Comparative study of Li, Na, K, Rb, Cs, Ca, Sr and Ba abundances in achondrites and in Apollo 11 lunar samples

Abstract

This study provides analytically precise abundance data for Li, Na, K, Rb, Cs, Ca, Sr and Ba in five lunar rocks, a breccia and the lunar soil. For comparison, analyses have also been made for eight eucrites, Angra dos Reis, and a terrestrial basalt. The chemical compositions of lunar rocks fall into two distinct groups which can be clearly distinguished by their K concentrations. A parallel grouping is also found for concentrations of Li, Rb, Cs and Ba which is the same as that observed for K. These elements plus many others can be shown to reside in interstitial phases. Gross abundance features, e.g. the Ba enhancement relative to carbonaceous chondrites, reflect the composition of the lunar crust. It is not possible to precisely form the soil by a mixture of the two rock types. A component with high Rb, Cs and Ba relative to K is required. Breccias form a well-defined chemical group whose composition can be described as a mixture of soil and high K rocks. There is no evidence for differential volatilization of alkalis between the rocks and the soil. Compared to carbonaceous chondrites, Ca, Sr, Ba and Li are strongly enhanced and Rb and Cs are systematically depleted relative to K in a similar manner in both lunar samples and achondrites. Similar abundance patterns are found in terrestrial basalts. The concentrations of K, Rb, Cs and Ba in the lunar samples are distinctly higher than the achondrites. The Na (and Ca) concentrations of achondrites and lunar samples are essentially equal. We conclude that the composition of the moon as a whole is characterized by relative abundances for these elements that are distinct from those found in chondrites.

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