Siderophile elements in Earth's upper mantle and lunar breccias: Data synthesis suggests manifestations of the same late influx

The platinum group elements (PGE; Ru, Rh, Pd, Os, Ir, Pt), Re and Au compri se the highly siderophile elements (HSE). We reexamine selected isotopic an d abundance data sets for HSE in upper mantle peridotites to resolve a long standing dichotomy. Re-Os and Pt-Os isotope systematics, and approximately chondritic proportions of PGE in these rocks, suggest the presence in undep leted mantle of a chondrite-like component, which is parsimoniously explain ed by late influx of large planetisimals after formation of the Earth's cor e and the Moon. But some suites of xenolithic and orogenic spinel lherzolit es, and abyssal peridotites, have a Cl-normalized PGE pattern with enhanced Pd that is sometimes termed "non-chondritic". We find that this observatio n is consistent with other evidence of a late influx of material more close ly resembling enstatite, rather than ordinary or carbonaceous, chondrites. Regional variations in HSE patterns may be a consequence of a late influx o f very large objects of variable composition. Studies of many ancient (>3.8 Ga) lunar breccias show regional variations i n Au/Ir and suggest that "graininess" existed during the early bombardment of the Earth and Moon. Reliable Pd values are available only for Apollo 17 breccias 73215 and 73255, however. Differences in HSE patterns between the aphanitic and anorthositic lithologies in these breccias show fractionation between a refractory group (Re, Os and Ir) and a normal (Pd, Ni, and Au) g roup and may reflect the compositions of the impacting bodies. Similar frac tionation is apparent between the EH and EL chondrites, whose PGE patterns resemble those of the aphanitic and anorthositic lithologies, respectively. The striking resemblance of HSE and chalcogen (S, Se) patterns in the Apoll o aphanites and high-Pd terrestrial peridotites suggest that the "non-chond ritic" abundance ratios in the latter may be reflected in the composition o f planetisimals striking the Moon in the first 700 Ma of Earth-Moon history . Most notably, high Pd may be part of a general enhancement of HSE more vo latile than Fe suggesting that the Au abundance in at least parts of the up per mantle may be 1.5 to 2 x higher than previously estimated. The early lunar influx may be estimated from observed basin-sized craters. Comparison of relative influx to Earth and Moon suggests that the enrichmen t of HSE is limited to the upper mantle above 670 km. To infer enrichment o f the whole mantle would require several large lunar impacts not yet identi fied.