Highly siderophile element depletion in the Moon.

Coupled Os/Os and highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundance data are reported for Apollo 12 (12005, 12009, 12019, 12022, 12038, 12039, 12040), Apollo 15 (15555) and Apollo 17 (70135) mare basalts, along with mare basalt meteorites La Paz icefield (LAP) 04841 and Miller Range (MIL) 05035. The most magnesian samples have chondrite-relative HSE abundances and chondritic measured and calculated initial Os/Os, with mare basalts having consistently low HSE abundances at ~2 ×10 to 2 ×10 the chondritic abundance. The lower and more fractionated HSE compositions of evolved mare basalts can be reproduced with bulk-partition coefficients of ~2 for Os, Ir, Ru, Pt and Pd and ~1.5 for Re. Lunar mare basalt bulk-partition coefficients are probably higher than for terrestrial melts as a result of more reducing conditions, leading to increased HSE compatibility. The chondritic-relative abundances and chondritic Os/Os of the most primitive high-MgO mare basalts cannot be explained through regolith contamination during emplacement at the lunar surface. Instead, mare basalt compositions can be modelled as representing ~5-11% partial melting of metal-free sources with low Os, Ir, Ru, Pd (~0.1 ng g), Pt (~0.2 ng g) Re (~0.01 ng g) and S, with sulphide-melt partitioning between 1000 and 10000. Apollo 12 olivine-, pigeonite- and ilmenite normative mare basalts define an imprecise Re-Os age of 3.0 ±0.6 Ga. This age is within uncertainty of Sm-Nd ages for the same samples and the isochron yields an initial Os/Os of 0.109 ±0.008. The Os isotopic composition of the Apollo 12 source indicates that the lunar mantle source of these rocks evolved with Re/Os within ~10% of chondrite meteorites from the time that the mantle source became a system closed to siderophile additions to the time that the basalts erupted. The similarity in absolute HSE abundances between mare basalts from the Apollo 12, 15 and 17 sites, and from unknown regions of the Moon (La Paz mare basalts, MIL 05035) indicates relatively homogeneous and low HSE abundances within the lunar interior. Low absolute HSE abundances and chondritic Re/Os of mare basalts are consistent with ~0.02% late accretion addition that was added prior to the formation of the lunar crust and significantly prior to cessation of lunar mantle differentiation (>4.4 Ga) to enable efficient mixing and homogenization. The HSE abundances are also consistent with the observed, small W excess (20 ppm) in the bulk silicate Moon relative to the bulk silicate Earth.