Lunar Regolith: Petrology of the <10 μm fraction

Abstract

It has been known for some time that the part of the lunar regolith smaller than 10 micrometers (<10 μm) is chemically distinct from the bulk soil. Specifically, it is enriched in Al, Ca, Na, K, light rare earth elements, and Th, and depleted in Mg, Fe, Mn, and Sc. Three models have been proposed to explain those systematics. The Exotic Component model holds that a fine‐grained, KREEPy exotic component was distributed on a global scale, perhaps by the Imbrian impact. In the Simple Comminution model, the <10 μm chemical systematics are due to differential comminution processes with fine‐grained melt‐rock mesostasis and feldspar concentrating in this fraction. The Soil Mixing model states that the chemical systematics are due to mixing of compositionally distinct soils that have different grain‐size distributions. To determine which model is correct, modal petrographic data were obtained by scanning transmission electron microscopy with energy dispersive X ray analysis for the <10 μm fractions of four Apollo soils: 10084, 14163, 15271, and 64501. Modal results are consistent with chemical data regarding feldspathic and KREEP enrichments in the finest fraction. Soil glass contents, crystalline silica, and plagioclase/mafic mineral ratios are greater in the <10 μm fraction than in the coarser fractions for all soils. Grain‐size distributions are inconsistent with a fine‐grained exotic component derived from distant sources. Comminution of local lithologies, vertical mixing, and local lateral transport control the <10 μm soil composition.