Volatile and siderophile element geochemistry of the Moon: a reappraisal

Abstract

Lunar volcanic glasses (LVG) contain high abundances of many volatile elements as compared to mare basalts. Whereas the latter were derived from devolatilized source regions in the outer few hundred kilometers of the Moon, LVG are believed to come from a globally extensive source region at greater depths, which is relatively rich in volatile components. Abundances of volatile elements in LVG and their source regions are reviewed and in many cases found to resemble those in the Earth's mantle. Likewise, siderophile element abundances (including the highly siderophile elements Ir, Os, Au and Re) are similar in the Earth's mantle and in the source regions of LVG. The close geochemical similarity between the LVG source region, (i.e., the lunar lower mantle) and the terrestrial mantle constitutes strong evidence that protolunar material was derived primarily from the Earth's mantle. Since highly siderophile elements are believed to have been added to the Earth as an oxidised veneer at the end of its accretion, and then convectively mixed into the mantle, the removal of protolunar material from the Earth must have occurred after this late addition which may represent carbonaceous chondritic or cometary material from the outer solar system. It is widely believed that the Moon was formed by the impact of a giant planetesimal on the protoearth. The planetesimal is presumed to have formed in the inner solar system and to have differentiated internally into a mantle and core. Numerical modelling of the giant impact shows that most of the material now in the Moon would have been derived from the mantle of the impactor. However, this is contradicted by the geochemical evidence provided by LVG which implies that the Moon is composed mainly of material derived from the terrestrial mantle. A possible answer to this dilemma is provided by the observation that the LVG source region reflects the geochemistry of the “late, oxidised veneer” which accreted on the Earth. This suggests that protolunar material may have been removed from the Earth's mantle by impacts from one or more high-velocity (∼ 30 km/s) planetesimals composed mainly of ices, and derived from the outer solar system. The volatiles from the impacting planetesimals were lost after these collisions. The impacts probably ejected material from the Earth's mantle as jets of molten matter which was rapidly quenched, thereby retaining a large proportion of its terrestrial volatile inventory.