Lunar Agglutinate Glass Composition and Implications for Agglutinate Formation

When micrometeorites (<1 mm interplanetary "dust") strike the surface of the Moon at high speed, the transfer of kinetic energy causes melting. Small aggregates of mineral and lithic clasts are welded together by melt that chills to glass, forming particles called agglutinates. Agglutinates can compose more than 50% of mature lunar soils and are important for understanding space weathering of airless bodies. There are several models for agglutinate formation; most notable is the Fusion of the Finest Fraction (F³) model (Papike, 1981, Proc. Lunar Planet. Sci. Conf. 12, 805-807), according to which the composition of agglutinate glass should approach that of the <10 μm fraction of soil because smaller particles with high surface-to-volume ratios should melt more efficiently.

As part of a study of the effect of agglutinates on soil reflectance spectra, we used electron probe microanalysis (EPMA) to measure the composition of agglutinitic glass in eight regolith samples with different maturities from Apollo 14, 15, 16, and 17 and compared the compositions to data for different grain size fractions of the same soils. We found that agglutinitic glass most closely matches the composition of the bulk soil (<1 mm) in which it formed (see figure). Like the bulk soil, agglutinitic glass is richer in FeO and MgO and depleted in Al₂O₃ and CaO compared to the <10 μm fraction of soil. These findings are contrary to the F³ model; rather, they suggest that melting in micrometeorite impacts does not favor the <10 μm soil grains, and the formation of agglutinate glass is dominated by a process of indiscriminate melting of the target regolith.

Agglutinates are characteristically intricate in form; to study their morphology, we imaged 88 agglutinates using EPMA and/or an optical microscope. We found that agglutinate content is, on average, around 40 vol% mineral/lithic clasts, 40% vesicles, and 20% glass. The proportion of glass is surprisingly low; this is significant because agglutinate glass contains nano- to micro-phase Fe metal which forms during space weathering processes.