DIOGENITES AS ASTEROIDAL CUMULATES - INSIGHTS FROM ORTHO-PYROXENE MAJOR AND MINOR ELEMENT CHEMISTRY

Diogenites appear to be cumulates formed from one or more igneous rese rvoirs in the interior of asteroid 4 Vesta. Magmatism in this parent b ody gave rise to a series of related lithologies designated howardites , eucrites, and diogenites or ''HED.'' Eucrites are pigeonite/plagiocl ase basalts, and diogenites are orthopyroxenites. Howardites are brecc iated mixtures of eucrites and diogenites. The major objective of this paper is to characterize the major and minor element chemistry of ort hopyroxene in diogenites and to interpret these data in terms of a pet rogenetic model. This study, which involves interpretation of similar to 1,200 high-quality electron microprobe analyses, demonstrates that the systematics of Ca, Fe, and Mg in orthopyroxene were largely reset by reaction with melt and by subsolidus exchange. However, the systema tics of the minor elements Al, Cr, and Ti still effectively record the ir igneous history. Aluminum and Ti are highly correlated with the inc ompatible trace elements (e.g. Yb, Y) and thus are useful fractionatio n indicators. As many as twenty of the twenty-three diogenites studied may have formed from one igneous system based on Al, Cr, and Ti syste matics in orthopyroxene. The Al content of the orthopyroxene in the tw enty diogenites forms the basis for a fractionation sequence with Peck elsheim the least fractionated and LEW 88008 the most fractionated. Ro da and Manegaon, and possibly ALHA77256, fall off the main composition al trends and may belong to different melt systems. The coupled substi tutions that incorporate Al, Cr, and Ti into the orthopyroxene crystal structure are Cr-VI(3+)-Al-IV(3+), Al-VI(3+)-Al-IV(3+), and Ti-VI(4+) -2(IV)Al(3+). The dominant substitutional couple in the early stages o f crystallization is Cr-VI(3+)-Al-IV(3+) while Al-VI(3+)-Al-IV(3+) is, dominant in the late stages. These findings demonstrate the importanc e of understanding the coupled substitutions for minor elements when a ssessing the appropriate mineral/melt partition coefficients (D values ) or when discussing whether an element is compatible or incompatible. For example, in diogenitic orthopyroxene, Al has three different comp atibilities (D values) with Cr-VI(3+)-Al-IV(3+) more compatible than A l-VI(3+)-Al-IV(3+) which is more compatible than (TI4+)-T-VI-2(IV)Al(3 +). Based on the assumption that D-Al increases during crystallization from 0.05 to 0.1 corresponding to the crystallization interval repres ented by Peckelsheim to LEW 88008, 60% crystallization is required. Th e melt parental to LEW 88008 is estimated to contain similar to 12 wt% Al2O3, very near plagioclase saturation.