Rubidium and cesium in the Earth and the Moon

Abstract

The volatility of the alkali elements makes them potential tracers of high-temperature geochemical processes. Specifically, the abundances of Rb and Cs in the silicate portions of the Earth and the Moon may be used to evaluate theories of lunar origin which involve energetic events and, hence, high temperatures. In particular, theories which involve derivation of the Moon substantially from the Earth's mantle immediately following terrestrial core formation predict that the Moon would have a higher Rb Cs ratio than the silicate portion of the Earth. Determining Rb Cs ratios existing 4.5 Ga ago is complicated by differentiation and alteration processes. In general, ancient terrestrial “basaltic” rocks have lower Rb Cs ratios than modern MORE and OIB. This apparent secular change is plausibly attributed to low-grade metamorphism and exchange of Rb and Cs with the surrounding crust. An acceptable mass balance for the alkalis appears to be provided by the continental crust ( Rb Cs ~ 25 ), MORB mantle ( Rb Cs ~ 80 ), and OIB mantle ( Rb Cs ~ 80 ). Presently, these are the only major reservoirs of Rb and Cs that have been well characterized. Since only 50% of the Rb resides in the continental crust, the silicate portion of the Earth must have a Rb Cs ratio between 25 and 80. We conclude that the bulk Earth has a Rb Cs ratio 1.5× that of the continental crust, ~ 40. Lunar mare basalts have Rb Cs ratios of about 20, whereas lunar terrae rocks have lower Rb Cs ratios. The Moon appears to have a Rb Cs ratio between 15 and 20. The Rb Cs ratio of the Moon is lower than the silicate portion of the Earth, contrary to the prediction of theories which derive the Moon entirely from the Earth. For the giant impact hypothesis to be correct, there have been alkali element contributions to the Moon from other sources (e.g., the impactor). Depending on the Earth's contribution to the mass of the Moon, these additional sources of alkalis must have had a combined Rb Cs ratio comparable to or lower than that of the Moon.