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 (H 2 ,aq) release upon oxidation of ferrous iron in primary phases to ferric iron in secondary minerals by H 2 O. We have compiled and evaluated thermodynamic data for Fe^Ni^Co^O^S phases and computed phase relations in fO 2 ,g^fS 2 ,g and aH 2 ,aq^aH 2 S,aq diagrams for temperatures between 150 and 4008C at 50 MPa. 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 15820 0 N Fracture Zone area (Ocean Drilling Program Leg 209). Petrographic observations suggest a systematic change from awar-uite^magnetite^pentlandite and heazlewoodite^magnetite^pentlandite assemblages forming in the early stages of serpentinization to millerite^pyrite^polydymite-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 highsulfur-fugacity sulfides, such as polydymite and pyrite^vaesite 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 ,g^fS 2 ,g in the system follows an isopotential of H 2 S,aq, indicating that H 2 S in vent fluids is buffered. In contrast, H 2 in vent fluids is not buffered by Fe^Ni^Co^O^S phases, which merely monitor the evolution of H 2 activities in the fluids in the course of progressive rock alteration.The co-occurrence of pentlan-dite^awaruite^magnetite 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.

Abiotic methane formation during experimental serpentinization of olivine

“…These experiments were performed using methods very similar to those of this study, except that the reaction vessel was periodically rotated during heating. After heating for [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] (Table 1). However, isotopic labeling indicated that the CH 4 in our experiments was not derived from reduction of dissolved inorganic carbon but instead, was derived entirely from background sources (i.e., composed of 99 mol % 12 CH 4 ).…”

“…This situation might arise, for instance, at the serpentinization reaction front, where localized consumption of H 2 O and production of H 2 (Reaction 1) can lead to saturation of the fluid and exsolution of H 2 -rich vapor bubbles. Indeed, the relatively common occurrence of the NiFe alloy awaruite in many serpentinites may be an indication that such conditions may be widespread, because formation of this mineral requires highly elevated H 2 concentrations that may approach saturation levels (32,35). Because NiFe alloys can also catalyze the reduction of inorganic carbon to CH 4 (16), it may make these environments even more favorable for CH 4 synthesis.…”

Raman spectroscopy of hydrous inclusions in olivine and orthopyroxene in ophiolitic harzburgite: Implications for elementary processes in serpentinization

“…These reactions indicate the coexistence of magnetite with highly reduced gas phases (cf., Klein and Bach, 2009). …”

Silicate and Sulphide Mineralogy, and Conditions of Equilibration of Ultramafic Rocks of the Indo-Myanmar Ophiolite Belt between Tusom, Manipur and Shomra Village, Myanmar

“…In general, dihydrogen (H 2,aq ) is released during hydrothermal alteration/serpentinization of ultramafic rocks due to the oxidation of Fe 2+ in primary silicates (via addition of water), producing Fe 3+ in secondary minerals (e.g. magnetite commonly forms during serpentinization) 15 . A high degree of serpentization would thus evolve a significant amount of H 2,aq , and metamorphic conditions would become reducing.…”

“…This desulphurization process is triggered by the release of H 2,aq during serpentinization. At higher oxygen and sulphur fugacities that may occur due to more intense serpentinization/steatization, awaruite breaks down to millerite and magnetite instead 15 . Experimental study shows that the development of pentlandite + awaruite + magnetite assemblages imply hydrogen concentrations close to (or at) the solubility of dihydrogen in water: between 200C and 300C (ref.…”