Trace Element Characteristics of Minerals in the Apollo 14 High-al Basalts: Implications for an Igneous versus an Impact Origin

Introduction: Previous trace element and isotopic analyses of the Apollo 14 high-Al basalts, suggest that these samples represent the earliest stages of basaltic volcanism on the Moon and that they are not random samples from the lunar regolith [1,2,3,4,5,6,7]. One of the primary goals of this work is to determine if these small, fragmented samples are the result of igneous or impact processes. The chemical information locked within the individual minerals will be used to define the characteristics and determine the potential mechanisms of early lunar volcanism. This abstract reports the results from in-situ mineral analyses of the Apollo 14 high-Al basalts. Scientific Rationale and Analytical Approach: Petrography, mineral compositions, and bulk major element compositions of the Apollo 14 high-Al basalts are similar, yet there is approximately an 8 to 10 fold increase in REE and other incompatible elements [1,2,3,4]. Variation in compatible trace elements (e.g. Ni and Co) for the bulk fragments appears to be minimal [1]. Previous studies have divided these basalts into five groups based on their incompatible element characteristics (REE patterns and incompatible element abundance) [2,3]. Petrogenetic models devised to account for significant variations in incompatible elements range from the simple (fractional crystallization, varying degrees of melting of a late-stage LMO source) to complex (cyclical assimilation of KREEP, melting of a hybrid-ized source) [2,3,4,5,8], to production of melts via impact processes [9]. However, these fragmented Apollo-14 basalts are quite small (< 1 to 3 mm in size) and contain variable amounts of mesostasis, which means that bulk rock analyses of these samples may not truly represent the composition of the basalt [1]. Therefore, analyses of individual olivine and plagio-clase grains were conducted in order to provide a detailed dataset that could be used to compare the fragments. The data summarized here are for samples Prior to trace element analysis, individual phases were imaged, mapped and analyzed using a JEOL 733 superprobe. Trace elements in olivine and plagioclase were measured using the Cameca ims 4f operated on the University of New Mexico campus by the Institute