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Title: Volatiles in the Moon : a sulfur and chlorine perspective
Author: Faircloth, Samantha Jane
ISNI:       0000 0004 9354 8867
Awarding Body: Open University
Current Institution: Open University
Date of Award: 2020
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Sulfur is a key volatile element in magmatic systems that exists in many phases (e.g. melt or gas), in multiple-oxidation states (S2−, S4+ and S6+), and has more than one stable isotope (e.g. 32S and 34S). Therefore, by measuring S, information regarding the conditions of a magma can be acquired. The aim of this work is to investigate what S can tell us about the behaviour of late-stage lunar basaltic magmas. An analytical protocol was developed to simultaneously measure S and Cl abundances and isotopes of lunar apatite in eleven lunar basalts with nano-scale secondary ion mass spectrometry (NanoSIMS). Additionally, a method was developed to measure the oxidation state of S in apatites of five mare basalts with X-ray absorption near-edge structure (XANES) spectroscopy at the S K-edge, making it possible to compare S oxidation state and S isotopes of lunar apatite for the first time. Lunar apatites contain ~20–2,800 ppm S, with δ34S values between -33.3 ± 3.8‰ and +36.4 ± 3.2‰ (2σ). The Cl abundance is ~350–7,230 ppm, with δ37Cl values of +6.5‰ ± 0.9‰ to +36.5‰ ± 1.1‰ (2σ). All of the apatites have S6+/ΣS(tot) ratios of >0, with average S6+/ΣS(tot) values between 0.05 and 0.55. An absence of correlation between S and Cl isotopes suggests a lack of evolutionary relationship between S and KREEP-rich components. The direction of S isotope fractionation, negative or positive, can be explained by degassing of H2S and SO2 from a relatively reduced (S2−) or oxidized (SO42−) late-stage silicate melt, respectively. The historical existence of relatively oxidized late-stage silicate melts is also evidenced by the presence of S6+ in lunar apatite. A positive trend is apparent between S6+/ΣS(tot) and δ34S which is indicative of the dependence of S isotope fractionation on the oxygen fugacity of the late-stage silicate melt.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral