Two Slowly Cooled and Chemically Diverse Basalt Clasts Identified in ANGSA Core 73001

Abstract

The Apollo 17 mission returned a double drive tube, 73001/73002, which was opened after ∼50 years and made available to the community through the Apollo Next Generation Sample Analysis (ANGSA) program. Here, we investigate the petrogenesis of two basalt clasts separated from the 73001 core, samples 73001,1095B and 73001,1234B. During the preliminary examination by NASA Curation using X-ray computed tomography, these clasts were categorized as high-Ti and low-Ti basalt clasts, respectively. Our subsequent investigation confirms a high-Ti affinity for 73001,1095B, but very low-Ti (VLT) affinity for 73001,1234B. We compare 73001,1095B to other high-Ti samples and show that the basalt represents a Type1B basalt. Based on the sample's low vesicularity in 3D, coarse-grained mineralogy in 2D and 3D, and complex exsolution features, this high-Ti basalt likely cooled slowly in the thermally insulated core of a lava flow. As VLT basalts are rare or undersampled in the lunar sample collection, the investigation of 73001,1234B with 3D techniques represents the first of its kind and offers an opportunity to more robustly understand the volcanic histories of VLT lavas. We observed low vesicularity in 3D, coarse-grained mineralogy, and coarse pyroxene exsolution in the VLT basalt in agreement with previous studies suggesting that VLT basalts likely represent samples from the slowest cooled lava flows on the lunar surface, or from a shallow intrusive body. We show from the combination of 2D and 3D studies applied to small returned samples lacking geologic context that a comprehensive interpretation of their crystallization histories can be obtained.