Oxidized slab fluids revealed in metasomatized eclogites: A case study from Syros, Greece
Authors: Walters Jesse , Marschall Horst , Lanari Pierre , Cruz-Uribe Alicia 
Published: March 2020

&lt;p&gt;Garnet-epidote oxybarometry, major element mineral compositions, and textural analysis of eclogites from Syros, Greece reveal the presence of high fO&lt;sub&gt;2&lt;/sub&gt; slab fluids. The four investigated samples from blocks hosted in the serpentinite matrix m&amp;#233;lange 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 &amp;#177; Ep &amp;#177; 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 Fe&lt;sup&gt;3+&lt;/sup&gt;-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 Fe&lt;sup&gt;2+&lt;/sup&gt; to Fe&lt;sup&gt;3+&lt;/sup&gt;. In contrast, the Adr + Acm + Ep + Chl + Hem veins in the Type III sample are sulfide absent and suggest fluids with &lt;em&gt;f&lt;/em&gt;O&lt;sub&gt;2&lt;/sub&gt; above the Hem-Mag and Hem-Py buffers.&lt;/p&gt;&lt;p&gt;Garnet-epidote oxybarometry revealed elevated &lt;em&gt;f&lt;/em&gt;O&lt;sub&gt;2&lt;/sub&gt; in metasomatically altered samples. Calculations were performed using a new oxybarometry Matlab code written by the authors. Our code utilizes the latest thermodynamic database, &lt;em&gt;A-X&lt;/em&gt; models, and equations of state implemented in THERMOCALC. The code was also implemented in the XMapTools software package for quantitative visualization of &lt;em&gt;f&lt;/em&gt;O&lt;sub&gt;2&lt;/sub&gt; using EPMA X-ray maps. Fugacity calculations were conducted at 550 &amp;#176;C, 2.0 GPa, and an &lt;em&gt;a&lt;/em&gt;H&lt;sub&gt;2&lt;/sub&gt;O 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 &lt;em&gt;f&lt;/em&gt;O&lt;sub&gt;2&lt;/sub&gt; calculation in the Ep-bearing Type II sample. These garnet-epidote pairs exhibit &lt;em&gt;f&lt;/em&gt;O&lt;sub&gt;2&lt;/sub&gt; of QFM+1.9 to +2.0. A minimum &lt;em&gt;f&lt;/em&gt;O&lt;sub&gt;2&lt;/sub&gt; of QFM+4 calculated from the Hem-Mag buffer is estimated for Type III veins. The remarkably high &lt;em&gt;f&lt;/em&gt;O&lt;sub&gt;2&lt;/sub&gt; of Type III veins contrasts with prograde &lt;em&gt;f&lt;/em&gt;O&lt;sub&gt;2&lt;/sub&gt;conditions of QFM+1 to +2 estimated for epidote inclusions in garnet cores from the same sample at 400-450 &amp;#176;C and 1.0-1.5 GPa. While elevated &lt;em&gt;f&lt;/em&gt;O&lt;sub&gt;2&lt;/sub&gt; and acmite inclusions in pyrite are consistent with a SO&lt;sub&gt;x&lt;/sub&gt;(aq)-Fe&lt;sup&gt;2+&lt;/sup&gt; redox pair in Type II samples, &lt;em&gt;f&lt;/em&gt;O&lt;sub&gt;2&lt;/sub&gt; above the Hem-Mag buffer require the bulk addition of Fe&lt;sup&gt;3+&lt;/sup&gt; or Mn&lt;sup&gt;3+&lt;/sup&gt; as a carrier of oxidation. These data demonstrate that slab fluids may impose &lt;em&gt;f&lt;/em&gt;O&lt;sub&gt;2&lt;/sub&gt; well above the sulfur-sulfur oxide buffer.&lt;/p&gt;