Lunar basalt genesis: the origin of the europium anomaly

Abstract

The trace element abundances of lunar basalts are examined and the implications of the near-chondritic relative abundances of incompatible elements are discussed. It is inferred that in the source regions of lunar basalts the incompatible elements, e.g. Ba, U, RE 3+, are residing in accessory minerals that enter the first liquid quantitatively during the earliest stage of partial melting. On the other hand, the abundance relationships of europium in lunar basalts imply that it is present in different phases, and probably occurs as a compatible or dispersed element within principal silicate mineral(s) of the source region. It is shown that with this distribution of trace elements in the source region, and under appropriate conditions of partial melting, most of the europium may be retained within the principal host mineral(s) in the source region, and the observed trace element abudance patterns in lunar basalts, including the europium anomaly, may thereby be explained. Specifically, it is demonstrated that the observed abundance patterns are consistent with varying degrees of single stage partial melting of a pyroxenite source material possessing near-chondritic absolute abundances of incompatible trace elements. Relations between partial melting processes in the earth's mantle and the lunar interior are discussed. The model of partial melting developed herein provides a possible explanation for the observed Rb/Sr whole rock ages of the mare basalts and for the highly radiogenic strontium often observed in terrestrial alpine ultramafic rocks.