Petrogenetic relationships between diogenites and olivine diogenites: Implications for magmatism on the HED parent body

Diogenites are orthopyroxenites and harzburgites that are petrogenetically associated with basaltic magmatism linked to the earliest stages of asteroidal melting on the parent body for the howardite-eucrite-diogenite (HED) meteorites. There are several models proposed for their origin: (1) accumulation of orthopyroxene (OPX) + chromite (CHR) ± olivine (OL) during the crystallization of a magma ocean during the initial stages of asteroidal differentiation, (2) accumulation of OPX + CHR ± OL during the crystallization of compositionally distinct basaltic magmas emplaced into the crust of the HED parent body, and (3) the orthopyroxenites formed by the crystallization of basaltic magmas within the HED parent body crust, whereas the harzburgites represent the mantle of the HED parent body. Although OL and OPX experienced varying degrees of subsolidus reequilibration (1100-700 °C), their minor and trace element characteristics appear to partially preserve magmatic signatures that provide insights into distinguishing among different models for the origin of diogenites. The OPX exhibits a continuous and very systematic variation in incompatible elements such as Al, Ti, Zr, Y, and Yb. Polymict diogenites (i.e. Roda, EET 79002) can contain distinct lithologies with both different incompatible element characteristics and different model abundances of OL. There appears to be no relationship between the appearance and abundance of OL and the incompatible element characteristics of the OPX. The OL exhibits a range in Mg# and systematic variations Ni, Co, Ni/Co, and Mn. For examples, low Ni/Co appears to be closely associated with the harzburgites and Ni and Mn exhibit a negative correlation. Surprisingly, incompatible element concentrations in OPX are not negatively correlated to Ni concentrations in OL. The continuous nature of the minor and trace element characteristics of the OPX and OL is consistent with the all the diogenite lithologies forming through a single process such as crystallization within a magma ocean or a series of layered intrusions. Further, the range in incompatible element variability in the OPX, the Ni and Co systematics in the OL, and the association of distinctly different lithologies within polymict diogenites are most consistent with the diogenites representing lithologies from diverse layered intrusions. Alternatively, they may represent crystallization products of a magma ocean much more complex than has been thus far proposed (i.e. multiple MOs). There are some distinct differences between diogenites and the OL-rich achondrite QUE 93148 that was also analyzed during this study. These differences (such as Ni/Co in OL, estimated conditions of fO2) suggest that QUE 93148 is closely related to main-group pallasites rather than the parent bodies for the HED meteorites.