Water in the Mantle: Effects of Hydration on Physical Properties of Mantle Minerals

University Of Colorado At Boulder, Boulder CO

Investigators

Abstract

One tenth of one weight percent H2O in the minerals of the Transition Zone of the Earth's mantle (400 to 670 km depth) is an amount roughly equal to 800 m. of liquid water over the surface of the planet. It is indeed possible that the volume of Earth's oceans has not been constant, but has been maintained through geologic time by a dynamic exchange with water stored as hydroxyl in the solid silicate phases of the mantle. The nominally anhydrous silicate minerals that make up the upper mantle and Transition Zone (enstatite, olivine, wadsleyite, and ringwoodite) can incorporate up to several weight percent H2O, and seismic velocities in reference Earth models are consistent with significant hydration (~1 percent by weight H2O) of wadsleyite and ringwoodite in a Transition Zone of pyrolite composition. This is a proposal to investigate the stability, crystal chemistry, and physical properties of hydrous enstatite, olivine, wadsleyite, ringwoodite, and perovskite phases that could occur in a hydrous upper mantle and Transition Zone in order to refine estimates of the water content of the Earth's interior. In collaboration with colleagues at the Bavarian Geological Institute (Bayerisches Geoinstitut, BGI), samples of hydrous olivine, enstatite, wadsleyite, ringwoodite and perovskite with a range of hydrogen contents and Mg/Fe ratios will be synthesized. Samples will be characterized by single-crystal X-ray and neutron diffraction, electron microprobe, IR, Raman, and Mossbauer spectroscopies, and transmission electron microscopy. Isothermal compressibilities will be measured by single-crystal X-ray diffraction in the diamond anvil cell at University of Colorado, and by synchrotron powder diffraction at the Advanced Photon Source. The broader impacts of the proposed research include training of graduate and undergraduate students in laboratory geophysics in a university that is distant from major centers of mineral physics research in the US and introducing these students to experiments at larger facilities such as APS. The proposed research project is complementary to on-going field observational geophysics programs (seismology), geophysical modeling and planetary sciences at the University of Colorado. The project has a strong and on-going international component of collaboration of US students and faculty with scientists at the Bavarian Geological Institute in Bayreuth, Germany, University of Oxford, and ISIS neutron diffraction facility (UK). Domestic collaborations of the proposed study include University of Hawaii and Advanced Photon Source (APS) at Argonne National Laboratory.