Accessory phases as recorders of subduction redox: Sulfide-oxide-silica equilibria during high-pressure metamorphism

Examination of a global suite of eclogite-facies metabasites and metasediments suggests that eclogites tend to exhibit reduced mineral assemblages relative to their protoliths. High-pressure rocks tend to lack sulfides and Fe³⁺-bearing oxides in the eclogite facies. We suggest that eclogite-facies mineral assemblages are consistent with prograde reactions that balance the oxidation of S^2- or S^- to S⁶⁺ by reducing Fe³⁺in silicates or oxides:

(1) 8Fe³⁺Si_aO_b(OH)_c+ S^2-= 8 Fe²⁺Si_dO_e+ SO₄^2-+ (H₂O)_f

The oxidation of one mole of S^2-or S^-is balanced by the reduction of 7 to 8 moles of Fe³⁺, and typical S concentrations in the oceanic crust are capable of fully reducing the entire Fe³⁺ budget of metabasites.

As most eclogite facies rocks do not preserve peak metamorphic sulfides, petrographic evidence for prograde S oxidation reactions are cryptic; however, textures associated with sulfate reduction in response to influx of external fluids are common (reaction 1 in reverse). These reactions produce Fe³⁺-rich phases and are observed in both metasedimentary and metabasic rocks across a range of retrograde P-T paths (blueschist to granulite facies). For example, high-P calc-schists exhibit reaction textures that suggest the breakdown of garnet and white mica to produce pyrite + chalcopyrite + epidote + biotite + magnetite. Our thermodynamic models of aS₂ and aO₂ at subduction zone P-T conditions suggest assemblages of this type are indicative of aO₂0.7 to 4.5 log units above the quartz-fayalite-magnetite buffer.

In rehydrated eclogites, pyrite is commonly associated with the breakdown of garnet + omphacite to amphibole + pyrite. Additionally, direct precipitation of sulfide from sulfate is observed in two samples: 1) The retrograde assemblage pyrite + ilmenite + gypsum occurs in one retrogressed metagabbroic eclogite, and 2) Coronas of secondary pyrite + barite + gypsum enclose early retrograde pyrite in a retrogressed garnet blueschist. In many eclogites, S^- is reduced to S^2- as pyrite is replaced by pyrrhotite, chalcopyrite, and mixed valence Co-Ni sulfides. These reactions are balanced by oxidation of divalent to trivalent Fe-Co-Ni. Reactions of this type are consistent with increasing aS₂ during retrograde metamorphism. Thus, ample evidence exists for oxidized S-bearing fluids released from subducting slabs.