Chemical and isotopic charateristics of the Didwana-Rajod (H5) chondrite

Abstract

The mineralogical and chemical characteristics of the Didwana-Rajod chondrite are described. The mean mineral composition is found to be olivine (Fo_83.2) and pyroxene (En_83.5Wo_0.7Fs_15.8), and feldspar is mainly oligoclase. Oxygen isotopic analysis shows δ ¹⁸O = +3.8%0 and δ ¹⁷O = +2.59%0. The nitrogen content of Didwana-Rajod is ≈2 ppm with δ¹⁵N ≅3.4%0. Based on microscopic, chemical, isotopic and electron probe microanalysis, the meteorite is classified as an H5 chondrite. Cosmogenic tracks, radionuclides and the isotopic composition of rare gases were also measured in this meteorite. The track density in olivines varies in a narrow range with an average value of (6.5±0.5) ×10⁵/cm² for four spot samples taken at the four corners of the stone. The cosmic-ray exposure age based on neon and argon is 9.8 Ma. ²²Na/²⁶Al ≅0.94 is lower than the solar-cycle average value of ≈1.5 and is consistent with irradiation of the meteoroid to lower galactic cosmic-ray fluxes as expected at the solar maximum. The track density, rare gas isotopic ratios, ⁶⁰Co activity and other radionuclide data are consistent with a preatmospheric radius of ∼15 cm, corresponding to a mass of ∼50 kg. The cosmogenic properties are consistent with a simple exposure history in interplanetary space.