Mukul Sharma Current Research Interests in Isotope Geochemistry

• Near surface geochemistry of osmium
• Generation of oceanic lithosphere: the role of mantle heterogeneities
• Magma Differentiation and Source Characteristics of Continental Flood Basalts
• Early evolution of the earth: the terrestrial record of 146Sm
• Study on continental weathering using osmium isotopes
• Variations in Solar Magnetic Activity and the Sun-Climate Connection
• Tracing the source of groundwater arsenic in Bangladesh Delta
• Geochemical and isotopic studies of cosmic dust
• Near surface geochemistry of osmium
• Osmium and Iridium Transport through Estuarine Environments

Generation of Oceanic Lithosphere: Role of Mantle Heterogeneity

The issue of the origin of the mid-ocean ridge basalt (MORB) is one of the most fundamental and yet poorly understood problems in mantle geochemistry. At the center is the question of the extent to which the mantle heterogeneities are involved in MORB generation. Important insights have been gained from the Re-Os isotopic studies of mid-ocean ridge basalt and abyssal peridotites, which show quite unambiguously that an Os isotopic contrast exists between these two end-members. Many hypotheses such as recycled crust in the mantle, seawater alteration of basalts, partial melting of pyroxenite veins have been posited to explain this discrepancy. This is issue is not yet resolved. On another hand, the relative behavior of Re and Os during magmatic processes is poorly understood. A specific issue is what are the phases controlling Re and Os during the early stages of melting and fractionation of a basaltic melt. The abyssal peridotites are residues of partial melting. While their Os isotopic compositions shed some light on the degree of mantle heterogeneity, their Os contents do not provide clues to partial melting processes on account of large KD(Os). The MORBs, on the other hand, have experienced several processes from extraction to eruption. It thus appears difficult a posteriori to investigate the behavior of Os by studying only basalts. Ophiolite complexes, where both mantle (peridotites) and crustal (basalts, dolerites and cumulate gabbros) lithologies are well exposed, bear witness to processes that take place during magma migration and differentiation. Additionally, studies of the ophiolite complexes may also provide clues to the nature of Os isotopic disequilibrium existing between erupted lavas and mantle peridotites.

My work on the Polar Urals ophiolite has identified harzburgites with the lowest (Al/Si)wt ratios, extreme light rare earth element depletions and very high 143Nd/144Nd ratios (Sharma and Wasserburg, 1996; Sharma et al., 1995b). The harzburgites along with the overlying basalts, and cumulate gabbros define an isochron and indicate that these rocks are related via partial melting and fractionation. This study has implied that to a large extent the gross structure of the oceanic lithosphere has resulted from the partial melting of a mantle lherzolite. The Re-Os isotope data in some of these rocks, however, indicate that the harzburgites are not related to the overlying crustal lithologies (Sharma et al., 1998b). This observation, if true globally, would indicate a major role of vein material (pyroxenites ± garnet) in the production of crustal section of the oceanic lithosphere. I will continue to analyze samples from Russia (both Polar and Southern Urals), Oman (Samail), Greece (Vourinos), and Canada (Table Mountain Massif) to address the issue of the relationship between crustal and mantle lithologies. The samples will be analyzed for Nd and Os isotopes and Sm, Nd, Re, Os, Ir, Os, Ru, Pt, Pd using the techniques I have developed/implemented over the years.