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Project Progress

We have synthesized samples of high pressure mineral phases that are likely hosts for H, and thus water, in planetary interiors, and measured physical properties including crystal structure, density, elasticity, and electrical conductivity to see if there is evidence of deep hydration in the Earth. The nominally anhydrous minerals (those without H in the formula) are likely significant host phases for much of the H in and on the Earth and may control the H balance in Terrestrial planets. Nominally hydrous phases may also play a significant role. Nominally anhydrous phases synthesized under hydrous conditions at Bayerisches Geoinstitut (University of Bayreuth, Germany) include olivine, wadsleyite, and ringwoodite (all (Mg,Fe)₂SiO₄), and clino-enstatite, majorite, akimotoite, and silicate perovskite (all (Mg,Fe)SiO₃). Nominally hydrous phases include Phases B and D.

Significant new results in 2011 include synthesis and compressibility of Phase D. Phase D (Mg_1.0Si_1.7H_3.0O₆ ) is the likely host for H, and thus water, in the Earth’s lower mantle and may be stable to depths of 1500 km. Phase D is very hydrous and can contain 15% H₂O by weight (Hushur et al., 2011), but is very incompressible so that the presence of 3000 ppm by weight H₂O in the lower mantle would be difficult to detect seismically. We have observed and documented the epitaxial relations and H distributions among coexisting ringwoodite, ringwoodite, and clinopyroxene (Smyth et al. submitted). We observed the coherent dehydration of hydrous wadsleyite at high temperature and ambient pressure (Ye et al 2011). We measured the thermal expansion of hydrous ringwoodite (Ye at al., 2012).