Peridotites from Another Planet? Osmium Isotope and Highly Siderophile Element Constraints on the Evolution of Diogenites and the Hed Parent Body

Diogenites and 4 Vesta: Diogenite meteorites are orthopyroxene ± olivine rich rocks that form part of the howardite-eucrite-diogenite (HED) meteorite suite [1, 2]. The HED parent-body is thought to be the ~530 km diameter asteroid 4 Vesta [3, 4], target of the recently launched DAWN mission. Homogeneous Oisotope systematics for HED meteorites are consistent with wholesale melting of their parent body [2] and initial studies indicate low abundances of highly siderophile elements (HSE) [5], implying crust-mantlecore differentiation. Thus, the HED meteorites are valuable samples for understanding early planetary differentiation processes, and as proxies for Earth, where materials formed prior to around 4 Ga are neither preserved nor sampled. Identification of olivinerich diogenites [e.g., 6] and evidence for a range of orthopyroxene [7] and spinel compositions [8], imply diogenites represent cumulates, and possibly even mantle peridotites from their parent body [6, 9, 10]. Diogenites may, therefore, represent the only sampling of bonafide mantle materials outside of Earth and provide the rationale for this study. Samples and Analytical Protocols: We present new osmium isotope and HSE (Os, Ir, Ru, Pt, Pd, Re) abundance data for 1 North-west African and 7 Antarctic diogenites. These include the recently discovered olivine diogenite MIL 07001, olivine diogenites NWA 1877, EETA 79002, olivine-rich diogenite LAP 05639 and olivine-poor diogenites ALHA 77256, LAP 91900, MET 00424 and SAN 03473. Textures of the samples range from cataclastic (e.g., LAP 05639, SAN 03473) through to granular aggregates (e.g., MIL 07001), but all have relatively homogeneous orthopyroxene (Fs22-27Wo1-2) and, where present, olivine (Fo7279) compositions. Spinel compositions vary within the samples, as does the presence and modal abundance of trace phases, including spinel, clinopyroxene, plagioclase, sulphides, FeNi metal and silica. Analytical protocols are outlined in [11]. Blank contributions are low (e.g., <0.2pg Os, <1.5pg Re) resulting in estimated HSE blank contributions that are variable for the samples. For example, estimated Os blank contributions are less than 0.1% for MIL 07001, but as high as 20% for NWA 1877. Highly Siderophile Element Abundances: Diogenites range nearly five orders of magnitude in HSE abundances (0.0018 to 3.6 ng g Os) and are characterized by broadly flat CI-chondrite normalized HSE patterns (Fig. 1). NWA 1877 (Ol diogenite) and MET 00424 (Ol-poor diogenite) have the lowest HSE abundances and highest Re/Os ratios. MIL 07001 and EETA 79002 (both Ol diogenites) have the highest HSE abundances. We find no clear relationship between the modal abundance of olivine in diogenites and their HSE abundances. Although olivine diogenites MIL 07001 and EETA 79002 have the most elevated abundances of HSE in the suite of diogenites analyzed so far, NWA 1877, another olivine diogenite [6], has one of the lowest HSE abundance inventories of the suite. Nevertheless, we note that the HSE abundances of olivine diogenites MIL 07001 and EETA 79001 are remarkably close to fertile terrestrial mantle peridotite estimates (PUM; Fig. 1), which requires explanation.