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Re‐Os isotope systematics and fractionation of siderophile elements in metal phases from CB a chondrites
Author(s) -
Nakanishi Nao,
Yokoyama Tetsuya,
Okabayashi Satoki,
Usui Tomohiro,
Iwamori Hikaru
Publication year - 2018
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/maps.13050
Subject(s) - chondrite , meteorite , analytical chemistry (journal) , metal , microanalysis , mass spectrometry , chemistry , thermal ionization mass spectrometry , rhenium , refractory metals , iron meteorite , laser ablation , allende meteorite , fractionation , ionization , inorganic chemistry , astrobiology , laser , environmental chemistry , physics , ion , organic chemistry , chromatography , optics
We report Os isotope compositions of metal grains in two CB a chondrites (Bencubbin and Gujba) determined using a micromilling sampling coupled with thermal ionization mass spectrometry, together with the abundances of major and trace siderophile elements obtained by electron probe microanalysis and femtosecond laser ablation inductively coupled plasma–mass spectrometry. The CB a metal grains presented 187 Os/ 188 Os ratios akin to carbonaceous chondrites with limited variations (0.1257–0.1270). Most of the CB a metal grains were scattered along a 187 Re‐ 187 Os reference isochron of IIIAB iron meteorites, indicating that the CB a metals experienced limited Re‐Os fractionation at the time of their formation. The Re/Os ratios of sampling spots for the CB a metals, recast from the observed 187 Os/ 188 Os ratios, had a positive correlation with their Os/Ir ratios. In addition, the metal grains showed a positive correlation in a Pd/Fe versus Ni/Fe diagram. These correlations suggest that the CB a metal grains have formed via equilibrium condensation or evaporation from a gaseous reservoir at ~10 −4 bar with enhanced metal abundances. Compared to the Bencubbin metals, the Gujba metals are characterized by having systematically lower Pd/Fe and Ni/Fe ratios that span subchondritic values. Such a difference was most likely induced by the compositionally heterogeneous impact plume from which the metals were condensed.