Isotopic anomalies of noble gases in meteorites and their origins—VII. C3V carbonaceous chondrites

Abstract

Noble gases were measured in bulk samples of the C3V chondrites Grosnaja, Vigarano, and Leoville, and in HF,HCl-insoluble residues before and after etching with HNO 3. The residues were characterized by INAA and SEM. Gas components were determined, directly or by subtraction, for the following fractions: HF, HCl- solubles (⪢98% of the meteorite), ‘ sphase Q’, a poorly characterized trace mineral that is insoluble in HCl-HF but soluble in HNO 3, and an insoluble residue, consisting of ferrichromite, carbonaceous matter, and spinel. Bulk meteorites show some correlation of the noble-gas pattern with McSween's subclasses: two ‘oxidized’ C3V's—Allende (LEWIS et al, 1975) and Grosnaja— have lower Ar/Xe but higher Ne/Xe ratios than the ‘reduced’ C3V's—Vigarano and Leoville—which are transitional to LL3's and C3O chondrites in both respects. An HCl-soluble mineral of high Ar/Xr ratio seems to be responsible. In other respects, the 3 C3V's of this study resemble Allende, with only moderate differences. Phase Q contains most of the Ar, Kr, Xe, but only small amounts of Ne; the etched residues contain planetary Ne ( Ne 20 Ne 22 ≅ 8.5 ) and the controversial CCFXe component, enriched in the heavy Xe isotopes ( Xe136 Xe132 ≅ 0.4–0.5 ). The CCFXe is accompanied by an ‘L-Xe’ component that is enriched in the light Xe isotopes. The proportion of the two is virtually constant in C3V's. as in all other C-chondrites. in contrast to the ~ 2-fold variation in ordinary chondrites. C3V's have systematically higher Xe 136 Xe 132 ratios, and hence higher ratios of CCFXe to planetary Xe, than do other chondrite classes. This may reflect some peculiarity in their formation conditions, favoring uptake of CCFXe.