Noble gas and oxygen isotope studies of aubrites : a clue to origin and histories
GCA_71_251.pdf 506 KB
Miura, Yayoi N.
Earth sciences. Geology
Noble gas measurements were performed for nine aubrites: Bishopville, Cumberland Falls, Mayo Belwa, Mt. Egerton, Norton County, Pena Blanca Spring, Shallowater, ALHA 78113 and LAP 02233. These data clarify the origins and histories, particularly cosmic-ray exposure and regolith histories, of the aubrites and their parent body(ies). Accurate cosmic-ray exposure ages were obtained using the 81Kr-Kr method for three meteorites: 52 ± 3, 49 ± 10 and 117 ± 14 Ma for Bishopville, Cumberland Falls and Mayo Belwa, respectively. Mayo Belwa shows the longest cosmic-ray exposure age determined by the 81Kr-Kr method so far, close to the age of 121 Ma for Norton County. These are the longest ages among stony meteorites. Distribution of cosmic-ray exposure ages of aubrites implies 4-9 break-up events (except anomalous aubrites) on the parent body. Six aubrites show “exposure at the surface" on their parent body(ies): (i) neutron capture 36Ar, 80Kr, 82Kr and/or 128Xe probably produced on the respective parent body (Bishopville, Cumberland Falls, Mayo Belwa, Pena Blanca Spring, Shallowater and ALHA 78113); and/or (ii) chondritic trapped noble gases, which were likely released from chondritic inclusions preserved in the aubrite hosts (Cumberland Falls, Pena Blanca Spring and ALHA 78113). The concentrations of 128Xe from neutron capture on 127I vary among four measured specimens of Cumberland Falls (0.5-76 × 10-14 cm3STP/g), but are correlated with those of radiogenic 129Xe, implying that the concentrations of (128Xe)n and (129Xe)rad reflect variable abundances of iodine among specimens. The ratios of (128Xe)n/(129Xe)rad obtained in this work are different for Mayo Belwa (0.045), Cumberland Falls (0.015), and Shallowater (0.001), meaning that neutron fluences, radiogenic 129Xe retention ages, or both, are different among these aubrites. Shallowater contains abundant trapped Ar, Kr and Xe (2.2 × 10-7, 9.4 × 10-10 and 2.8 × 10-10 cm3STP/g, respectively) as reported previously (Busemann and Eugster, 2005, MAPS 37, 1865-189
Geochimica et Cosmochimica Acta
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Copyright (c) 2007 Elsevier Ltd.
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Graduate School of Science