Colloquium: Alberto Saal

Abstract: The general consensus is that the Moon formed and evolved through a single or series of catastrophic heating events in which most of the highly volatile elements, especially hydrogen, were evaporated away. That notion has changed with the new reports showing evidence of indigenous water in lunar volcanic glasses [1], in lunar apatites from mare basalts [2, 3, 4], and lunar melt inclusions [6]. These results represent the best evidence for the presence of a deep source within the Moon relatively rich in volatiles. We compiled volatile data (C, H, F, S, Cl) for more than 360 individual Apollo 14, 15 and 17 lunar glasses and melt inclusions with composition ranging from very-low to high-Ti contents. Furthermore we measure D/H ratios on over 200 single volcanic beads. The glassy volcanic spherules range in size from ~100 µm to ~ 1 mm. We measure the volatile content by SIMS using a Cameca IMS 6F and NanoSIMS at DTM, CIW. Our new SIMS detection limits (~0.13 ppm C; ~0.4 ppm H2O, ~0.05 ppm F, ~0.21 ppm S, ~ 0.04 ppm Cl) represent at least 2 orders of magnitude improvement over previous analytical techniques.

Our data support the hypothesis that there were significant differences in the initial volatile content, and/or the mechanism of degassing and eruption among these glasses was different. Interestingly, a general correlation was found between the volatile enrichment and the incompatible trace elements contents of the lunar glasses. This suggests that degassing did not completely erase the initial difference in volatile contents between the distinct compositional groups of volcanic glasses Our results suggest not only a much wetter Moon’s interior than previously thought [1], but also suggest that the KREEP component, either through shallow assimilation by the melt or deep hybridization of the LMO cumulate, may influence the volatile composition of the erupted glasses. The δD of the highest-H2O lunar glasses after corrections for spallation and kinetic de-gassing suggest that the isotope ratios might not be that different from that of terrestrial MORB.

References: [1] Saal, A. E. et al. (2008) Nature 454, 192.[2]. [2] McCubbin, F.M. et al. (2010) PNAS, 107 (25), 11223. [3] Boyce, J. W. et al. (2010) Nature 466, 466. [4] Greenwood, J. P. et al. (2010) Proc. 41th LPSC Abs 2439. [5] Hauri, E. H. et al. (2006) EPSL 248, 715. [6] Spangler, R. et al. (1984) J. Geophys. Res. 89, 487. Proc. 40th LPSC Abs 2374.

Alberto Saal, Associate Professor,
Geological Sciences, Brown University