PSRD: Two Views of the Moon's Composition

Estimates of the chemical composition of the bulk (entire) Moon fall into two drastically different categories. One group of estimates claims that the Moon is enriched in refractory elements (those that boil at high temperatures, such as calcium and aluminum) by about 50% compared to Earth. The other group claims that the abundances of refractory elements are the same in the Earth and Moon. Two papers in the same issue of a geochemistry journal fall into those two categories. Each takes a completely different approach to the problem. In one, John Longhi (Columbia University) focuses on the genesis of basalts in the mantles of the Moon and Earth. He concludes that the total aluminum concentration cannot be higher than it is in the Earth because we should see more lunar basalts as rich in aluminum as are terrestrial basalts. The other paper, by S. Ross Taylor (Australian National University), G. Jeffrey Taylor (University of Hawaii), and L. August Taylor (University of Tennessee) focuses on a mass balance between the lunar crust and mantle and tries to match up the abundances of aluminum and thorium (another refractory element). The Taylors conclude that it is likely that the Moon is enriched by about 50% in refractory elements. The discrepancy among all the studies that have been done stems from a fundamental lack of information about the composition and compositional variations in the lunar crust and mantle.