Chromite in serpentine mud volcanoes of the Mariana forearc: Implications for abiotic organic reactions

Abstract: The Mariana convergent plate margin is a non- accretionary subduction system in the western Pacific Ocean. Subduction of the Pacific plate causes dissolution of carbonate minerals releasing inorganic carbon, and causes dehydration of hydrous minerals in the subducted slab, generating hydrous fluids. The hydrous fluids infiltrate the overriding plate and induce serpentinization of mantle peridotite. Serpentinized material is transported by the rising fluids, which are channeled along fractures and faults in the forearc region, consequently erupting at the seafloor forming serpentine seamounts. Manganese (IV) oxides that commonly occur in fractures near convergent plate margins are naturally occurring oxidants for chromite ((Al,Mg,Fe)Cr2O4), which is an accessory mineral in mantle peridotite. Chromite is known to be an efficient catalyst in Fischer- Tropsch type (FTT) reactions where molecular hydrogen produced by serpentinization reacts with dissolved inorganic carbon, producing organic hydrocarbons.   Samples were collected from two serpentine seamounts in the Mariana forearc during Ocean Drilling Program, Leg 125 and Leg 195. The aim of this study is to relate organic content in fluid inclusions to minerals that are known to be efficient catalysts in FTT- reactions. The results show that chromites have been altered, with hematite (Fe2O3) precipitated at the margins of the chromites. This is inferred to be caused by an oxidation reaction where manganese (IV) oxides acted as oxidants, oxidizing Fe (II) in chromites to Fe (III) which precipitated as hematite. No fluid inclusions were found in the samples. However, elemental carbon was detected in fractures of the chromites, suggesting that FTT- reactions has occurred where chromite probably acted as a catalyst