Magmatic evolution of KREEP‐free lunar meteorite Asuka‐881757 inferred from sector‐zoned clinopyroxene, pyroxene symplectites, and thermodynamic modeling

Abstract

AbstractLunar basaltic meteorite Asuka‐881757 (A‐881757), a member of the source crater paired YAMM meteorites (Yamato‐793169, A‐881757, Miller Range 05035 and Meteorite Hills 01210), provides information on potassium‐rare earth element‐phosphorous (KREEP)‐free magmatic sources within the Moon. Asuka‐881757 is an unbrecciated and Fe‐rich (Mg# 36) gabbro with coarse pyroxene (2–8 mm) and plagioclase (1–3 mm). The coarse pyroxene preserves mm‐scale, near‐complete hour‐glass sector zoning with strong Ca and Fe partitioning, similar to some Fe‐rich Apollo basalts. In contrast to the most Mg‐rich Apollo basalts, A‐881757 contains various types of symplectites (~8 vol%) formed by the breakdown of pyroxferroite due to slow cooling, resembling a few extreme Fe‐rich (Mg# 40) Apollo basalts. Petrographic observations and thermodynamic modeling suggest crystallizing in the order: Fe‐poor pyroxenes (Mg# 58–55) → co‐crystallized plagioclase and Fe‐rich pyroxenes (Mg# 49–20) → late‐stage assemblage including Fe‐augite, Fayalite, and Fe‐Ti oxides. Combining phase stability at variable P–T with petrographic observations, the minimum depth of formation of the A‐881757 parent magma can be constrained to between 60 and 100 km. KREEP‐free basalts (such as A‐881757 and the YAMM meteorites) originated from a relatively shallow mantle source and later underwent polybaric crystallization that occurred prior to eruption at the lunar surface. In contrast, the Apollo mare basalts mostly crystallized within lava flows from relatively deeper‐seated mantle sources. The crystallization of A‐881757 and other YAMM meteorites is unlike most Apollo basalts from the Procellarum KREEP terrane, and likely represent hidden cryptomare basalts close to lunar surface.