Microstructures, mineral chemistry, noble gases and nitrogen in the recent fall, Bhuka iron (IAB) meteorite

摘要

We report some chemical, petrological and isotopic studies of the Bhuka iron meteorite that fell in Rajasthan, India in 2005. Numerous silicate and graphite inclusions are visible on the surface of the hand specimen. In the polished and etched surface studied, irregular patches of graphite are found as the most dominant inclusion and commonly associated with pure corundum (95 wt% Al2O3), spinet, feldspar and Si-rich phases. Apart from typical lamellar intergrowth with kamacite (i.e. the Widmanstatten pattern), taenites are also commonly found to occur as a rim of the graphite inclusions. P-rich (up to 10 wt%) taenites are also found locally within the recrystallised kamacite matrix. Based on mineralogy, texture and bulk composition, Bhuka resembles the low-Ni IAB subgroup (ungrouped). Noble gas isotope studies suggest He, Ne and Ar are mostly of cosmogenic origin, while Kr and Xe are a mixture of cosmogenic, radiogenic and trapped components. A pre-atmospheric radius of 10 +/- 1 cm and a cosmic ray exposure age of 346 +/- 52 Ma are derived based on depth dependant (He-3/He-4)(c) and Ar-38(c) respectively, as per the production systematics of cosmogenic noble gas isotopes (Ammon et al., 2009). Cosmogenic Kr-83 and Xe-126 yield production rates of 12 and 0.335 (in 10(-15)ccSTP/g Ma) for Kr-83 and Xe-126 respectively. Presence of trapped Kr and Xe, with (Kr-84/Xe-132)(t)=2 and radiogenic Xe-129=120 x 10(-12) ccSTP/g are due to presence of graphite/silicate inclusions in the analysed sample. Over similar to 150% excess Xe-131(c), than expected from spallation suggests contribution from (n,gamma) reactions from Ba from inclusions and suggests irradiation of pre-atmospheric object in a larger body, indicative of complex irradiation. Trapped N of 24 ppm, with delta N-15= -10.7 +/- 0.8 parts per thousand observed in Bhuka, is heavier than the range observed hither to in IAB irons. (C) 2016 Elsevier Ltd. All rights reserved.