New noble gas data of primitive and differentiated achondrites including Northwest Africa 011 and Tafassasset

Abstract

This work reports on the noble gas inventory of 3 new acapulcoites, 3 brachinites, 2 new eucrites from the Dar al Gani region in Libya, the unique achondrite Dar al Gani (DaG) 896 from the same locality, the new eucrite-like achondrite Northwest Africa (NWA) 011, and the controversial sample Tafassasset. We determined cosmic ray exposure and gas retention ages, evaluated shielding conditions, and discuss the trapped noble gas component of the specimens. All exposure ages are within the known range of stony meteorites and partly confirm previously established age clusters. Shielding conditions vary, suggesting substantial shielding for all 3 brachinites and Tafassasset. We cannot exclude, however, that the Mg-rich composition of brachinites simply simulates heavy shielding. Regarding the trapped component, we found Q-like compositions only for the acapulcoite Thiel Mountains (TIL) 99002. The brachinite Elephant Moraine (EET) 99402 yields a high, subsolar 36Ar/ 132Xe ratio of ~400 along with a slightly elevated 84Kr/132Xe ratio, indicating minor atmospheric contamination. All the other samples, particularly the eucrite DaG 983, are characterized by clearly elevated Ar/Kr/Xe ratios due to significant terrestrial alteration. Tafassasset exhibits noble gas parameters that are different from those of CR chondrites, including a relatively high cosmic ray exposure age, the absence of a solar component, low 132Xe concentrations, a low trapped 36Ar/132Xe ratio of ~30, and a noticeable amount of radiogenic 129Xe. Similar attributes have been observed for some primitive achondrites. These attributes are also consistent with the metamorphic character of the sample. We, therefore, consider Tafassassets noble gas record to be inconclusive as to its classification (primitive achondrite versus metamorphosed CR chondrite).