A MODEL COMPOSITION OF THE BASALTIC ACHONDRITE PLANETOID

The basaltic achondrites, eucrites, diogenites, and howardites have co mpositions on a common oxygen isotope mass fractionation line and prob ably formed from a chondritic precursor also lying on that same line. No chondritic meteorite group has the same isotopic signature as the b asaltic achondrites, so the oxygen isotope ratios of several known cho ndritic groups were used to construct a two component mixing model for the composition of the precursor. This model does not provide a uniqu e solution, as several mixtures of ordinary and carbonaceous precursor s will satisfy the isotopic constraints. The Fe-Mn-Mg abundances of th e precursors and of the eucrites were used to provide an additional co nstraint. The precursor composition selected for study is a mixture of 70% (wt) H-chondrite with 30% (wt) CM-chondrite. This mixture generat es a slightly FeO-rich silicate precursor that, after reduction and se paration of an iron + sulfide core, is compatible with the mantle of t he basaltic achondrite planetoid (BAP) having a similar composition to that modeled by Dreibus and Wanke (1980). Partial melting experiments of this H-CM precursor composition suggest that eucritic magmas could be formed in such a mantle. These experiments also suggest that the m antle must have experienced metal loss to constrain the Fe/Mn ratios a nd probably significant olivine fractionation as well. Diogenite precu rsors may also be generated in this mantle composition as FeO reductio n and olivine fractionation lead to the formation of SiO2 enriched com positions from which diogenite source magmas may be extracted. If mixi ng of material from two very distinct chondritic reservoirs (I-I and C M-chondrites) is realistic, then an asteroid scale mixing process is n eeded to generate the achondrite precursor. Large impact events would provide a plausible method for mixing material from reservoirs with qu ite different oxygen isotope characteristics to assemble the basaltic achondrite planetoid. Copyright (C) 1997 Elsevier Science Ltd.