The genesis of diogenites and HED parent body petrogenesis

Abstract

Abstract Samples of clasts and whole rocks from eighteen of the twenty-four known diogenites (orthopyroxenite breccia meteorites) have been studied by combined INAA-EMPA in order to elucidate their petrogenetic history. The incompatible elements A12O3 and TiO2 are positively correlated in diogenite orthopyroxenes. These elements, determined by EMPA, are also positively correlated with incompatible trace elements, such as Yb, determined by INAA on bulk samples. These correlations indicate that most diogenites may form a single igneous fractionation sequence. The incompatible minor and trace elements are decoupled from the major elements; for essentially constant mg#, TiO2 varies by about 4 times and Yb by about 8 times. Hence, there is no strong evidence for igneous fractionation preserved in the major element composition of orthopyroxenes. Geothermometry calculations using two-pyroxene and spinelorthopyroxene thermometers show that these phases in diogenites maintained equilibrium to temperatures in the range 650–880°C, far below likely magmatic temperatures. In some diogenites, orthopyroxenes of differing composition are in breccia contact. Therefore, the low equilibration 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 likely have much lower diffusivities in orthopyroxene, and therefore may have largely preserved their magmatic distributions. The extensive variation in incompatible trace elements indicates that either the diogenites were formed through an extensive crystallization interval, or that partition coefficients greatly increased during crystallization. If the former were true, one would expect to find substantial amounts of plagioclase 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 mineralogie and trace element constraints.