Impact-melt Rock Populations in Apollo Double-drive Tubes

The lunar regolith is the boundary between the surface of the Moon and the dynamic space environment. As a result, the regolith preserves an archive of processes that have modified the surface of the Moon over the last ~4 billion years. Of particular interest to us is the impact history of the Moon, preserved as clasts of impact-melted materials created in impact crater events. These clasts date the creation of the impact crater and build a statistical view of the lunar bombardment history. However, the repeated burial, exhumation and transportation (i.e., 'gardening') of the regolith plays a major role in redistributing impact-melt samples. The gardening process may affect the location and preservation of impact-melt rocks in the regolith, which could bias the age distribution of samples collected at the surface. For example, many older (e.g., >3.9 Ga) impact-melt rocks experienced much longer residency in the regolith, potentially being diluted by subsequently-deposited material and making older rocks volumetrically rare at the surface. The opposite may be true for younger impact-melt rocks, particularly if the samples were collected near small, young craters.

Double-drive tubes present an opportunity to investigate the age distribution of populations of impact-melt rocks with depth in the regolith and to evaluate the effect of gardening. We have begun this work on Apollo sample 68001/68002. We separated 50-70 particles (250+ μm size fraction) from six compositionally distinct horizons extending to ~70 cm below the surface, including particles from the immediate surface. We grouped the particles from each horizon based on texture, major element, and trace-element, and determined the parent lithology for each group. We will measure the crystallization age (Ar-Ar dating), maturity and cosmic-ray exposure age to understand the regolith history of each particle during its lifetime in the lunar regolith.

We will apply these techniques to particles from unopened and unstudied Apollo double-drive tube 73001 as part of the Apollo Next Generation Sample Analysis (ANGSA) Program. Determining the regolith history for these two double-drive tubes will allow direct comparison of the parentage of impact-melt rocks and their exposure histories at the Apollo 16 and Apollo 17 landing site.