Dartmouth College Department of Earth Sciences

 

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Mukul Sharma Current Research Interests in Isotope Geochemistry

Magma Differentiation and Source Characteristics of Continental Flood Basalts

The continental flood basalts (CFBs) have been the subjects of numerous studies as they help constrain the convective structure of the mantle. The production of CFBs has been attributed to the arrival of plume heads from the core-mantle boundary. Initial rapid and voluminous tholeiitic eruptions are followed by decreasing eruption rates and, in some cases, the generation of a hotspot track as a lithospheric plate moves over a relatively stationary mantle plume. Alternatively, some CFBs have been attributed to the melting of metasomatized shallow mantle caused by lithospheric stretching, with or without a plume. A yet another model of CFB generation suggests that pull-apart of an asymmetric lithosphere could generate conditions conducive to CFB volcanism. The problem of evaluating the relative roles of different mantle sources in CFBs using Sr-Nd-Pb isotope systematics is complicated as at least three mantle reservoirs may be implicated in the generation of tholeiites: (1) a MORB-type mantle, (2) an OIB-type mantle which may also be undegassed, and (3) the continental lithospheric mantle (CLM). Whereas the compositional variations of the MORB-type and OIB-type mantle reservoirs are grossly identified, the CLM reservoir appears to be very heterogeneous. The problem is further compounded as magmas may evolve and may get contaminated by continental crustal material en route to the surface. In this context, Re-Os (187Re --> 187Os, t1/2 = 42 billion years) and Pt-Os (186Pt --> 186Os, t1/2 = 449 billion years) isotope systems may be able to discern between signals from the deep and shallow mantles.

During small degrees of partial melting of mantle, Re is moderately incompatible whereas Os is compatible. This feature has produced a continental crust with a time-integrated 187Os/188Os ratio of 1.26, much higher than the convecting upper mantle (~0.125) and CLM (< 0.120). The 186Os/188Os ratios in these reservoirs are expected to be the same, however. In contrast, plumes derived from core-mantle boundary may contain Os with correlated 187Os/188Os and 186Os/188Os ratios that are also higher than the convecting upper mantle and CLM, resulting from incorporation of a small amount of Os from the outer core. The extent to which Os isotopes can be used to establish the sources of the CFBs is critically dependent on the level of continental crustal contamination, which, in turn, depends upon the Os concentration of magma. A majority of the CFBs have MgO contents <8% and Os concentrations similar (within a factor of 2) to those of upper continental crust (20 to 50 ppt). This makes them vulnerable to continental crustal contamination. The lavas with >15% MgO (picrites) contain appreciable amounts of Os (~1000 to 3000 ppt) and other PGEs.

I have obtained, in collaboration with Dr. K. V. Subbarao (Indian Institute of Technology), the least altered samples of high Mg basalts and picrites (MgO = 10 to 22%) from Deccan Traps, southwestern India. I will analyze these samples for Re, Ir, Os, Ru, Pt, and Pd concentrations and Os isotopes. There are three goals of this work (1) to determine the mechanisms by which the Platinum Group Elements are fractionated in basaltic rocks, (2) to evaluate the relationship between continental crustal assimilation and sulfide melt segregation, and (3) to investigate if lavas associated with deep mantle plumes contain radiogenic 186Os and 187Os indicating contributions from a Pt/Os and Re/Os enriched source.

 

 

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