Chemical Evidence for Lunar Melting and Differentiation

Abstract

AN important chemical characteristic of lunar samples is the high concentration of refractory elements, coupled with depletion of volatile elements in comparison with primitive solar nebula element abundances1,2. The concentration of many refractory elements (such as rare earths) in lunar samples frequently equals or exceeds those in terrestrial crustal samples. It is clear that such high abundances are not typical of the whole Moon. For example, the contents of the heat-producing elements, K, U and Th in the surficial rocks are sufficient to generate the observed heat flow from a thin outer zone. If such concentrations persisted at depth, the Moon would be molten. Thus the concept of an outer zone of the Moon enriched in refactory elements has gained general acceptance. Two basic models have been proposed to account for these observations: (a) melting and differentiation of the outer layers, or of the whole Moon, to provide both a lunar crust enriched in “incompatible” elements, and an alumina rich highland crust3; (b) primary accretion of an outer layer enriched in refractory elements (heterogeneous accretion model)4,5.