Metal grains in lunar rocks as indicators of igneous and impact processes

1James M. D. Day
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13544]
1Scripps Institution of Oceanography, University of California San Diego, La Jolla, California, 92093 USA
Published by arrangement with John Wiley & Sons

Anhedral metal grains of >micrometer size occur in many lunar rock types, including mare basalts, magnesian suite rocks (MGS), ferroan anorthosites (FAN), and impact melt rocks and breccias. Some metal grains are inherited from, or modified by, impactors striking the Moon into crustal materials. These grains have high Ni/Co resulting from the addition of chondritic or iron impactors. Metal grains in mare basalts, FAN, and MGS have Ni/Co ranging from >20 to <1, being generally distinct from impactor compositions. Nickel and Co behave as compatible elements in lunar melts, with parental melts having between ~40–50 ppm Co, ~40–60 ppm Ni, and Ni/Co ~1. These compositions suggest a bulk silicate Moon (BSM) with Ni some three times lower than in bulk silicate Earth. Modeling of Ni and Co during fractional crystallization of mafic mare basalt parental melts originating from a BSM source predicts high Ni/Co metals form during early olivine fractionation. The combined effects of pyroxene ± plagioclase crystallization and increasing but variable compatibility of Ni and Co during basaltic melt evolution can explain the generation of low Ni/Co metals in more differentiated mare basalts. High‐Ti mare basalts have metal with low Ni/Co, but the crystallization of ilmenite and armalcoite restricts the range of Ni and Co in metal. Collectively, these results are consistent with metal grains in mare basalts forming solely through endogenous processes. Measurement of metal grains represents a rapid way for determining endogenous (e.g., lunar interior melts) versus exogenous (e.g., impact contamination) processes acting on lunar samples. In turn, the presence of low Ni/Co metal grains in mare basalts supports their origin as uncontaminated partial melts originating from lunar mantle sources that may have experienced loss of Ni to a small lunar core.

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