Highly siderophile element depletion in the Moon
James M.D. Day, Richard J. Walker
Earth and Planetary Science Letters
Volume 423, 1 August 2015, Pages 114–124
Apollo 12 olivine-, pigeonite- and ilmenite normative mare basalts define an imprecise 187Re–187Os age of 3.0±0.9 Ga3.0±0.9 Ga with an initial 187Os/188Os of 0.107±0.0100.107±0.010. This age is within uncertainty of 147Sm–143Nd ages for the samples. The initial Os isotopic composition of Apollo 12 samples indicates that the 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. 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 a late accretion addition of ∼0.02 wt.% of the Moon’s mass to the mantle, prior to the formation of the lunar crust. Late accretion must also have occurred significantly prior to cessation of lunar mantle differentiation (>4.4 Ga), to enable efficient mixing and homogenization within the mantle. Low lunar HSE abundances are consistent with proportionally 40 times more late accretion to Earth than the Moon. Disproportional late accretion to the two bodies is consistent with the small 182W excess (∼21–28 ppm) measured in lunar rocks, compared to the silicate Earth.