Volatiles in the Moon: A sulfur and chlorine perspective

Authors: Samantha Jane Faircloth (Open University)
Co-authored by industrial partner: No

Type: Thesis

State: Published (Approved)
Published: July 2020

Abstract: 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.

Journal Keywords: moon; lunar; volatiles; sulfur; chlorine; basaltic magma

Subject Areas: Earth Science, Chemistry

Instruments: I18-Microfocus Spectroscopy