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

Serpentinization of abyssal peridotites is known to produce extremely reducing conditions as a result of dihydrogen (H-2,aq) release upon oxidation of ferrous iron in primary phases to ferric iron in secondary minerals by H2O. We have compiled and evaluated thermodynamic data for FeNiCoOS phases and computed phase relations in fO(2),gfS(2),g and aH(2),aqaH(2)S,aq diagrams for temperatures between 150 and 400C at 50 MPa. We use the relations and compositions of FeNiCoOS phases to trace changes in oxygen and sulfur fugacities during progressive serpentinization and steatitization of peridotites from the Mid-Atlantic Ridge in the 1520N Fracture Zone area (Ocean Drilling Program Leg 209). Petrographic observations suggest a systematic change from awaruitemagnetitepentlandite and heazlewooditemagnetitepentlandite assemblages forming in the early stages of serpentinization to milleritepyritepolydymite-dominated assemblages in steatized rocks. Awaruite is observed in all brucite-bearing partly serpentinized rocks. Apparently, buffering of silica activities to low values by the presence of brucite facilitates the formation of large amounts of hydrogen, which leads to the formation of awaruite. Associated with the prominent desulfurization of pentlandite, sulfide is removed from the rock during the initial stage of serpentinization. In contrast, steatitization indicates increased silica activities and that high-sulfur-fugacity sulfides, such as polydymite and pyritevaesite solid solution, form as the reducing capacity of the peridotite is exhausted and H-2 activities drop. Under these conditions, sulfides will not desulfurize but precipitate and the sulfur content of the rock increases. The co-evolution of fO(2),gfS(2),g in the system follows an isopotential of H2S,aq, indicating that H2S in vent fluids is buffered. In contrast, H-2 in vent fluids is not buffered by FeNiCoOS phases, which merely monitor the evolution of H-2 activities in the fluids in the course of progressive rock alteration. The co-occurrence of pentlanditeawaruitemagnetite indicates H-2,aq activities in the interacting fluids near the stability limit of water. The presence of a hydrogen gas phase would add to the catalyzing capacity of awaruite and would facilitate the abiotic formation of organic compounds.