Fe-Ni-Co-O-S Phase Relations in Peridotite-Seawater Interactions

Abstract

Serpentinization of abyssal peridotites is known to produce extremely reducing conditions as a result of dihydrogen (H2,aq) release upon oxidation of ferrous iron in primary phases to ferric iron in secondary minerals by H2O.We have compiled and evaluated ther-modynamic data for Fe^Ni^Co^O^S phases and computed phase relations in fO2,g^fS2,g and aH2,aq^aH2S,aq diagrams for tem-peratures between 150 and 4008C at 50MPa.We use the relations and compositions of Fe^Ni^Co^O^S phases to trace changes in oxygen and sulfur fugacities during progressive serpentinization and steatitization of peridotites from the Mid-Atlantic Ridge in the 158200N Fracture Zone area (Ocean Drilling Program Leg 209). Petrographic observations suggest a systematic change from awar-uite^magnetite^pentlandite and heazlewoodite^magnetite^pentlan-dite assemblages forming in the early stages of serpentinization to millerite^pyrite^polydymite-dominated assemblages in steatized