Joseph R. Smyth1, C. M. Holl1, D. J. Frost2, and S. D. Jacobsen2
1University of Colorado, Boulder;
2Bayerisches Geoinstitut, Bayreuth
joseph.smyth@colorado.edu
The oceans cover more that 70% of the surface area but compose only 0.025% of the mass. The nominally anhydrous silicate phases thought to compose the upper 660 km of Earth can incorporate more than ten times this much H. Hydrogen is thus the most poorly constrained compositional variable in the Earth. In order to evaluate the possibility of there being a very large reservoir of H in the interior we have conducted experiments to measure the effects of H on the 410 km discontinuity, to synthesize hydrous silicates at pressures up to 20 GPa, and to measure the effect of H incorporation on the physical properties of these minerals. We have also measured H contents in natural high-pressure eclogite and peridotite xenoliths. At depths less than 410 km, olivine can incorporate 2000 ppm or more H2O by weight at pressures above 10 GPa. Clinopyroxene can incorporate a similar amount of H as hydroxyl, and petrologic evidence in natural high-pressure samples suggests it may incorporate 5000 ppm or more. Measurements indicate that garnet is probably not a significant carrier of H in the mantle. In the subducting basaltic slab after all hydrous phases break down, the pyroxene can carry down 0.1 to 0.2 H2O by weight of the slab. If the ultramafic portion below the crustal portion becomes hydrated, the olivine can carry as much or more water into the interior. At 410 km, the presence of H will shallow the transition and broaden the phase transition interval, but in regions of little vertical movement across the discontinuity, H diffusion may act to sharpen the boundary. In the Transition Zone (TZ) the silicate spinel and spinelloid phases can incorporate 2 to 3.5 percent H2O by weight, but H incorporation strongly reduces bulk modulus and P and S velocities in this region. Observed seismic velocities in this region are consistent with significant hydration of these phases. In regions distant from subduction zones, hydration has a much larger effect on velocity than does temperature within reasonable ranges of these parameters. In tomographic images of the TZ in regions distant from active subduction, red is more likely to mean 'wet' than it is to mean 'hot'. At 660 km, the presence of H should deepen and broaden the transition of ringwoodite to perovskite plus ferropericlase. In summary, a hydrous Transition Zone is likely to be thick and slow, whereas a dry TZ will be thin and fast. A hot, dry Transition Zone will be thin and slow, whereas a cool, dry TZ will be thick and fast.