Mantle hydrogen isotope composition

H2O in MORE and the upper mantle. Water is not so readily lost by vesiculation, with measurements of unaltered MORE glasses of 0.12% to 0.33 wt.% H2O (Pineau and Javoy, 1994 Jambon, 1994 Sobolev and Chaussidon, 1996). Water behaves incompatibly during melting (see Jambon and Zimmermann, 1990), and so a concentration of 120-330 ppm is obtained for the mantle source region (see also Thompson, 1992). The upper mantle, therefore, contains at least as much water as the hydrosphere. The flux at ridges totals (1-3) X 10 " g The hydrogen isotopic composition is distinctive from that of the surface, with 5D = —l %c to —91%o (Craig and Lupton, 1976 Kyser and O Neil, 1984 Poreda et al, 1986). 

The deuterium to protium (D/H) ratio of the deep mantle may be a remnant of the hydrogen isotopic composition of Earth forming planetesimals. The mass of asteroids and comets incident on Earth since the time of its accretion is estimated to be 4 X 10 -2 X 10 kg. The combined use of water D/H ratios, the lunar cratering record, and terrestrial mantle siderophiles would favor a rather low mass fraction of comets among impacting bodies (<0.01). Asteroids, comets, and the early Earth contributed to 0-0.5, 0-0.1, and 0.5-0.9 of Earth s water inventory, respectively (Dauphas, Robert, and Marty, 2000 [94]). 

Bell D. R. and Ihinger P. D. (2000) The isotopic composition of hydrogen in nominally anhydrous mantle minerals. Geochim. Cosmochim. Acta 64, 2109-2118. 

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