Magmatic Degassing and the Volatile Depletion of the Moon

Abstract:

The detection of highly volatile elements in lunar volcanic glasses and melt inclusions has provided the first definitive evidence for the accretion and retention of these elements in the Moon’s interior^1,2. Measurement of H in lunar apatite, at levels similar to terrestrial apatite, has added weight to this discovery^3,4. These results are at odds with the longest-standing observations that the abundances of highly- and moderately-volatile elements in lunar basalts are as much as 1000 times more depleted than in terrestrial basalts⁵. We will show that most of these apparent contradictions have arisen due to the previously unappreciated importance of a single widespread process, magmatic degassing. Degassing occurs in all eruptions of magma, with the consequent release of volatile elements into an exsolved vapor phase. We use ours and previously published results to evaluate lunar magmatic degassing and to show that A) volatile element contents for the bulk silicate Moon (BSM) are only moderately depleted compared with the bulk silicate Earth (range 0.5-0.1, avg. 0.25 x BSE), B) they essentially overlap the composition of the terrestrial depleted MORB source and C) the volatile depletion pattern for the BSM is largely flat, and so does not correlate with condensation temperature at 10⁻⁴ bars, nor with bond energy for likely ligands. Published high-precision Sr and Pb isotope ratios on well-dated lunar rocks^6-8 reveal ⁸⁷Rb/⁸⁶Sr and ²³⁸U/²⁰⁴Pb ratios of the lunar mantle a factor of 0.3-0.5 and 0.28-0.85 depleted compared to those of the BSE, respectively; lending support to our estimates for the abundances of Rb (0.245 x BSE) and Pb (0.187 x BSE) in the BSM. Before the Moon’s extent of volatile depletion can be confidently attributed to the accretion processes, magmatic degassing must be examined and critically evaluated.

References

[1] Saal et al., 2008. Nature 454, 192. [2] Hauri et al., 2015. FEPS 409, 252. [3] Boyce et al., 2014. Sc. 344, 400. [4] Anand et al., 2014. PTRS A 372. [5] Albarede et al., 2014. MAPS.12331. [6] Carlson et al., 2014. Philos. Trans. R. Soc. A 372. [7] Misawa et al., 1993. GCA 57, 4687. [8] Tera & Wasserburg, 1976. PSCP 7, 858.