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

Abstract

Abstract-The platinum group elements (PGE; Ru, Rh, Pd, Os, Ir, Pt), Re and Au comprise the highly siderophile elements (HSE). We reexamine selected isotopic and abundance data sets for HSE in upper mantle peridotites to resolve a longstanding dichotomy. Re-0s and Pt-0s isotope systematics, and approximately chondritic proportions of PGE in these rocks, suggest the presence in undepleted mantle of a chondrite-like component, which is parsimoniously explained by late influx of large planetisimals after formation of the Earth's core and the Moon. But some suites of xenolithic and orogenic spinel lherzolites, and abyssal peridotites, have a CI-normalized PGE pattern with enhanced Pd that is sometimes termed "non-chondritic". We find that this observation is consistent with other evidence of a late influx of material more closely resembling enstatite, rather than ordinary or carbonaceous, chondrites. Regional variations in HSE patterns may be a consequence of a late influx of very large objects of variable composition.Studies of many ancient (>3.8 Ga) lunar breccias show regional variations in A d I r 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, 0 s and Ir) and a normal (Pd, Ni, and Au) group and may reflect the compositions of the impacting bodies. Similar fractionation 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 Apollo aphanites and highPd terrestrial peridotites suggest that the "non-chondritic" abundance ratios in the latter may be reflected in the composition of 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 volatile than Fe suggesting that the Au abundance in at least parts of the upper mantle may be 1.5 to 2x 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 enrichment of HSE is limited to the upper mantle above 670 km. To infer enrichment of the whole mantle would require several large lunar impacts not yet identified.

186Os and 187Os enrichments and high-3He/4He sources in the Earth’s mantle: Evidence from Icelandic picrites

“…The Os concentrations and isotopic compositions for present-day PM are well constrained and are used for the primitive mantle component present in the plume (Table 4; Meisel et al, 2001;Morgan et al, 2001;Brandon et al, 2006). The DPS component represents a source with a slightly lower long-term Re/Os resulting from melt removal and loss of slightly incompatible Re relative to Os in the convecting mantle, consistent with the outgassing He model of Class and Goldstein (2005).…”

Os isotopic evidence for a carbonaceous chondritic mantle source for the Nagqu ophiolite from Tibet and its implications

“…From the above results, the Os-isotope characteristics of the mantle source forthe Nagquophioliteare consistent with those of carbonaceous chondrites, which mean that there is a carbonaceous chondrite-type mantle [77]. In contrast to primitive mantle values [78,79], the Os-isotope characteristics of the mantle source for the magmas in the Nagquo phioliteare slightly lower, which could imply that the source is a slightly depleted asthenospheric mantle.…”

Siderophile element constraints on the origin of the Moon

“…It is also possible the 'missing' HSE reside in the lunar crust, which is both ancient and thick, and may have served as an effective barrier against HSE addition to the mantle from late stage impactors [36,63]. However, unless the late accretionary impactors were largely metal, the bulk Finally, the presumed low HSE abundances in the lunar mantle could also mean that either the Moon received proportionally much less late accreted material compared with the Earth, or that the late accretionary clock for the Earth began prior to that for the Moon.…”

“…Consequently, it has been argued that the present inventory of the HSE, as well as a small portion of the MSE in the BSE, were added as a 'late veneer' of late accreted materials, subsequent to the termination of core segregation [35,36] 0.8% of the total mass of the Earth [37]. This hypothesis predicts the imposition of chondritic relative abundances of HSE onto the mantle and is consistent with the chondritic 187 Os/ 188 Os and 186 Os/ 188 Os projected for the BSE [19,38,39].…”