Diogenites as asteroidal cumulates: Insights from orthopyroxene major and minor element chemistry

Abstract

Diogenites appear to be cumulates formed from one or more igneous reservoirs in the interior of asteroid 4 Vesta. Magmatism in this parent body gave rise to a series of related lithologies designated howardites, eucrites, and diogenites or “HED.” Eucrites are pigeonite/plagioclase basalts, and diogenites are orthopyroxenites. Howardites are brecciated mixtures of eucrites and diogenites. The major objective of this paper is to characterize the major and minor element chemistry of orthopyroxene in diogenites and to interpret these data in terms of a petrogenetic model. This study, which involves interpretation of ~ 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 systematics of the minor elements Al, Cr, and Ti still effectively record their igneous history. Aluminum and Ti are highly correlated with the incompatible trace elements (e.g. Yb, Y) and thus are useful fractionation indicators. As many as twenty of the twenty-three diogenites studied may have formed from one igneous system based on Al, Cr, and Ti systematics in orthopyroxene. The Al content of the orthopyroxene in the twenty diogenites forms the basis for a fractionation sequence with Peckelsheim the least fractionated and LEW 88008 the most fractionated. Roda and Manegaon, and possibly ALHA77256, fall off the main compositional trends and may belong to different melt systems. The coupled substitutions that incorporate Al, Cr, and Ti into the orthopyroxene crystal structure are VICr 3+- IVAl 3+, VIAl 3+- VIAl 3+, and VITi 4+-2 IVAl 3+. The dominant substitutional couple in the early stages of crystallization is VICr 3+- IVAl 3+ while VIAl 3+- IVAl 3+ is dominant in the late stages. These findings demonstrate the importance of understanding the coupled substitutions for minor elements when assessing 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 compatibilities (D values) with VICr 3+- IVAl 3+ more compatible than VIAl 3+- IVAl 3+ which is more compatible than VITi 4+-2 IVAl 3+. Based on the assumption that D Al increases during crystallization from 0.05 to 0.1 corresponding to the crystallization interval represented by Peckelsheim to LEW 88008, 60% crystallization is required. The melt parental to LEW 88008 is estimated to contain ~12 wt% Al 2O 3, very near plagioclase saturation.