The MacAlpine Hills lunar meteorite and implications of the lunar meteorites collectively for the composition and origin of the Moon

Abstract

The MAC88104/MAC88105 meteorite is a lunar highlands regolith breccia even more anorthositic than previously available samples of highlands regolith. Clasts studied include two unusual pristine rocks. One, a 2.5-mm, slightly granulitic clast rated as probably pristine, contains extraordinarily Fe-rich (Fo 40) olivine. The other, a 5-mm clast with clear vestiges of a poikilitic cumulate texture, has silicate compositions that extend the range of the Mg-suite in the direction of the high-mg ∗ end of the ferroan-anorthositic suite. The pyroxene of the latter clast is relatively Ca-rich and poorly equilibrated by lunar cumulate standards, suggesting that it may have formed in an uncommonly shallow intrusion. The four lunar highlands meteorites are all regolith breccias (or, in one case, a marginal type between regolith breccia and “ordinary” polymict breccia), and they probably represent three to four separate lunar source craters. The proportion of regolith breccias is higher among lunar highlands meteorites than among rock samples from the classic highlands site, Apollo 16 (~9% regolith breccias). For the majorelement composition of the crust, the highlands meteorites confirm that the two low-Th central nearside sites, Luna 20, and especially Apollo 16, are approximately representative. The consistently high-Al 2O 3 composition indicated for the upper crust supports the magmasphere hypothesis. For the trace-element composition of the crust, the highlands meteorites indicate that the central nearside Apollo and Luna sites are in several respects grossly unrepresentative. Concentrations of siderophile elements are far lower in highlands-meteoritic regolith breccias than in their central nearside counterparts. The high overall siderophile levels and hyperchondritic Ni Ir and Au Ir ratios characteristic of highlands materials from Apollo 16 and Apollo 14 are evidently idiosyncracies of the central nearside. Concentrations of incompatible elements, including K, Th, and U, are far lower in the highlands meteorites than in regolith samples from the central nearside. This trend implies that certain lower limits on the bulk-Moon content of U (and associated refractory lithophile elements) should be relaxed. Models of lunar origin implying large enrichments of refractory lithophile elements are not favored by the new constraints from these meteorites.