THE NORMAL PLANETARY NOBLE GAS COMPONENT IN PRIMITIVE CHONDRITES: COMPOSITIONS, CARRIER, AND METAMORPHIC HISTORY

Abstract

In order to study the characteristics of P1, the dominant ''normal planetary'' noble gas component in chondrites, we prepared HF/HCl residues for fourteen chondrites from seven chondrite classes. We chose relatively unmetamorphosed meteorites to study the primary characteristics of the P1 component, but we also selected meteorites covering a range of petrologic types to study the effects of mild metamorphism on the P1 component. The noble gases from each residue were measured by stepped pyrolysis. Data from the Orgueil (CI), Semarkona (LL3.0), Bishunpur (LL3.1), and Krymka (LL3.1) residues were used to derive precise compositions for Ar-P1, Kr-P1, and Xe-P1. Our compositions differ only slightly from those obtained by closed-system chemical oxidation of HF/HCl residues (Wieler et al., 1991, 1992), which indicates that the compositions are now reasonably well known. The isotopic compositions of P1 gases are similar to, but distinct from, those of the low-temperature P3 component in presolar diamonds. By comparing the characteristics and compositions of P1 gases and their carrier from meteorites with different metamorphic histories, we found several systematic trends. With increasing metamorphic grade, P1 abundance decreases, median release temperature increases, the isotopic composition evolves as gases from other carriers are retrapped into the P1 carrier, the elemental ratios change slightly, and the P1 carrier becomes more resistant to chemical oxidation. Some small systematic differences were also observed between unmetamorphosed meteorites of different classes, including lower matrix-normalized P1 abundances in EH and CV chondrites compared to Orgueil and the unequilibrated ordinary chondrites, and small differences in elemental ratios between classes. Some of the differences between classes may reflect thermal processing of the P1 carrier and associated material in the accretion disk. Our observations indicate that the characteristics of the P1 noble-gas component, in most if not all meteorites, no longer exclusively reflect the processes that originally trapped the gases into their carrier.