Controls on greigite preservation in a gas hydrate system of the Krishna-Godavari basin, Bay of Bengal

Abstract

Shallow sediments across large parts of Krishna-Godavari (K-G) basin offshore east central Indian contain abundant methane and gas hydrates. In this study, we carried out rock-magnetic and transmission electron microscope (TEM) analyses on the samples from a sediment core (MD161/Stn-8) to constrain the formation and preservation of greigite in shallow sediments and how this might link to reactions involving methane. Here, we report for the first time the occurrence of silicate-hosted iron sulfide (greigite) inclusions which is an important observation for understanding the preservation of magnetic minerals in gas hydrate systems. The magnetization of the greigite zone (17-23 mbsf) is carried by complex magnetic mineral assemblages of detrital iron oxides (titanomagnetite), diagenetic iron sulfide (greigite) occurring as nano-inclusions within larger silicate particles, and biogenic minerals. Elevated concentrations of dissolved pore water silica and alkalinity within the magnetically enhanced greigite zone suggest that silica diagenesis and silicate weathering triggered by paleo-methane seepage played a key role in crystallizing the diagenetically formed iron sulfide (greigite) into silicate matrix. The silicate-hosted magnetic inclusions protected the ferrimagnetic greigite from further diagenetic dissolution and prevented its conversion into stable pyrite. Three scenarios explaining the potential controls on the greigite preservation in gas hydrate marine sedimentary system have been proposed.