Primordial noble gases in "phase Q" in carbonaceous and ordinary chondrites studied by closed-system stepped etching

The HF/HCl-resistant residues of the chondrites CM2 Cold Bokkeveld, CV3 (ox .) Grosnaja, CO3.4 Lance, CO3.7 Isna, LL3.4 Chainpur, and H3.7 Dimmitt have been measured by closed-system stepped etching (CSSE) in order to better c haracterise the noble gases in "phase Q", a major carrier of primordial nob le gases. All isotopic ratios in phase Q of the different meteorites are qu ite uniform, except for (Ne-20/Ne-22)(Q). As already suggested by precise e arlier measurements (Schelhaas et al., 1990; Wieler et al., 1991, 1992), (N e-20/Ne-22)(Q) is the least uniform isotopic ratio of the Q noble gases. Th e data cluster similar to 10.1 for Cold Bokkeveld and Lance and 10.7 for Ch ainpur, Grosnaja, and Dimmitt, respectively. No correlation of (20Ne/22Ne)( Q) with the classification or the alteration history of the meteorites has been found. The Ar, Kr, and Xe isotopic ratios for all six samples are iden tical within their uncertainties and similar to earlier Q determinations as well as to Ar-Xe in ureilites. Thus, an unknown process probably accounts for the alteration of the originally incorporated Ne-Q. The noble gas eleme ntal compositions provide evidence that Q consists of at least two carbonac eous carrier phases "Q(1)" and "Q(2)" with slightly distinct chemical prope rties. Ratios (Ar/Xe)(Q) and (Kr/Xe)(Q) reflect both thermal metamorphism a nd aqueous alteration. These parent-body processes have led to larger deple tions of Ar and Kr relative to Xe. In contrast, meteorites that suffered se vere aqueous alteration, such as the CM chondrites, do not show depletions of He and Ne relative to Ar but rather the highest (He/Ar)(Q) and (Ne/Ar)(Q ) ratios. This suggests that Q(1) is less susceptible to aqueous alteration than Q(2). Both subphases may well have incorporated noble gases from the same reservoir, as indicated by the nearly constant, though very large, dep letion of the lighter noble gases relative to solar abundances. However, th e elemental ratios show that Q(1) and Q(2) must have acquired (or lost) nob le gases in slightly different element proportions. Cold Bokkeveld suggests that Q(1) may be related to presolar graphite. Phases Q(1) and Q(2) might be related to the subphases that have been suggested by Gros and Anders (19 77). The distribution of the Ne-20/N-22 ratios cannot be attributed to the carriers Q(1) and Q(2) The residues of Chainpur and Cold Bokkeveld contain significant amounts of Ne-E(L), and the data confirm the suggestion of Huss (1997) that the Ne-22-E(L) content, and thus the presolar graphite abundan ces, are correlated with the metamorphic history of the meteorites.