The lunar regolith: Chemistry and petrology of Luna 24 grain size fractions

Chemical data are reported for the first time for lunar soil size fractions smaller then 2 μm. We report chemical data for 30 elements by INAA in eight size fractions (370?200, 200?94, 94?74, 74?40, 40?10, 10?5, 5?2 and <2 μm) and petrology of five size fractions (down to 40?10 μm) in two Luna 24 soils, 24176 and 24214. Consistent with our previous results for lunar soils, the compositions of coarser fractions (>10 μm) are quite similar to each other but quite different from the fine fractions (<10 μm). The finer fractions (10–5, 5–2, <2 μm) become increasingly feldspathic and enriched in large-ion lithophile elements (LILE) with decreasing grain size. Chemical data for the finer fractions provide direct evidence in favor of efficient comminution of rock mesostasis and feldspar leading to their preferential incorporation into the finer fractions. High concentrations of meteoritic indicator elements (Ni, Au, Ir) in the finer fractions are consistent with the comminution process by micrometeorite impacts. The chemical data strongly support the F³ (fusion of the finest fraction) model for agglutinate formation.Based on grain size distribution, petrology, and LILE patterns of size fractions, the Luna 24 soils are less reworked than most lunar soils. The Luna 24 regolith appears to have formed as a result of mixing more mature and fine grained material with less mature coarse material in different proportions at different depth intervals.