FORMATION OF SYMPLECTITES BY LIQUID-SOLID AND SOLID-SOLID REACTIONS DURING COOLING OF AN FEO-ENRICHED MAGMA ON THE MOON

Northwest Africa 773 (NWA 773) is a lunar meteorite composed of an olivine cumulate lithology and a heterolithic breccia. Many fragments in the breccia are plausibly linked with the cumulate and appear to have formed by Fe-enrichment during igneous differentiation of a single magmatic system (Fagan et al., 2003, Meteoritics and Planetary Science v. 38, 529-554). Accepting this interpretation, lithic and mineral clasts in this single sample preserve a record of igneous differentiation on the Moon. Lithic clasts in the breccia with hedenbergite, fayalite and silica or a silica-rich phase formed from a late stage liquid with extremely enriched FeO/(FeO+MgO). In many clasts, these minerals are intricately intergrown, forming symplectite textures. Most of the symplectites are characterized by uniform distributions of hedenbergite, fayalite and silica—all having curved, finely intergrown crystal boundaries, such that individual “grains” appear to be no more than 2 or 3 µm across. Hedenbergite-fayalite-silica-symplectites have been reported from lunar and martian meteorites and are frequently, although not universally, interpreted as the breakdown products of metastable pyroxferroite (Ca1/7Fe6/7SiO3). The solid state breakdown of pyroxferroite is a plausible origin for the fine-grained symplectites in NWA 773; however, a slightly coarser grained symplectite-like texture apparently formed by direct interactions with late-stage liquid. In this texture, fayalite, which may be in direct contact with fine-grained symplectite, hosts multiple amoeboid SiO2-rich inclusions, resulting in a symplectite-like appearance. Several of the inclusions are wide enough for electron microprobe analyses and show significant deviations from silica stoichiometry (Al2O3 ~ 12 wt.%; K2O ~ 9 wt.%). Some inclusions contain silica, as well as the non-stoichiometric phase. These amoeboid symplectite-like inclusions are interpreted as quenched melt trapped by fayalite that crystallized directly from FeO-rich silicate liquid. The silica within the inclusions also crystallized directly from the melt. This symplectite-like texture did not form by pyroxferroite breakdown, but rather from direct crystallization and quenching of silicate liquid.