, Germany
  • 3University of Bern, Institute of Geological Sciences, Switzerland

Garnet-epidote oxybarometry, major element mineral compositions, and textural analysis of eclogites from Syros, Greece reveal the presence of high fO2 slab fluids. The four investigated samples from blocks hosted in the serpentinite matrix mélange on Syros fall into three categories: unmetasomatized eclogite (Type I, n = 1), heavily metasomatized garnet-clinopyroxene bearing rocks (Type II, n = 2), and an eclogite which hosts veins of Andr + Acm + Ep + Hem + Chl (Type III, n = 1). Type I samples of metagabbroic eclogite are characterized by a peak assemblage of Grt + Cpx + Rt with abundant clinozoisite after lawsonite. In addition to Grt + Cpx, Type II samples contain Chl + Ilm + Py + Ap ± Ep ± Cam. Clinopyroxene compositions within Type I samples display a prograde trend of increasing jadeite and decreasing acmite. In contrast, the matrix clinopyroxene in one Type II sample exhibits compositions up to 60 % acmite component. Alternatively, clinopyroxene in the second Type II sample investigated exhibit an increase in acmite component during metasomatism. Type II samples also contain epidote and hematite-rich ilmenite as opposed to clinozoisite and rutile. The association of Fe3+-rich phases with sulfides, such as inclusions of acmitic pyroxene in pyrite, in Type II samples suggests a temporal link between sulfide deposition and oxidation of Fe2+ to Fe3+. In contrast, the Adr + Acm + Ep + Chl + Hem veins in the Type III sample are sulfide absent and suggest fluids with fO2 above the Hem-Mag and Hem-Py buffers.

Garnet-epidote oxybarometry revealed elevated fO2 in metasomatically altered samples. Calculations were performed using a new oxybarometry Matlab code written by the authors. Our code utilizes the latest thermodynamic database, A-X models, and equations of state implemented in THERMOCALC. The code was also implemented in the XMapTools software package for quantitative visualization of fO2 using EPMA X-ray maps. Fugacity calculations were conducted at 550 °C, 2.0 GPa, and an aH2O of unity, unless otherwise stated. Oxygen fugacities for clinozoisite-garnet pairs in the Type I sample were calculated in XMapTools and fall within 0.5 log units of the quartz-fayalite-magnetite (QFM) buffer. Inclusions of garnet in epidote and epidote overgrowths on garnet were selected for fO2 calculation in the Ep-bearing Type II sample. These garnet-epidote pairs exhibit fO2 of QFM+1.9 to +2.0. A minimum fO2 of QFM+4 calculated from the Hem-Mag buffer is estimated for Type III veins. The remarkably high fO2 of Type III veins contrasts with prograde fO2conditions of QFM+1 to +2 estimated for epidote inclusions in garnet cores from the same sample at 400-450 °C and 1.0-1.5 GPa. While elevated fO2 and acmite inclusions in pyrite are consistent with a SOx(aq)-Fe2+ redox pair in Type II samples, fO2 above the Hem-Mag buffer require the bulk addition of Fe3+ or Mn3+ as a carrier of oxidation. These data demonstrate that slab fluids may impose fO2 well above the sulfur-sulfur oxide buffer.

How to cite: Walters, J., Marschall, H., Lanari, P., and Cruz-Uribe, A.: Oxidized slab fluids revealed in metasomatized eclogites: A case study from Syros, Greece, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20622, https://doi.org/10.5194/egusphere-egu2020-20622, 2020.

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