HR: 11:20h
AN: MR52A-05    [Abstracts]
TI: Single-crystal elasticity of hydrous wadsleyite to 12 GPa
AU: * Mao, Z
EM: zhumao@princeton.edu
AF: Princeton University, Guyot Hall Department of Geosciences, Princeton, NJ 08544, United States
AU: Jacobsen, S D
EM: steven@earth.northwestern.edu
AF: Northwestern University, Department of Geological Sciences, Evanston, IL 60208, United States
AU: Jiang, F
EM: fumingj@princeton.edu
AF: Princeton University, Guyot Hall Department of Geosciences, Princeton, NJ 08544, United States
AU: Smyth, J R
EM: joseph.smyth@colorado.edu
AF: University of Colorado, Department of Geological Sciences, Boulder, CO 80309, United States
AU: Holl, C M
EM: chrish@earth.northwestern.edu
AF: Northwestern University, Department of Geological Sciences, Evanston, IL 60208, United States
AU: Frost, D J
EM: Dan.Frost@Uni-Bayreuth.DE
AF: Universität Bayreuth, Bayerisches Geoinstitut, Bayreuth, DEU 95440,
AU: Duffy, T S
EM: duffy@princeton.edu
AF: Princeton University, Guyot Hall Department of Geosciences, Princeton, NJ 08544, United States
AB: Wadsleyite, β-Mg₂SiO₄, is expected to be the dominant mineral in the Earth's transition zone from 410 to 520 km depth. This mineral has the greatest water storage capacity among the olivine polymorphs (e.g. Smyth et al., 1987; Kohlstedt et al., 1996) and could contain up to 0.9 wt% H₂O under transition zone conditions (Demouchy et al., 2005). Previously, we reported that the elasticity of wadsleyite decreases strongly with increasing water content at ambient conditions (Mao et al., 2007). Pressure derivatives of bulk and shear moduli are needed to extrapolate elastic moduli to high-pressure conditions. Static compression studies suggest that the presence of hydroxyl might increase the pressure derivative of the bulk modulus for wadsleyite (Smyth et al., 2005; Holl et al., 2007). Here, we conducted high-pressure Brillouin measurements to 12 GPa to determine the single-crystal elasticity of wadsleyite with 0.84 wt% H₂O. Three platelets were used in the Brillouin measurements. For each platelet, we collected spectra at 10⁰ steps in a total of 19 directions over a range of 180 degrees. With the exception of C₅₅, the elastic constants, C_ij, of hydrous wadsleyite follow the similar trends as anhydrous wadsleyite with increasing pressure (Zha et al., 1997). Pressure derivatives of the bulk and shear moduli of wadsleyite with 0.84 wt% H₂O are 4.2(1) and 1.4(1) respectively. These values are not significantly different from the corresponding values of anhydrous wadsleyite (e.g. Zha et al., 1997). Thus, the presences of 0.84 wt% H₂O has no detectable effect on the pressure derivatives of the bulk and shear moduli. We calculate the effect of water on the compressional and shear wave velocities for wadsleyite at 410 km (~13.8 GPa, along a 1400°C adiabat). If earth's transition zone is water saturated (0.9 wt% H₂O), this could lead to a 3.0% reduction in compressional wave velocities and a 3.3% reduction in shear wave velocities.
DE: 3630 Experimental mineralogy and petrology
DE: 3900 MINERAL PHYSICS
DE: 3909 Elasticity and anelasticity
DE: 3924 High-pressure behavior
DE: 3934 Optical, infrared, and Raman spectroscopy
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting