Kreutzberger, M. E. | Search NASA

NASA NTRS1986

Origin of the earth's moon - Constraints from alkali volatile trace elements

Although the moon is depleted in volatile elements compared to the earth, these depletions are not in accord with simple volatility. For example, the Cs/Rb ratios of the earth and moon inferred from basalt are approximately one seventh and one half of the CI ratio, respectively. Volatility considerations alone predict that the lunar Cs/Rb ratio should be equal to or lower than the terrestrial ratio if the moon was derived entirely from earth mantle material. Thus hypotheses such as rotational fission which invoke derivation of lunar material entirely from the earth's mantle may be excluded. The collisional ejection hypothesis of lunar origin requires at least 18 percent of lunar material to be derived from a projectile with dehydrated CI composition to match the lunar Cs/Rb ratio, and 25-50 percent to match both the lunar Cs/Rb ratio and absolute concentrations of Cs and Rb. It remains to be demonstrated that this relatively large contribution of projectile material is consistent with other elemental abundances and element ratios in the moon.

Kreutzberger, M. E.↗

NASA NTRS1984

Origin of the Moon: Constraints from volatile elements

Supporting arguments for the lunar fusion hypothesis include the low density of the Moon corresponding to the density of the Earth's mantle and the low volatile content of the lunar rocks vs. those of terrestrial origin. Vapor pressures of the alkali elements and their oxides increase in the following order: Na, K, Rb and Cs. The Moon should, therefore, be more depleted in Cs relative to Rb, Rb relative to K, and K relative to Na than the Earth if the fission model is correct. Analyses of lunar mare basalts and terrestrial mid-ocean ridge and other young basalts indicate that this behavior is not observed. It is possible that monovalent alkali elements might be lost from silicate materials in a different order than that inferred from elemental and oxide vapor pressures, as a result of differences in the way they are bound in silicate materials. To test this hypothesis a series of experiments was conducted to investigate alkali loss at high temperatures. Analyses indicate that the behavior of volatiles dissolved in a silicate melt is similar to that inferred from elemental and oxide vapor pressures. It is concluded that alkali element ratios in the Earth and Moon are not readily interpreted in terms of the fission hypothesis.

Kreutzberger, M. E.↗