• S Sahijpal

      Articles written in Journal of Earth System Science

    • 26Al as a heat source for early melting of planetesimals: Results from isotopic studies of meteorites

      S Sahijpal M A Ivanova L L Kashkarov N N Korotkova L F Migdisova M A Nazarov J N Goswami

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      Ion microprobe studies of magnesium isotopic composition in igneous components from several chondritic meteorites have been carried out to look for26Mg excess that may be attributed to the presence of the now-extinct radionuclide26Al(τ ∼ 1 Ma) at the time of formation of these objects. A positive evidence for the presence of26Al in the analysed objects will strengthen its case as the primary heat source for the early thermal metamorphism/melting of meteorite parent bodies. Based on calculated temperature profiles inside chondritic objects of different sizes and initial26Al/27Al ratios, we have estimated the initial abundances of26Al needed to provide the heat necessary for the wide range of thermal processing seen in various types of meteorites. The magnesium isotopic data obtained by us do not provide definitive evidence for the presence of26Al at the time of formation of the analysed igneous phases in different chondritic meteorites. Experimental evidence for a planetary scale distribution of26Al in the early solar system to serve as a significant heat source for the thermal metamorphism and melting of meteorite parent bodies (planetesimals) remains elusive.

    • Determination of rare earth and refractory trace element abundances in early solar system objects by ion microprobe

      S Sahijpal K K Marhas J N Goswami

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      Experimental and analytical procedures devised for measurement of rare earth element (REE) abundances using a secondary ion mass spectrometer (ion microprobe) are described. This approach is more versatile than the conventional techniques such as neutron activation analysis and isotope dilution mass spectrometry by virtue of its high spatial resolution that allows determination of REE abundances in small domains (10-20 micron) within individual mineral phases. The ion microprobe measurements are performed at a low mass-resolving power adopting the energy-filtering technique (Zinner and Crozaz 1986) for removal and suppression of unresolved complex molecular interferences in the REE masses of interest. Synthetic standards are used for determining various instrument specific parameters needed in the data deconvolution procedure adopted for obtaining REE abundances. Results obtained from analysis of standards show that our ion microprobe may be used for determining REE abundances down to ppm range with uncertainties of ∼ 10 to 15%. Abundances of rare earth and several other refractory trace elements in a set of early solar system objects isolated from two primitive carbonaceous chondrites were determined using the procedures devised by us. The results suggest that some of these objects could be high temperature nebular condensates, while others are products of melting and recrystallization of precursor nebular solids in a high temperature environment.

    • The role of impact and radiogenic heating in the early thermal evolution of Mars

      S Sahijpal G K Bhatia

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      The planetary differentiation models of Mars are proposed that take into account core–mantle and coremantle–crust differentiation. The numerical simulations are presented for the early thermal evolution of Mars spanning up to the initial 25 million years (Ma) of the early solar system, probably for the first time, by taking into account the radiogenic heating due to the short-lived nuclides, 26Al and 60Fe. The influence of impact heating during the accretion of Mars is also incorporated in the simulations. The early accretion of Mars would necessitate a substantial role played by the short-lived nuclides in its heating. 26Al along with impact heating could have provided sufficient thermal energy to the entire body to substantially melt and trigger planetary scale differentiation. This is contrary to the thermal models based exclusively on the impact heating that could not produce widespread melting and planetary differentiation. The early onset of the accretion of Mars perhaps within the initial $\sim$1.5 Ma in the early solar system could have resulted in substantial differentiation of Mars, provided, it accreted over the timescale of $\sim$1 Ma. This seems to be consistent with the chronological records of the Martian meteorites.

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