Lunar monazite: A late-stage (mesostasis) phase in mare basalt

Abstract

Thorium-poor monazite occurs as an inclusion in a ferrohedenbergite grain within a mesostasis area of the relatively coarse-grained Apollo 11 basalt 10047,68. Only a single grain of monazite (∼ 4 × 15 μm) has been observed but it is possible that monazite may exist as a more significant phase at smaller grain sizes. Electron probe analyses indicate chondrite-normalized REE fractionation patterns for both lunar and terrestrial monazites in which the light REE's (La to Sm) are highly enriched relative to the heavy REE's (Gd to Lu). Lunar monazite shows a distinct Eu anomaly which is absent from the terrestrial sample. Crystallization of monazite from late-stage liquids forming during crystallization of lunar igneous rocks could lead to these liquids becoming increasingly depleted in the light REE's relative to the heavy REE's.