Understanding the textures of Apollo 11 high-Ti mare basalts: A quantitative petrographic approach

1,2,3Zhuqing Xue,3Donald F. Welsh,3Clive R. Neal,4Long Xiao
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13767]
1School of Marine Sciences, University of the Chinese Academy of Sciences, Beijing, 100049 China
2Center of Deep-Sea Research, Institute of Oceanology, Chinese Academy of Sciences, Qingdao, 266071 China
3Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, Notre Dame, Indiana, 46556 USA
4School of Earth Sciences, China University of Geosciences, Wuhan, 430074 China
Published by arrangement with John Wiley & Sons

This paper represents a comprehensive crystal size distribution (CSD) study of ilmenite and plagioclase from 12 Apollo 11 basalts from four of the five compositional groups (Groups A, B1, B2, B3, and one unclassified basalt—Group “U” basalt 10062). Ilmenite was saturated in the magma at/before eruption, resulting in subsurface growth of phenocrysts (Group B1) and many small crystals upon eruption. Plagioclase always exhibits linear CSDs representing a single cooling regime in each sample, which is interpreted as crystallizing within isolated magma pockets late in the cooling of the erupted lava flow. Latent heat of crystallization and insulating effects of crystallized phases produced slower cooling and lower plagioclase nucleation densities. Exceptions are the Group B2 and B3 basalts, indicating relatively earlier crystallization of plagioclase on the lunar surface. Our study demonstrates that textures of the Apollo 11 basalts are a product of the interplay among cooling rate, bulk composition, and nucleation density during crystallization. Group A basalts have the highest cooling rates compared to the other Apollo 11 samples (except 10072,53), and were erupted through high effusion rates producing thick flows that underwent extended cooling that induced textural coarsening in both early crystallizing ilmenite and late-stage plagioclase. Group B1 lavas had the lowest effusion rates producing the thinnest flows. The Groups B2, B3, and U basalts are intermediate between these end members. Our approach can be used to define eruption environment, crystallization sequence, and cooling rate of samples collected on the Moon from non-bedrock sources.

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