Water partitioning between coexisting olivine (ol) and wadsleyite (wads) and the effect of the OH-defect concentrations on the P-T-x coordinates of the ol-wads phase transformation
Final Report Abstract
The project focused on the incorporation of hydrogen in three important constituents of the Earth upper mantle: coesite (cs), olivine (ol) and wadsleyite (wad). They belong to the category of Nominally Anhydrous Minerals (NAMs), which may incorporate water as hydrogen in their structure via point defects. Major aim of the work was to 1) analyze the water storage capacity of Mg2SiO4 wadsleyite and the H incorporation mechanisms; 2) determine how water affects the phase transition of olivine to wadsleyite in the system MgO-SiO2- H2O and in the system MgO-FeO-SiO2-H2O; 3) study the coupled boron and hydrogen substitution in coesite and compare it with the hydrogarnet substitution. 1) In the first part we provide a calibration to quantify H in wadsleyite by FTIR spectroscopy and focus on its incorporation mechanism. Hydrous wadsleyite was synthesized at 13.3-13.5 GPa and 1150-1200°C. Raman Spectroscopy and SIMS water quantifications along with the results of FTIR polarized measurements on single oriented crystals give the first absorption coefficient ei,tot of 73000 (± 7000) (L mol-1H2O cm-2) for water in wadsleyite. Typically the oxygen site O1 of the M3 site is protonated via Mg vacancies. The single crystal X-ray refinements of hydrous wadsleyite suggest that the hydration of wadsleyite occurs along the 01•••04 and/or O3•••O4 edges of a vacant M3 octahedron. H is bounded either on two O1 , two O3, or on one O1 and one O3 sites of a vacant M3 site. 2) In the second part we analyzed in which extent H affects the P-T-x coordinates of the 410-km discontinuity. Two sets of experiments were performed one in the system MgO-SiO2±H2O and one in the system MgO-FeO-SiO2±H20 at 13-13.7 GPa and 1025-1300 °C and 11-12.4 GPa and 1200 °C, respectively. In both systems wadsleyite incorporates a much higher amount of water than olivine. In the MgO-SiO2±H2O system the stronger fractionation of water in wadsleyite causes a shift of the olivine-wadsleyite phase boundary to lower P values (0.6 GPa). In the MgO-FeO-SiO2±H2O system H broadens unexpectedly the loops where respectively wadsleyite-ringwoodite (ring) and olivine-ringwoodite coexist. The stability field of hydrous wad widens in both directions, to lower (ol-wad loop) and higher (wad-ring loop) pressures. 3) Coesite can incorporate H via the hydrogarnet substitution, i.e. a vacant Si site with four coordinating OH groups instead of four oxygens for charge balancing and via Al or B based defects. In the latter substitution B3+ (or Al3+) enters the Si site and one of the tetrahedral oxygens forms a hydroxyl group for charge balancing. In this study coesite that stores H only via B based defects were synthesized for the first time at 9-12 GPa and 1000-2000 °C with water in excess. The experimental methods used provide important information on the amount of B and H incorporated in the structure and on B location. All the results lead to the conclusion that the relatively high pressures and temperatures of the experiments tend to favour the B based defect, instead of the hydrogarnet substitution, as the B-based defect is accompanied by a general decrease of the size of the tetrahedral site compared to the hydrogarnet substitution (approximately 20%). Thus, coesite with the B- based defect have a smaller volume than those with the hydrogarnet substitution and their formation is favoured at high pressure.
Publications
- (2009) Breakdown of hydrous ringwoodite to pyroxene and spinelloid at high P and T and oxidizing conditions. Phys Chem Mineral, 36, 329 - 341
Koch-Müller M., Rhede D., Schulz R., Wirth R.
- (2009) Coupled Boron and hydrogen incorporation in coesite. Europ. Journal Mineralogy, 21, 9 - 16
Deon F., Koch-Müller M., Hövelmann J., Rhede D., Thomas S.-M.
