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