MARE BASALT PETROGENESIS REVISITED: Rb/Sr, Sm/Nd and Lu/Hf FRACTIONATION FACTORS, MANTLE SOURCE REGIONS AND CRUSTAL CONTAMINATION

Introduction: Many questions regarding the petrogenesis of mare basalts remain unresolved. Trace element and isotope geochemists tend to view the range of mare basalt compositional groups as independent primary magmas and develop mantle melting models to explain the compositional diversity [e.g. 1, 2, 3]. On the other hand, petrologists view the mare basalt suites as non-primary and suggest that no primary magmas have been sampled by the Apollo missions or lunar meteorites [4]. Since isotopic ratios (Sr, Nd, Hf, Pb) are not fractionated by magmatic processes, these elements can be used to “see” at least part way through the haze imposed by post-melting processes. Isotopic compositions of mare basalts can be used to address aspects of mare basalt petrogenesis. Recent data on early lunar chronology obtained in part from increased precision in ages of events in the first 100 million years of solar system evolution, coupled with new data on Hf-W systematics, permit a more precise calculation of the compositional and isotopic evolution of the Moon and its mantle reservoir. We have calculated the fractionation factors (Rb/Sr, Sm/Nd and Lu/Hf) for mare basalts using the following equation: (equivalent equations are used to calculate f Sm/Nd and f Lu/Hf):