THE GENESIS OF DIOGENITES AND HED PARENT BODY PETROGENESIS

Samples of clasts and whole rocks from eighteen of the twenty-four kno wn diogenites (orthopyroxenite breccia meteorites) have been studied b y combined INAA-EMPA in order to elucidate their petrogenetic history. The incompatible elements Al2O3 and TiO2 are positively correlated in diogenite orthopyroxenes. These elements, determined by EMPA, are als o positively correlated with incompatible trace elements, such as Yb, determined by INAA on bulk samples. These correlations indicate that m ost diogenites may form a single igneous fractionation sequence. The i ncompatible minor and trace elements are decoupled from the major elem ents: for essentially constant mg#, TiO2 varies by about 4 times and Y b by about 8 times. Hence, there is no strong evidence for igneous fra ctionation preserved in the major element composition of orthopyroxene s. Geothermometry calculations using two-pyroxene and spinel-orthopyro xene thermometers show that these phases in diogenites maintained equi librium to temperatures in the range 650-880-degrees-C, far below like ly magmatic temperatures. In some diogenites, orthopyroxenes of differ ing composition are in breccia contact. Therefore, the low equilibrati on temperatures were achieved in the diogenites prior to brecciation. Decoupling of major and trace elements was most likely caused by slow, subsolidus cooling in the diogenite pluton which allowed Ca, Fe, and Mg to reach diffusive equilibration. Incompatible trace elements likel y have much lower diffusivities in orthopyroxene, and therefore may ha ve largely preserved their magmatic distributions. The extensive varia tion in incompatible trace elements indicates that either the diogenit es were formed through an extensive crystallization interval, or that partition coefficients greatly increased during crystallization. If th e former were true, one would expect to find substantial amounts of pl agioclase in the diogenites if they are comagmatic with eucrites. This is contrary to observations. A model in which diogenites crystallized from one or more melts of approximately orthopyroxenitic composition can satisfy both the mineralogic and trace element constraints.