NOBLE GASES IN THE DIAMOND-FREE UREILITE, ALHA 78019: THE ROLES OF SHOCK AND NEBULAR PROCESSES

Abstract

Primordial noble gases were measured in the unshocked ureilite ALHA #78019 (hereafter 78019) and in the moderately shocked ureilite Kenna. Much of the gas (> 25%) in 78019 is contained in fine-grained, amorphous carbon, where it is enriched 60-fold relative to that in the more abundant crystalline graphite. Isotopically, the gas in the gas-rich C resembles the planetary component (“phase Q”) in chondrites, but it is not lost upon etching with HNO3 and hence is not surface sited, unlike the chondritic gas. In most other respects, the noble gases are in the normal range for ureilites: bulk 132Xe = 0.30 × 10−8 ccSTP/g; 36Ar132Xe ~ 600 (this value is at the high end for ureilites, however); and trapped 20Ne22Ne and 21Ne22Ne are 10.5 ± 0.7 and 0.032 ± 0.003.

These results suggest that the gases—and carbon—predate the shock event that produced the diamond, hence the gas-rich C in 78019 appears to be a good candidate for the hypothetical pre-shock, noble-gas-bearing C in diamondiferous ureilites. Furthermore, the results suggest that the shock event has no bearing on the origin of carbon in these meteorites. More importantly, though, the gas-rich C in 78019 has a 132XeC = 4.2 × 10−10, which falls in the chondritic range, within 30% of C30's and unequilibrated ordinary chondrites. Thus, the gas-rich C in 78019 appears to have acquired its gases while exposed to the solar nebula and the similarity between the 78019 and chondritic 132XeC ratios implies fairly similar P and T at the times and places of trapping. The problem then reduces to explaining how the carbon and its noble gases survived incorporation into the ureilites.