Petrology of soils from the landslide deposit at Taurus-Littrow: Context for the ANGSA double drive tube 73001/73002 from Apollo 17

Abstract:

The Apollo Next Generation Sample Analysis (ANGSA) initiative involves the study of previously unopened or uniquely processed lunar samples returned by Apollo missions. Among these samples is the Apollo 17 double drive tube 73001/73002, collected at Station 3 in the landslide deposit (“light mantle”, LM) at the base of South Massif.

Grain mounts from 73001/73002 are not yet available but to improve our understanding of its geologic context and the geology of the Apollo 17 site we studied four surface soils collected during lunar roving vehicle (LRV) traverses. Two (72141 and 74121, from stations LRV-2 and LRV-6, resp.) are from the LM and two (72150 and 75111, from LRV-3 and LRV-7, resp.) are from the mare floor immediately adjacent to the LM. A polished grain mount of the 90-150 µm fraction of each soil was studied with a scanning electron microscope and electron probe. Until now only modal, maturity, and bulk chemical data were available for these samples.

We found that 72141 surprisingly lacks the Mg-rich, Ca-poor pyroxenes found in the other soils considered here. Most of the plagioclase grains in the LM soils are more calcic than An₈₉ and are probably derived from highland rocks. Both LM soils have olivine more forsteritic than Fo₉₀, suggesting a troctolitic source. One grain of pink spinel was found in 74121, also indicative of a plutonic component. Pink spinel lithologies have also been found at other locations in the LM (Stations 2 and 3). The pyroxene populations in the valley floor soils extend to more Fe-rich compositions than those from the LM, owing to their larger mare basalt components, and are comparable to that of Sculptured Hills soil 78221. The minimum highland fraction of pyroxene, on the basis of quadrilateral components, is very low in all soils studied. Orange glass beads with compositions indistinguishable from that of 74220 were found in all samples except 74121.

In addition to the glasses and monomineralic grains the samples contain fragments of high-Ti mare basalts and various non-mare lithologies such as recrystallized noritic breccias, crystalline matrix breccias, and feldspathic plutonic rocks. The lithic fragment populations are typical of Apollo 17 regolith. These samples and those from the Station 3 trench will provide a valuable basis for comparison with the new samples from the 73001/73002 drive tube.