Diogenites as asteroidal cumulates: Insights from spinel chemistry

The chemical composition of spinel was determined for a suite of 19 diogeni tes (orthopyroxenites) thought to be from asteroid 4 Vesta. Previous studie s (Fowler et al. 1994, 1995) of orthopyroxene demonstrated that these dioge nites are Linked genetically, perhaps through fractional crystallization, i n one or more crustal intrusions. The present study focuses on spinel to se e if it also retains some chemical signatures of an igneous history. The ch emical compositions across spinel grains reveal flat concentration profiles , indicating major subsolidus exchange with orthopyroxene. Nevertheless, si gnificant chemical differences exist among the average spinel compositions from individual diogenites in the suite. A chemical continuum exists from h igh-Cr, low-Al spinel in diogenite LAP 91900 (Cr2O3 60.7 wt%;Al2O3 6.1 wt%) to low-Cr, high-Al spinel in diogenite ALHA 77256 (Cr2O3 44.7 wt%; Al2O3 2 1.8 wt%), which may represent one or more fractionation series. In these tr ends, Al and Ti behave as incompatible elements whose abundances increase w ith crystallization. These systematics differ from those in spinel in terre strial or lunar basaltic systems because of the extremely Al-depleted natur e of the diogenite parental melts. In terrestrial and lunar basalts, the in crease in Al concentration in spinel is interrupted when plagioclase crysta llizes. In diogenite parental melts, plagioclase does not come onto the liq uidus until the very end of spinel crystallization.