New interpretations of lunar mare basalt flow emplacement from XCT analysis of Apollo samples

Abstract

The study of basaltic samples returned half a century ago through the Apollo missions has provided unparalleled insights into the magmatic processes associated with volcanism on planetary bodies. Traditional study of these samples has been conducted in two dimensions (2D) via interrogation of thin sections and subsequent in-situ chemical analyses, yet returned samples are three-dimensional (3D) objects and therefore preserve evidence of lunar volcanic processes in 3D. Here, X-ray computed tomography (XCT) was used for the first time to evaluate lava emplacement mechanisms on the lunar surface. A total of six samples from the Apollo 11, 12, 15, and 17 missions were studied. From volumetric mineralogies, textures, and petrofabrics, lunar lava cooling histories were inferred. Collectively, these physical characteristics were then correlated with pāhoehoe lava flow lobe stratigraphy. Samples 10057 and 15556 are inferred to have crystallized in the lobe crust of their respective lava flow lobes. Samples 12038, 12043, and 70017 may mark the transition between the vesiculated, fine-grained lobe crust, and the dense, coarse-grained lobe core in their respective flows. Finally, coarse-grained, and non-vesiculated sample 15085 is inferred to have crystallized in a lobe core. No statistically significant petrofabric is preserved in any of the samples, indicating that the basalt samples studied here, and the lava flows they originated from, experienced minimal strain during emplacement and solidification on the lunar surface. This is consistent with the low viscosities attributed to lunar mare lavas. Future in-situ sampling of extraterrestrial basaltic products should focus on detailed documentation of, and collection from, stratigraphically well-characterized lava flows to further evaluate the interpretations presented here.