Add time:08/31/2019         Source:sciencedirect.com

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 RbCs ratio than the silicate portion of the Earth. Determining RbCs ratios existing 4.5 Ga ago is complicated by differentiation and alteration processes. In general, ancient terrestrial “basaltic” rocks have lower RbCs 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 (RbCs ~ 25), MORB mantle (RbCs ~ 80), and OIB mantle (RbCs ~ 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 RbCs ratio between 25 and 80. We conclude that the bulk Earth has a RbCs ratio 1.5× that of the continental crust, ~ 40. Lunar mare basalts have RbCs ratios of about 20, whereas lunar terrae rocks have lower RbCs ratios. The Moon appears to have a RbCs ratio between 15 and 20. The RbCs 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 RbCs ratio comparable to or lower than that of the Moon.

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