HR: 09:12h
AN: DI51B-07    [Abstracts]
TI: Elasticity of Hydrous Olivine Polymorphs: Implications for Seismic Structure of the Transition Zone
AU: Duffy, T S
EM: duffy@princeton.edu
AF: Department of Geosciences, Princeton University, Princeton, NJ 08544, United States
AU: * Mao, Z
EM: zhumao@princeton.edu
AF: Department of Geosciences, Princeton University, Princeton, NJ 08544, United States
AU: Jacobsen, S D
EM: steven@earth.northwestern.edu
AF: Department of Geological Sciences, Northwestern University, Evanston, IL 60208, United States
AU: Jiang, F
EM: fumingj@princeton.edu
AF: Department of Geosciences, Princeton University, Princeton, NJ 08544, United States
AU: Smyth, J R
EM: Smyth@colorado.edu
AF: Department of Geological Sciences, University of Colorado, Boulder, CO 80309, United States
AU: Holl, C M
EM: chrish@earth.northwestern.edu
AF: Department of Geological Sciences, Northwestern University, Evanston, IL 60208, United States
AU: Frost, D J
EM: Dan.Frost@Uni-Bayreuth.DE
AF: Bayerisches Geoinstitut, Universitat Bayreuth, Bayreuth, 95440, Germany
AB: The presence of water in the upper mantle and transition zone has the potential to explain various phenomena such as shear velocity anomalies or uplift and broadening of the 410-km discontinuity. The presence of H₂O in the transition zone has also been frequently invoked to reconcile laboratory elasticity data on olivine polymorphs with seismic data for the amplitude of the 410-km discontinuity (Li et al., 2001; Chambers et al., 2005). Recently, we have measured the single-crystal elastic properties of hydrous olivine (Jacobsen et al., 2006) and a suite of hydrous wadsleyites (Mao et al., 2007a) at ambient conditions and one hydrous wadsleyite composition (0.84 wt% H₂O) up to 12 GPa (Mao et al., 2007b). These data provide new constraints on elastic moduli and their pressure derivatives for hydrous olivine and wadsleyite. Using this data, we first examine the effect of H₂O on bulk sound velocities under transition zone conditions because anelastic effects can be neglected in this case. At 410 km depth (~13.8 GPa, along a 1400°C adiabat), the bulk sound velocity of wadsleyite with 1 wt% H₂O is 3.1% lower than for dry wadsleyite. Comparison of the seismic velocity jump across the 410-km discontinuity with the measured velocity contrast between wadsleyite and olivine provides a means to estimate the olivine abundance at 410-km depth. For mantle wadsleyite with 0.1-0.2 wt% H₂O (Huang et al., 2005) and using experimentally determined olivine- wadsleyite H₂O partition coefficients, the olivine abundance is found to be 40%, much lower than a pyrolite model. In order for a pyrolite composition to satisfy the seismic data, 1.2 wt. % H₂O is needed in wadsleyite- a value greater than its maximum solubility under these conditions. The anomalously steep seismic gradient in the transition zone has been another feature of the region that has long defied explanation. We show that the seismic gradient can be matched if there is a gradient in H₂O concentration across the transition zone such that the H₂O content drops, for example, from 0.3 wt% at 410 km to 0.1 wt% at 520 km dpeth. For compressional and shear wave velocities, 0.1 wt% H₂O in wadsleyite would lead to 0.3% and 0.4% reductions in V_P and V_S, respectively, neglecting any anelasticity. If the water content of wadsleyite was instead 1.0 wt. %, then the corresponding velocity reductions would be 3.3% and 3.6%. Following the work of Karato and Jung (1998), we have implemented a preliminary model accounting for the effect of H₂O on anelasticity. This model indicates that 0.1 wt% H₂O in wadsleyite could be responsible for reductions in shear velocities up to 1.0%.
DE: 1025 Composition of the mantle
DE: 3909 Elasticity and anelasticity
DE: 4465 Phase transitions
DE: 7208 Mantle (1212, 1213, 8124)
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting