In a paper recently published in the journal Icarus, researchers studied the basaltic samples recovered from the Apollo 17, 15, 12, and 11 missions using X-ray computed tomography (XCT) to evaluate the emplacement mechanism of lava on the surface of the moon.
Study: New interpretations of lunar mare basalt flow emplacement from XCT analysis of Apollo samples. Image Credit: Vadim Sadovski/Shutterstock.com
Background
Scientific investigations spanning half a century have resulted in a comprehensive characterization of the earth’s moon, identifying the geochemical and microstructural diversity of the lunar basalts. Although it is well understood that the distinct sources from the lunar mantle generated the lunar magmas and diversified into basalt suits, the lava emplacement dynamics on the lunar surface and the contribution of the vesicle and crystal content to the emplacement mechanisms and flow rheology are still less understood.
Recent studies to map and investigate lunar flows have used remote sensing methods. Since traditional analysis studies involve certain degrees of sample destruction, the study of lunar materials necessitates their studies through non-destructive techniques. One of the best approaches is XCT, which allows sample measurement and assessment in three dimensions (3D), thereby resulting in accurate records.
About the Study
In this study, XCT was used to evaluate the petrofabrics of the lunar basalts and their eruptive environments. 3D observations of the samples were used to quantify the textures and shape preferred orientations and determine the preservation of lineations or foliations. Furthermore, the cooling history of each sample was determined through the quantification of the vesicle and particle size distributions. Collectively, the data from these analyses were used to investigate the flow stratigraphy, as well as the history of emplacement of the lava flow over the lunar surface.
Samples assessed in this study were associated with the Apollo 17 (sample 70017), Apollo 15 (sample 15085), Apollo 12 (sample 12038), and Apollo 11 (sample 10057) missions. All samples mostly consisted of abundant plagioclase feldspar, pyroxene, and minor olivine with accessory phases present in trace amounts. During the scan, the samples were triple bagged in Teflon. The sample textures were assessed by looking along the orthogonal x, y, and z axes since XCT allows the samples to be viewed in multiple directions. Blob3D was used to segment and separate the sample components.
The extracted data was used for the strain analysis by plotting the shape-preferred orientation data on stereographic projections and rose diagrams. Further investigation of the sample fabrics was carried out by the Woodcock and Naylor technique. Moreover, particle size distributions were examined using the calculated particle volumes.
Observations
All the extracted volumetric mineralogies were in line with the previously reported ranges of modal mineralogies. Furthermore, textural observations determined the sample 10057 to be fine-grained with varying vesicle shapes and no crystal orientation in any phase. Sample 12038 with an average grain size of about 0.6 mm was holocrystalline while the vesicles/vugs were irregularly shaped. Sample 12043 was a medium-grained, holocrystalline pigeonite basalt with small vugs defining a diktytaxitic texture, while sample 15085 was the coarsest grained, least vesiculated, and micrograbbroic. Sample 15556 was determined to be fine-grained, holocrystalline, and highly vesiculated, while sample 70017 was medium-grained, holocrystalline, and vesicular.
The XCT characterizations of the sample textures were consistent with the characteristics exhibited by terrestrial pahoehoe lava flows. Furthermore, the abundant, well-rounded vesicles were typical of a cooling environment in a thicker lava flow while the diktytaxitic textures observed in sample 12038 could indicate a transition between the core and the crust of the lava lobe. Additionally, the coarse-grained, less vesiculated sample 15085 was observed to be in line with an insulating and cooling magmatic environment.
The strain evaluation on the rose diagrams for the samples 10057, 15556, 70017, and 15085 showed that neither vesicles nor oxides were strongly lineated or foliated, thus indicating no sufficient strain to impart a petrofabric to these samples. The lack of significant preferred orientations indicated lower viscosity of the lunar magmas. The lower vesiculation of the samples could be due to efficient initial degassing leading to rapid exsolution and removal of volatiles as well as a lower magma volatile content.
Conclusions
To summarize, researchers used XCT analysis on six basaltic samples from the Apollo missions, which determined that the lunar lava flows showed textural consistency with the stratigraphy of terrestrial pahoehoe lava flows, indicating their formation through effusive, low-viscosity flows. This was evident from the lack of strain-induced petrofabrics that were defined by the vesicle or oxide populations.
According to the authors, further work in characterizing lunar volatiles can provide additional understanding of the lava eruption and emplacement mechanisms, in addition to the stratigraphic correlation of the vesicular samples. Moreover, XCT can support investigations involving lava flows on other extra-terrestrial bodies using the constraints on lava characteristics.
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Source
A.J. Gawronska, C.L. McLeod, E.H. Blumenfeld, et al., New interpretations of lunar mare basalt flow emplacement from XCT analysis of Apollo samples, Icarus (2022), https://doi.org/10.1016/j.icarus.2022.11521
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