Physics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p41a..03s&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P41A-03
Physics
[1026] Geochemistry / Composition Of The Moon, [1041] Geochemistry / Stable Isotope Geochemistry, [1060] Geochemistry / Planetary Geochemistry, [6250] Planetary Sciences: Solar System Objects / Moon
Scientific paper
The chlorine isotope composition of primitive terrestrial basalts and carbonaceous chondrites cover a narrow range centered around 0‰ with a total variation of ± 0.5‰. In contrast, the chlorine isotope composition of bulk samples and in situ ion microprobe analyses of lunar basalts and glasses cover a range of 25‰. Three possibilities were considered to explain the large spread: 1) initial isotopic heterogeneities, 2) devolatilization from solar wind/micrometeorite bombardment, 3) degassing under anhydrous conditions. The first of these possibilities is rejected because the Moon went through an magma ocean stage which would have homogenized any isotopic heterogeneities. To examine surface effects, we chose samples that have extremely different degrees of surface exposure. We find no correlation between the Cl isotope composition and surface exposure. We also conducted a laboratory experiment in which a thin film of NaCl was bombarded with a proton source for 24 hours with no change in Cl isotope composition. The third possibility is that the fractionation is explained by the anhydrous character of the Moon. On Earth, the volatiling Cl species is HCl. HCl is known to preferentially incorporate 37Cl relative to 35Cl due to the high bond strength of the molecule. This is offset by the higher translational velocity of H35Cl, so that overall, there is very little Cl isotope fractionation during degassing. We propose that lunar basalts were anhydrous and the volatile Cl species were metal chlorides, such as ZnCl2, NaCl, FeCl2, etc. The bond strength of metal chlorides and Cl dissolved in a basalt are similar, so that fractionation is caused mainly by volatilization, with the light isotopologue preferentially lost to the vapor phase. This idea is supported by the consistent lower Cl isotope ratios of water soluble salt fraction (~10 ‰ lower) and the lowest lunar Cl isotope values close to those of bulk Earth. The H content of lunar magmas must have been lower than the Cl content or HCl would have been the volatilizing Cl species. The calculated initial Cl concentrations of three undifferentiated lunar basalts are all close to 40 parts per million. Back calculation of primitive mantle H contents are ~ 10 ppb. We suggest that the Moon was anhydrous as initially proposed, and that the high H contents calculated for some lunar samples are atypical. High H contents are found only in anomalous samples with corresponding low Cl contents. Presumably these samples lost HCl during devolatilization leading to an extremely low Cl content and relatively high H content. The fumarolic glass beads are likely the product of igneous processes that resulted in extreme and anomalous volatile enrichment.
Barnes Joshua
McKeegan Kevin D.
Sharp Zachary D.
Shearer Jr. C.
Wang Yadong
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