Noble gases in ureilites: Cosmogenic, radiogenic, and trapped components

摘要

Abundances and isotopic compositions of Ne (in bulk samples only), Ar, Kr, and Xe have been investigated in 6 monomict, 3 polymict, and the diamond-free ureilite ALH78019 and their acid-resistant, C-rich residues. Isotopic ratios of Kr and Xe are very uniform and agree with data for ureilites from the literature. The measured ratio Ar-38/Ar-36 showed large variations due to an experimental artifact. This is shown to be connected to the pressure dependence of the instrumental mass discrimination, which for ureilites with their low abundance of Ar-40 is different from that of the usual air standard. This observation necessitates a reassessment for the recently reported 36 Ar excesses due to possible decay of extinct Cl-36 in the Efremovka meteorite. Trapped Ne-22 in the range of (1.4-2.5) X 10(-8) cc STP/g is present in bulk ureilites. A Ne three-isotope plot for polymict ureilites indicates the presence of solar Ne. Ne-21-based cosmic ray exposure ages for the 10 ureilites studied range from 0.1 Ma (for ALH78019) to 46.8 Ma (for EET83309). All ureilites may have started with nearly the same initial elemental ratio (Xe-132/Ar-36)(0), established in the nebula during gas trapping into their carbon carrier phases (diamond, amorphous C) by ion implantation. Whereas diamonds are highly retentive, amorphous C has suffered gas loss due to parent body metamorphism. The correlation of the elemental ratios Xe-132/Ar-36 and Kr-84/Ar-36 along the mass fractionation line could be understood as a two-component mixture of the unaffected diamond gases and the fractionated (to varying degrees) gases from amorphous C. In this view, the initial ratio (Xe-132/Ar-36)(0) is a measure of the plasma temperature in the nebula at the formation location of the carbon phases. Its lack of correlation with A Delta(17)O (a signature of the silicate formation location) indicates that carbon phases and silicates formed independently in the nebula, and not from a carbon-rich magma. The elemental ratios Xe-132/Ar-36 and Kr-84/Ar-36 in carbon-rich acid residues show a decreasing trend with depth (inferred from carbon consumption during combustion), which can be interpreted as a consequence of the ion implantation mechanism of gas trapping that leads to greater depth of implantation for lighter mass ion. The similarity between trapped gases in phase Q in primitive chondrites and the C phases in ureilites-for both elemental and isotopic compositions-strongly suggests that phase Q might also have received its noble gases by ion implantation from the nebula. The slight differences in the elemental ratios can be explained by a plasma temperature at the location of phase Q gas loading that was about 2000 K lower than for ureilite C phases. This inference is also consistent with the finding that the trapped ratio Xe-129/Xe-132 (1.042 +/- 0.002) in phase Q is slightly higher, compared to that of ureilite C phases (1.035 +/- 0.002), as a consequence of in situ decay of I-129, and becomes observable due to higher value of I/Xe in phase Q as a result of ion implantation at about 2000 K lower plasma temperature. Copyright (C) 2003 Elsevier Ltd. [References: 67]