Interpreting Ilmenite Crystal Size Distributions Derived from CT Images of Basalt Clasts in Previously Unopened Apollo 17 Drive Tube 73002

Abstract:

Station 3 of Apollo 17 was positioned in the light mantle landslide deposit nearly 50m east of Lara Crater in the Taurus-Littrow valley. Among the samples collected from this station was a double-drive tube, which comprises drive tube samples 73002 above 73001. These form part of the Apollo Next Generation Sample Analysis (ANGSA) project. Recently, drive tube 73002 was imaged in its entirety through use of computerized tomography (CT), from which three basalt clasts (,51A, ,51C, and ,80A) were identified. The full CT results will be presented in another abstract in this session

Images of these clasts were used to perform ilmenite crystal size distribution (CSD) analyses, a non-destructive method to determine the crystallization history for igneous samples. The CSD data collected for this study uses a process similar to that in Neal et. al, 2015 (GCA 146, 62-80). Several frames captured from the CT scans were imported into Corel Paintshop Pro 2020. Between 800 - 1800 ilmenite crystals were traced and processed for each clast to determine the size distribution and population density of the crystals in each.

Each basalt clast exhibited a linear CSD, indicative of a constant cooling rate. The striking similarity of ilmenite CSDs for clast 51C and 80A suggest that these could have been derived from the same basalt flow. When compared to previously analyzed A-17 basalt samples, those from 73002 have a relatively faster cooling rate (Fig. 1) and are likely to have been in the chilled upper portion of a lava flow or a thin lobe of a pahoehoe flow (cf. Donohue & Neal, 2015, GCA 149, 115-130). A-17 high-Ti basalts contain more TiO₂ than the A-11 variants (Neal & Taylor, 1992, GCA 56, 2177-2211), but Fig. 1 shows the ilmenite CSDs overlap, indicating cooling rate, not bulk composition, is the primary control on texture.