Dwijesh Ray

Geochemical implications of gabbro from the slow-spreading Northern Central Indian Ocean Ridge, Indian Ocean

Gabbro samples ( c . −1 ) include mostly olivine gabbro and Fe–Ti oxide gabbro. The cumulate olivine gabbro shows ophitic to subophitic texture with early formed plagioclase crystals in mutual contact with each other, and a narrow range of compositions of olivine (Fo 80–81 ), clinopyroxene (magnesium number: 85–87) and plagioclase (An 67–70 ). This olivine gabbro could be geochemically cogenetic with the evolved oxide gabbro. These gabbro samples are geochemically distinct from the CIR gabbro occurring along the Vema, Argo and Marie Celeste transform faults and can further be discriminated from the associated NCIR basalts by their clinopyroxene (augite in gabbro, and diopsidic in basalts) and olivine (gabbro: Fo 80–81 , basalts: Fo 82–88 ) compositions. Our major oxide, trace element and REE geochemistry analyses suggest that the gabbro and the NCIR basalts are also not cogenetic and had experienced different trends of geochemical evolution. The clinopyroxenes of the present NCIR gabbros are geochemically similar to primitive melt that is in equilibrium with mantle peridotite, and do not show any poikilitic texture with resorbed plagioclase; these results negate the possibility of these gabbros being a pre-existing cumulate that has been brought up to the shallower oceanic crust and interacted with the NCIR basalt. The Sr, Pb and Nd isotopic data of the gabbro substantially differ from those of the NCIR basalts and suggest significant contamination of the depleted mantle source of the gabbro, most likely by the Indian Ocean pelagic sediments. The Pb-isotope data suggest that the proportion of pelagic sediment that mixed in the depleted mantle source of the NCIR gabbro is much higher than the level of contamination observed for the Indian Ocean MORBs.

Evidence of shock pressure above 600 kilobar and post-shock annealing in Nyaung IIIAB octahedrite

Nyaung iron is structurally a medium octahedrite (IIIAB) with several large inclusions of troilite-daubreelite aggregates and rare small inclusions of rhabdites. Primary cooling related textures include Widmanstätten structures on mesoscopic scale and large variety of plessites (net, comb and black variety); the latter formed under low temperature through martensitic transformations. Neumann lines (<130 kb) and widely spread matte kamacites (>600 kb) refer to shock- induced secondary changes. Post- shock annealing event is well documented with a large number of microtextures which include disappearance of older Neumann lines and development of decorated Neumann lines, sub-grain boundaries associated with triple point bearing polygonised kamacite, polygonised plessite and spherodised taenite particles; local recrystallisation coupled with degeneration of e — kamacite, plastic flowage of taenite lamellae and moving grain boundary pinned with taenite beads. Mean cooling rate for Nyaung octahedrite is estimated ~385°C/ Ma with cooling rate variation by a factor of 3.2. Rapid cooling rate is explained in terms of undisturbed fractional crystallisation of small molten core insulated within a very thin silicate mantle.

Astrobiological implications of dim light phototrophy in deep-sea red clays

Red clays of Central Indian Basin (CIB) under influence of trace of Rodriguez Triple Junction exhibited chemoautotrophy, low temperature hydrothermal alterations and photoautotrophic potential. Seamount flank TVBC-08, hosting such signatures revealed dominance of aerobic anoxygenic phototroph Erythrobacter, with 93% of total 454 pyrosequencing tags. Subsequently, enrichments for both aerobic (Erythrobacter) and anaerobic anoxygenic phototrophs (green and purple sulphur bacteria) under red and white LED light illumination, with average irradiance 30.66Wm-2, were attempted for three red-clay sediment cores. Successful enrichments were obtained after incubation for c.a. 120 days at 4°± 2°C and 25°± 2°C, representing ambient psychrophilic and low temperature hydrothermal alteration conditions respectively. During hydrothermal cooling, a microbial succession from anaerobic chemolithotrophy to oxygenic photoautotrophy through anaerobic/aerobic anoxygenic phototrophic microbes is indicated. Spectral absorbance patterns of the methanol extracted cell pellets showed peaks corresponding to metal sulphide precipitations, the Soret band of chlorosome absorbance by photosystem II and absence of peaks at Qy transition band. Dendritic nano-structures of metal sulphides are common in these sediments and are comparable with other sulphidic paleo-marine Martian analogues. Significant blue and redshifts have been observed for the experimental samples relative to the un-inoculated medium. These observations indicate the propensity of metal-sulphide deposits contributing to chemiluminiscence supporting the growth of phototrophs at least partially, in the otherwise dark abyss. The effects of other geothermal heat and light sources are also under further consideration. The potential of phototrophic microbial cells to exhibit Doppler shift in absorbance patterns is significant towards understanding planetary microbial habitability. Planetary desiccation could considerably influence Doppler effects and consequently spectral detection techniques exo-planetary microbial life.

Mineral chemistry and alteration characteristics of spinel in serpentinised peridotites from the northern central Indian Ridge

Serpentinites (frequently cross cut by gabbroic dikelet), collected from Northern Central Indian Ridge (NCIR), Indian Ocean, contain both Cr-rich (Group I) and Cr-poor (Group II) variety of spinel. Based on mineralogy they can be classified as non-residual and residual spinel, respectively. While, non residual spinel (TiO2 up to 0.5 wt %; Group I) display the evidences of peridotite-gabbro interaction, residual spinel (TiO2<0.1 wt%; Cr# ~24; Group II), suggests intermediate degree of partial melting (~10%) of mantle peridotite, which falls between most depleted mantle peridotite (~15%) and most fertile peridotite (~5%). Alteration of spinel porphyroclast is conceded by the presence of relict fresh interior, intermediate ferritchromit zone and secondary magnetite at the margins. Due to the effect of serpentinisation, marginal areas of spinel porphyroclasts display increasing Fe, but decreasing Al and Mg, while their Cr content remains unaffected throughout the spinel.

Disseminated sulphides in basalts from the northern Central Indian Ridge: implications on late-stage hydrothermal activity

This study examined the mineralogy and mineral chemistry of disseminated sulphides (mainly chalcopyrite-pyrite) in partly altered basalts from the northern Central Indian Ridge, Indian Ocean in order to understand the role of hydrothermal alterations and infer possible sulphide formation history. Pyrite and chalcopyrite are dominant sulphide minerals and generally associated with the oxide phases including magnetite and often ilmenite. Close association of sulphide and oxide minerals suggests that they are paragenetically related. Sulphides also occur as late impregnated veins cutting through the basaltic hosts. The chemical compositions of pyrite (avg. Fe 46.3 wt%, S 53.7 wt%) and chalcopyrite (avg. Cu 34.4 wt%, Fe 30.7 wt%, S 34.7 wt%) are almost uniform, while the secondary ilmenite often shows MnO enrichment (up to 3.0–3.4 wt%). The associated altered minerals typically resemble the greenschist facies mineral assemblages—e.g. chlorite±epidote. Evidence of albitisation and silicification suggests low-temperature hydrothermal alteration processes. This is supported by the bulk Au content (up to 60 ppb) of host-altered basalts with pyrite mineralisation. Au is usually associated with late-stage pyrites and thus related with low-temperature hydrothermal activity. Close to the dredge location, tectonic activity around the Vityaz megamullion might have promoted hydrothermal circulation and subsequent alteration of the mineral constituents in basalts, eventually inducing the formation of late-stage disseminated sulphide minerals in these rocks.