Physics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p41a..01h&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P41A-01
Physics
[5480] Planetary Sciences: Solid Surface Planets / Volcanism, [6063] Planetary Sciences: Comets And Small Bodies / Volcanism, [6250] Planetary Sciences: Solar System Objects / Moon, [8450] Volcanology / Planetary Volcanism
Scientific paper
The presence of magmatic water in lunar volcanic glasses (LVGs) [1] requires a re-evaluation of conventional wisdom that the Moon was thoroughly dehydrated following its formation via giant impact. The LVGs are the most primitive melts erupted on the surface of the Moon, and their post-eruptive degassing and thermal histories are exceedingly simple. The presence of water and chlorine in these magmas indicates the presence of a deep volatile-bearing mantle source within the Moon. New volatile abundance data were obtained for over 200 individual lunar glasses, contained in three samples recovered by the Apollo 15 mission (15426,32; 15426,138 and 15427) with eruption ages of 3.35 to 3.65 Ga; H2O and D/H ratios were measured by SIMS. Yellow-brown volcanic glasses contain the highest concentrations of H2O (up to 70 ppm) which is two times higher than our previous measurements, while green glasses contain smaller amounts of water (4 - 17 ppm H2O). D/H ratios range from +180‰ to +5400‰ and are inversely correlated with water contents. The presence of tritium in lunar samples [2] requires the presence of a cosmogenic component of volatile isotopes from interactions with solar and galactic cosmic rays [3]. After correction for cosmogenic contributions, our data exhibit a systematic negative correlation of δD with water content. The systematic nature of the data correlation, and the heterogeneous H2O concentrations and D/H data, indicate that hydrogen isotopes were fractionated in these lunar magmas by kinetic degassing during eruption. The average δD of the five highest-H2O glasses is +340‰ (+180‰/-240‰); this δD range overlaps the range of carbonaceous chondrites and terrestrial water. Furthermore, it is very likely that the original pre-eruptive δD value of these lunar magmas was significantly lower, and that kinetic D/H fractionation has resulted in preferential loss of H during magmatic degassing. As a result, we conclude that juvenile magmatic water in the lunar interior has a D/H ratio that is indistinguishable from terrestrial water. This study is the first to identify a planetary body with a hydrogen isotope composition that is the same as the Earth, and imply a common origin for the water contained in the interiors of the Earth and Moon. [1] Saal et al. (2008) Nature 454, 192-195. [2] Bochsler et al. (1971) LPSC v. 2, 1803-1812. [3] Merlivat et al. (1976) LPSC v. 7, 649-658.
Hauri Erik H.
Rutherford Malcolm J.
Saal Alberto E.
van Orman James A.
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