HIGHLY SIDEROPHILE ELEMENT COMPOSITION OF THE EARTH’S PRIMITIVE UPPER MANTLE: CONNECTION TO LUNAR IMPACT MELT ROCKS AND LATE ACCRETION

Osmium, Ru, Ir, Pt, Pd and Re abundances and ¹⁸⁷Os/¹⁸⁸Os data were determined in peridotites by improved analytical techniques (isotope dilution and Carius tube digestion at 345ºC) in order to precisely constrain the highly siderophile element (HSE) composition of the primitive upper mantle (PUM) and the origin of the HSE excess in Earth's mantle. Samples include lherzolite and harzburgite xenoliths from Archean and post-Archean continental lithosphere, peridotites from ultramafic massifs, ophiolites, and other samples of oceanic mantle. Os/Ir and Ru/Ir are constant in lherzolites, whereas, Pd and Re abundances, and their ratios with Ir, Os and Ru, and ¹⁸⁷Os/¹⁸⁸Os show positive correlations with Al2O3, indicating incompatible behavior of Pt, Pd and Re during mantle melting. Secondary igneous processes such as silicate melt percolation or refertilization appear to have had little effect on HSE abundances in fertile lherzolites. Based on fertile lherzolite compositions, we infer that PUM is characterized by chondritic ratios involving Os, Ir, Pt and Re (Os/Ir_PUM of 1.12 ± 0.09, Pt/Ir_PUM = 2.21 ± 0.21, Re/Os_PUM = 0.090 ± 0.002) and suprachondritic ratios involving Ru and Pd (Ru/Ir_PUM = 2.03 ± 0.12, Pd/Ir_PUM = 2.06 ± 0.31, uncertainties 1σ). Planetary processes such as metal-silicate equilibration may have difficulties accounting for the combination of chondritic and modestly suprachondritic HSE ratios in PUM. Comparison with HSE patterns of chondrites shows that no known chondrite group perfectly matches the PUM composition. Similar HSE patterns, however, were found in Apollo 17 impact melt rocks from the Serenitatis impact basin (Norman et al. 2002, Puchtel et al. 2005), which represent mixtures of chondritic material from the basin-forming impactor and a component that may be either of meteoritic or indigenous origin. The similarities between the HSE composition of PUM and the bulk composition of lunar impact melt rocks establish a connection between the late accretion history of the lunar surface and the HSE composition of the Earth's mantle. We infer that, like on the Moon, late accretion and mixing played a major role in setting HSE abundances of the Earth's early crust. HSE-enriched crust may have been returned to the Earth's mantle by some form of Hadean and early Archean crustal recycling.