High-Ti melts from the Taurus-Littrow Valley (TLV). A product of volcanism or impact? An ANGSA investigation using the station 3 double drive tube 73001/73002

The ANGSA initiative examined an unopened Apollo 17 double drive tube that penetrated a “light mantle” surface feature that represents a landslide deposit originating from the South Massif in the Taurus Littrow Valley. Within this double drive tube are several lunar lithologies not identified in the Apollo, Luna, Chang’e 5 or lunar meteorite collections. One such lithology is lithic fragment 73002,27G. It consists of a fine-grained, high-Ti melt lithology which hosts lithic clasts and mineral fragments derived from a variety of high-Ti basalts. This lithology represents either a quickly cooled mare basalt with xenocrysts produced by thermal erosion of older crystalline high-Ti basalts, or a high-Ti impact melt with remnants of the target lithologies. Both types of lithologies are rare to non-existent, so this new sample has the potential to shed light on either the dynamical near-surface interactions between erupting magmas and previously erupted flows, or impact processes involving high-Ti mare basalt targets.

Although not entirely unambiguous, numerous lines of evidence support an impact origin for this rock type. Based on crystal size distribution,  the interclast melt experienced rapid cooling, exceeding rates documented in most high-Ti mare basalts. Mineral and bulk chemistry and preferred orientations of matrix grains indicate that the rapidly-cooled portion of this lithology contains remnants of a variety of mare basalts. It seems unlikely that basalt flow would contain such a variety of xenoliths at its chilled surface. Although the interclast melt falls on an appropriate liquid-line-of-descent, its composition has characteristics distinct from other high-Ti basalts.

Methods
A variety of lithic fragments were separated from the 73001 and 73002 cores during dissection and preliminary examination (PE). These curation activities were done in a nitrogen glove box at the Pristine Lunar Sample Laboratory Facility at the Johnson Space Center (JSC).  In the first two passes of each core, the core was dissected in 0.5 cm sampling intervals, and each segment was sieved into <1 mm and >1 mm size fractions (Fig. 3a,b). The >1 mm lithic fragments were divided into 1-2 mm, 2-4 mm, and >4 mm size fractions (Fig. 3c). Individual >4 mm lithic fragments were triple-sealed in Teflon bags (Fig. 3d), removed from the nitrogen glove box, and scanned by X-ray Computed Tomography in the X-FaCT Lab at NASA JSC with a Nikon XTH 320 equipped with a 180 kV nano-focus transmission target source (Fig. 3e). See Gross et al. (2024) and Zeigler et al. (2024) (both in this special issue but not in the reference list) for detailed descriptions about the PE and XCT scanning, respectively, of 73001 and 73002. The sample identified by XCT imaging and considered in this study was lithic fragment 27G (Fig. 3f). Lithic fragment 27G was recovered from the 1.5-2.0 cm depth interval from the top of drive tube 73002. Its initial sample mass was 0.035g. An XCT video of this lithic fragment is available (presented in supporting information). The XCT data of 27G were used to guide the preparation of a thick section (73002,480) at Johnson Space Center that specifically exposed the two largest high-Ti clasts. Mineral data collected by SEM and EMPA is also available. This data was collected at the University of New Mexico SEM - EPMA lab.