Hydrous Spinelloids and the Hydration State of the Mantle

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SmythEAR-0229315Hydrogen is perhaps the most poorly constrained compositional variable in the Earth, yet it has a controlling effect on many if not most geologic processes. This is a proposal to improve the understanding of the hydration state of the interior by investigation of the stability, crystal chemistry and equation of state of hydrous olivine, spinel, and spinelloid phases that would occur in and near a hydrous Transition Zone. In collaboration with colleagues at the Bavarian Geological Institute (Bayerisches Geoinstitut), we will perform synthesis experiments on hydrous peridotite compositions at temperatures and pressures corresponding to depths of 300 to 700 km. At the University of Colorado, we will characterize these materials by single crystal X-ray diffraction, electron microprobe and IR spectroscopy. We will use single-crystal X-ray diffraction measurements in the diamond anvil cell (DAC) at the University of Colorado to measure compressibilities (isothermal bulk moduli) and thermal expansion of the hydrous olivine, spinel and spinelloid phases observed in these experiments. With a single year of funding, we expect to be able to complete characterization of a series of hydrous ringwoodites, wadsleyites and olivines and to perform high-pressure compression experiments on several of these samples. Because the bulk modulus is directly related to seismic velocity, if the compressibilities of these phases differ greatly from the anhydrous modifications, it may be possible to place some constraint on the amount of OH actually present in the minerals of the Transition Zone. In addition, we will continue our collaborative studies on measurement of elastic properties by Brillouin spectroscopy and GHz ultrasonic interferometry, and make samples available to other groups.