Consortium study of labile trace elements in some Antarctic carbonaceous chondrites: Antarctic and non-Antarctic meteorite comparisons

Abstract

We report data for Ag, Au, Bi, Cd, Co, Cs, Ga, In, Rb, Sb, Se, Te, Tl, U and Zn determined by radiochemical neutron activation analysis in consortium samples of Belgica (B)-7904,Yamato (Y)-82042,Yamato-82162 and Yamato-86720 carbonaceous chondrites. These trace elements cover a wide volatility/mobility range and give unique information on thermal histories of meteorites. The results indicate the unique nature of these carbonaceous chondrites. Y-82042 proves to have the volatile element pattern of a C2 (≡CM) chondrite and the petrologic characteristics of a C1 (≡CI) chondrite. These must be primary nebular condensation/accretion features, unaffected by post-accretionary processes. The other three meteorites were thermally metamorphosed in &ge;2 parent regions over the 600-700℃ range, at relative temperatures B-7904<Y-82162<Y-86720. Before heating, B-7904 and Y-86720 had C2-levels of volatile elements : Y-82162 had uniquely high volatile element concentrations, at about C1-levels. The data require a new classification scheme for such chondrites. Belgica-7904 and Y-82162 and -86720 seem to be derived from one or more thermally altered carbonaceous asteroids and their spectral characteristics should be compared with those of B-, F-, G-, or T-asteroids. These results indicate substantial differences in the thermal histories of Antarctic and non-Antarctic C1 and C2 chondrite populations. In reviewing all that is known about the Antarctic and non-Antarctic meteorite populations, the overwhelming weight of evidence supports the view that these populations sample different extraterrestrial source materials, differing in thermal histories. It may be that over the extended collecting period of the Antarctic ice sheet, it has sampled a considerable greater proportion of near-Earth asteroids than do current falls