Modal Petrology and Mineral Chemistry of the <1 mm Size Fraction of Lunar Regolith in Apollo 17 Drive Tube Section 73002

Abstract

The Apollo 17 mission returned samples from the Taurus‐Littrow Valley of the Moon. Key features of the site are a basaltic valley floor partially enclosed by South Massif and North Massif mountains and the Sculptured Hills, which consist of feldspar‐rich highland lithologies. A recently opened soil core sampled an inferred landslide deposit at the base of South Massif. Study of a suite of polished grain mounts of six size fractions of <1 mm material from 14 depth intervals of the ∼18 cm soil column shows that all size fractions from the upper 5–6 cm are richer in agglutinates, an indicator of surface exposure, than the deeper material, which is among the most agglutinate‐poor Apollo 17 regolith reported. Regolith breccia, crystalline melt breccia, and noritic igneous rocks are the most abundant fragment types. Nonmare rock types found range from anorthosites to ultramafic rocks and include rare types such as dunites, pyroxenites, and granitic fragments rich in potassium feldspar and silica. Mare basalt, glasses, and agglutinates are minor components throughout. The observations are consistent with the deposition of the highland‐dominated material in a landslide, which did not preserve any previous stratigraphy, followed by in situ maturation, with only minor additions of mare lithologies from nearby basaltic regolith, due to the inefficiency of lateral transport on the Moon.

Plain Language Summary

We examined samples of lunar soil from 14 depth intervals of a recently opened ∼18 cm core from the Apollo 17 landing site. It is thought to have sampled a landslide deposit at the base of a mountain. Samples were sieved into six size fractions, mounted, polished, and studied using a scanning electron microscope. The soils are mixtures of igneous and impact‐generated rocks, mineral and glass fragments, and lithified and impact glass‐fused soils. The upper 5–6 cm of the soil column are richer in the latter than the deeper material due to surface exposure. Rock types found range from anorthosites to ultramafic rocks plus basalts and include rare types such as dunites, pyroxenites, and granitic fragments rich in potassium feldspar and quartz. Overall, the core is less mature (fresher) and more uniform in composition than most other lunar soil cores, consistent with deposition in a chaotic landslide, as opposed to episodic deposition of various layers.

Key Points

• Drive tube 73002 sampled a lunar landslide deposit. The core is more uniform and less mature than most other lunar soil cores

• Impact melt rocks and feldspathic crustal rocks are the most abundant lithic fragment types, with mare basalts in minor abundance throughout

• Rare rock types such as granites and dunites are also found