Evaluating the crystallization and eruptive histories of low-titanium basalts with 2D and 3D studies

Abstract

Volcanic products returned from the Apollo missions over 50 years ago provide a unique perspective into the magmatic evolution of the Moon. However, questionsremain regarding the volatile loss, crystallization, and emplacement histories of lunar lavas. To address gaps in our understanding of the eruptive histories of lunar lavas, we investigate phase chemistry and 3D morphologies of low-titanium Apollo 15 basalts belonging to the olivine-normative and quartz-normative suites. We report the 2D and 3D petrography, mineral chemistry, and 3D void space morphologies of 15499, 15555, 15556, and the lesser studied 15495 and 15608 basalts. Quantitative apatite chemistry shows a wide range of apatite volatile compositions and that low-Ti basalt 15495 may contain the most OH-rich compositions measured from the Moon. Analyses of metal grains within the low-Ti basalts have expanded the field of expected Ni and Co metal concentrations for Apollo 15 mare basalts and are used to determine the petrogenesis of two of the studied samples. Coupling 2D chemistry with nondestructive 3D morphologic analyses provides critical insights on the relative timing of volatile exsolution in low-titanium lavas. Through the analysis of vesicles and vugs from X-ray computed tomographic data, we report the first 3D void space volume percentages for a suite of low-Ti basalts and show that these basalts degassed before the onset of mesostasis (e.g., apatite) crystallization. We use calculated cooling rates and 3D morphologic analyses to show that the studied basalts crystallized at various depths in separate lava flows, and 15608 represents the quenched margin of a mare flow. Our work highlights the value of combining 2D and 3D analytical techniques to study the emplacement history of basalts that lack geological context.