Crystal storage and transfer in lunar magmatic systems

Abstract

Mare basalts collected at the Apollo 15 landing site can be classified into two groups, the quartz-normative basalt suite and the olivine-normative basalt suite, based on differences in whole-rock major element chemistry. Two main theories have been proposed for the origin of the chemical difference between the two suites: (1) they are from different sources or degrees and depth of partial melting in the lunar mantle, and (2) they are from the same source but the quartz-normative basalts experienced a considerably different magmatic history. The main objective of this thesis is to use quantitative petrological techniques to investigate the magmatic history of the Apollo 15 mare basalts. The results will be evaluated and discussed with respect to the outstanding petrogentic theories. Crystal size distribution (CSD) analysis is a non-destructive, quantitative method providing insights into the crystallisation histories of magmas. CSD analysis and spatial distribution pattern (SDP) analysis of pyroxene, olivine and plagioclase crystals was performed on fifteen thin sections from eight Apollo 15 mare basalt samples. The CSD and SDP results show similarities between the quartz-normative and olivine-normative suites, especially for pyroxene CSD trends. The pyroxene CSD trends likely represent samples with similar cooling histories from comparable depths within respective olivine-normative and quartz-normative lava flows. Traditional crystal size distribution data collection requires the manual tracing of crystal boundaries, which is often time-intensive. The feasibility of using the Quantitative Evaluation of Minerals by SCANing electron microscopy (QEMSCAN) software for semi-automated crystal size distribution analysis was investigated. The CSD data collected by QEMSCAN, in most cases, shows an offset compared to the manual CSD data for the same samples, despite having broadly similar trends. The source of the discrepancy is the ability of the processors in the QEMSCAN software to correctly separate touching crystals of the same mineral, which is dependent on sample texture. The final method, used to gain insights into the timescales of magmatic processes experienced by the two suites of mare basalts, was diffusion modelling of Fe-Mg in zoned olivine crystals. Diffusion timescales are similar for both suites, with a range of 5 to 24 days for quartz-normative samples, and 6 to 91 days for olivine-normative samples. The short diffusion timescales suggest that the zoning profiles record the final stages of cooling within lava flows at the lunar surface. Using diffusion timescales to constrain cooling rates, we estimate that Apollo 15 lava flows are a minimum of 1.5 to 6 m thick. The results of both CSD analysis and diffusion chronometry, highlight similarities between the magmatic and eruptive histories of the two Apollo 15 mare basalt suites. We found no textural or chemical evidence to suggest the quartz-normative suite experienced a different magmatic history to the olivine-normative suite. Overall, the findings of this thesis are in agreement with mechanisms for the origin of the quartz-normative and olivine-normative suites being due to differences in magma source composition, depth or degree of partial melting. This study demonstrates that non-destructive quantitative petrological analysis of Apollo mare basalt thin sections is a valuable method for understanding lunar magmatic systems.

Date of Award	31 Dec 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Katherine Joy (Supervisor) & Margaret Hartley (Supervisor)