Trace element and textural evidence favoring lunar, not terrestrial, origin of the mini-granite in Apollo sample 14321

Abstract

The 14321 lunar granite (14321g) has recently been reinterpreted (Bellucci et al., 2019) as a piece of the Hadean Earth, impact-transported to the Moon. In principle, samples of such derivation may afford important insights into the nature of Earth's Hadean crust. We have tested the terrestrial provenance hypothesis by comparing trace-element data from 14321g versus other lunar evolved rocks and a large data base for terrestrial granites. Volatile trace metals Zn, Ga and Ge are depleted in 14321g below the terrestrial granite ranges; and relative to the terrestrial granite averages by factors of 27, 2.0 and 18, respectively. Evidence from shocked chondrites, martian meteorites, and impact-shock studies in general, indicates that such major depletions are unlikely to develop without near-complete shock-melting, which clearly did not occur in the mostly still-crystalline 14321g. Moreover, other aspects of compositional disparity between 14321g and terrestrial granites involve exclusively refractory trace elements. Compared to terrestrial granites of similarly high Ba content, 14321 is enriched in Ta by a factor of 10; and the few terrestrial granites that are as Ta-rich as 14321 have 10 times lower Ba. The refractory-element ratio Lu/Sm is also close to 10 times higher in 14321g than in terrestrial granites of similarly high Ba content. Other highly evolved lunar rocks, “felsites”, strongly resemble 14321g in all these respects. We conclude that 14321g is probably of wholly lunar derivation. This finding stands in contradiction to a recent inference from Ti-in-quartz modeling (Bellucci et al., 2019) that 14321g crystallized at a pressure of 0.69 GPa. The geodynamically limited Moon was presumably never capable of forming, or burying, such a highly granitic material ~100 km below the base of its crust, nor of excavating material from such a depth to the surface.