Mare basalt genesis: Modeling trace elements and isotopic ratios

Abstract

A model of the mare basalt source region and mare basalt genesis has been developed in which the primary magmas were produced by the partial melting (30±2%) of shallow (≲200 km deep), olivine‐dominated (73% to 80% olivine) source regions. They then assimilated small amounts of urKREEP residuals as they cooled and underwent varying degrees of olivine±pyroxene fractionation in shallow magma chambers [Binder, 1982]. This model was based on the mineralogical, major oxide, Ni, Co, Ba, and REE data on the basaltic units and a set of distribution equations developed to calculate the trace element evolution in the moon as it crystallized, in the crust and mare basalt source region as they simultaneously formed, and in urKREEP as it formed and evolved. This model has now been extended by (1) including the data for K, Rb, and Sr, as well as the isotopic data for the Rb‐Sr and Sm‐Nd systems in the modeling, (2) considering the effects of melt trapped during the formation of the source regions, and (3) by the development of an additional distribution equation that allows the evolution of the trace elements to be calculated in the proposed sequence where crystals are both partially remelting in, and being deposited from, a convecting magma system. The results of this study show that the proposed model does accurately account for the compatible siderophile trace element (Ni and Co), incompatible trace element (K, Rb, Sr, Ba, and the REE), and isotopic (Rb‐Sr and Sm‐Nd systems) characteristics of the mare basalts and the pyroclastic glass units.