Petrography, classification, oxygen isotopes, noble gases, and cosmogenic records of Kamargaon (L6) meteorite: The latest fall in India

1D. Ray,1R. R. Mahajan,1A. D. Shukla,2T. K. Goswami,3S. Chakraborty
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12875]
1Physical Research Laboratory, Ahmedabad, India
2Department of Applied Geology, Dibrugarh University, Assam, India
3Department of Chemistry, University of California, San Diego, California, USA
Published by arrangement with John Wiley & Sons

A single piece of meteorite fell on Kamargaon village in the state of Assam in India on November 13, 2015. Based on mineralogical, chemical, and oxygen isotope data, Kamargaon is classified as an L-chondrite. Homogeneous olivine (Fa: 25 ± 0.7) and low-Ca pyroxene (Fs: 21 ± 0.4) compositions with percent mean deviation of <2, further suggest that Kamargaon is a coarsely equilibrated, petrologic type 6 chondrite. Kamargaon is thermally metamorphosed with an estimated peak metamorphic temperature of ~800 °C as determined by two-pyroxene thermometry. Shock metamorphism studies suggest that this meteorite include portions of different shock stages, e.g., S3 and S4 (Stöffler et al. 1991); however, local presence of quenched metal-sulfide melt within shock veins/pockets suggest disequilibrium melting and relatively higher shock stage of up to S5 (Bennett and McSween 1996). Based on noble gas isotopes, the cosmic-ray exposure age is estimated as 7.03 ± 1.60 Ma and nitrogen isotope composition (δ15N = 18‰) also correspond well with the L-chondrite group. The He-U, Th, and K-Ar yield younger ages (170 ± 25 Ma 684 ± 93, respectively) and are discordant. A loss of He during the resetting event is implied by the lower He-U and Th age. Elemental ratios of trapped Ar, Kr, and Xe can be explained through the presence of a normal Q noble gas component. Relatively low activity of 26Al (39 dpm/kg) and the absence of 60Co activity suggest a likely low shielding depth and envisage a small preatmospheric size of the meteoroid (<10 cm in radius). The Kr isotopic ratios (82Kr/84Kr) further argue that the meteorite was derived from a shallow depth.

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