Diagenetic dissolution, maghemitization and sulphidization of magnetic minerals in rapidly deposited gas hydrate bearing sediments from the Bay of Bengal

Abstract

We conducted a comprehensive rock magnetic investigation complemented by a scanning electron microscopy analyses on a 148.8 m sediment core (NGHP-01-20A) from the methane-hydrate bearing site in the Bay of Bengal to systematically evaluate how magnetic minerals selectively respond to a range of diagenetic processes in the sulfidic and methanic sediments. For diagenetic analysis, we defined five distinct sediment magnetic zones (I–V) based on the sediment magnetic signatures. Rock magnetic property is mainly carried by complex magnetic mineral assemblages of detrital (titanomagnetite, silicate hosted magnetic inclusions), altered (titanomaghemite, titanohematite) and fine-grained diagenetic (pyrite) and authigenic (greigite) iron sulfide minerals. Three distinct bands of enhanced magnetic susceptibility in Z-II, Z-III and Z-V mainly comprised of detrital magnetite-rich minerals delivered by the peninsular river systems as a result of higher sedimentation events in the Krishna-Godavari (K-G) basin. Progressive diagenetic dissolution of primary iron oxide and consequent formation of iron sulfide in the uppermost sulfidic zone (Z-I) is clearly mirrored in the magnetic susceptibility (χlf) profile. Presence of moderately altered coarse-grained detrital Fe–Ti particles in the deeper methanic and magnetically enhanced zones (Z-II, Z-III, Z-V) suggest that sediment diagenesis at this site was significantly constrained by the multiple events of rapid sedimentation which might have restricted the complete dissolution of the primary minerals. Decline in magnetic grain size diagnostic proxy (ARM/IRM2.5T) in the uppermost sulfidic zones (Z-I, Z-II) can be attributed to the preferential dissolution of fine-grained magnetite. Further, downcore increases could be linked with the authigenic formation of fine-grained magnetite in methanic zones (Z-III to Z-V). Ferrimagnetic iron sulfide (greigite) indicative (SIRM/χlf) parameter showed sudden rise in Z-II and remain fairly constant throughout, suggesting the presence of authigenic greigite. Scanning electron microscopy in-line with energy dispersive X-ray spectroscopy analyses of magnetic particles from different sediment magnetic zones revealed a range of ferrimagnetic (titanomagnetite, titanomaghemite), antiferromagnetic (titanohematite) iron oxides and iron sulfide (pyrite). Detrital titanomagnetite with angular, irregular, spherical and well-defined octahedral with micron and submicron sized particles mainly derived from Deccan Volcanic Province (DVP) dominates the rock magnetic signal of bulk sediments in the studied core. We noticed several evidence of diagenetic dissolution, sulphidization and maghemitization of magnetic particles in the studied sediment core. Few nanometre-sized detrital magnetic particles occur as inclusions within the silicate minerals and remains protected against the sulfidic dissolution fronts through geologic times. Optical microscopy revealed the dominance of diagenetic pyrite occurring as overgrowth on the ferrimagnetic particles. We demonstrate that the rockmagnetic methods in combination with focussed electron microscope analyses provide valuable insights into the mineralogy, genesis and state of preservation of magnetic particles in sulfidic and methanic environments and thereby enhances the interpretative value of rock magnetic proxies.