Presentation on theme: "Single-crystal elasticity of hydrous wadsleyite and implication for the Earth’s transition zone Zhu Mao 1, Steven D. Jacobsen 1, Fuming Jiang 1, Joseph."— Presentation transcript:
Single-crystal elasticity of hydrous wadsleyite and implication for the Earth’s transition zone Zhu Mao 1, Steven D. Jacobsen 1, Fuming Jiang 1, Joseph R. Smyth 3, Christopher Holl 2, Daniel J. Frost 4 Thomas Duffy 1 1 Princeton University, Department of Geosciences, Princeton, NJ, 08540 2 Northwestern University, Department of Geological Sciences, Evanston, IL 60208 3 University of Colorado, Department of Geological Sciences, Boulder, CO 80309 4 Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Germany
2 Estimate of water content in the mantle MORB source from 0.005 to 0.02 wt% (e.g. Saal et al.2002) OIB source from 0.03 to 0.1 wt% (e.g. Dixon et al.2002) Shear wave velocity anomaly 0.3 wt% from 300-500 km in central and eastern Europe (Nolet and Zielhuis 1994) Width of 410-km discontinuity 0.02-0.07 wt% (Wood 1995; van de Meijde et al., 2003) Transition zone electrical conductivity 0.1-0.2 wt% (Huang et al., 2005)
3 Model for global water circulation Ohtani 2005
4 transition zone Li et al., 2001 Fei and Bertka 1999 ri
5 Wadsleyite ( -Mg 2 SiO 4 ) Has the greatest hydrogen storage capacity among the olivine polymorphs (e. g., Smyth 1987). Including ringwoodite, transition zone could contain a large water reservoir perhaps exceeding the mass of the hydrosphere (Smyth et al., 2006). Small amounts of water can strongly influence physical properties of mantle minerals (e. g., Wood 1995). The bulk modulus of hydrous wadsleyite was studied by static compression studies (Yusa and Inoue 1997; Smyth et al., 2005; Holl 2006). Pressure derivatives of bulk and shear moduli are needed to extrapolate elastic moduli to high pressures.
6 1.Ambient conditions measurements: 0.37 wt%, 0.84 wt% and 1.67 wt% H 2 O content 2.High-pressure measurements to 12 GPa: 0.84 wt% H 2 O content Single-crystal elasticity measurements for hydrous wadsleyite by Brillouin scattering:
7 Example of Brillouin spectrum at ambient conditions for sample containing 0.84 wt% H 2 O. 1. Ambient conditions measurements
8 RMS: 49 m/s 0.84 wt% water: Measured compressional and shear wave velocities as a function of direction by Brillouin scattering.
9 Single-crystal elastic moduli of Mg 2 SiO 4 hydrous wadsleyite as a function of water content. Elasticity of anhydrous wadsleyite: Sawamoto et al., 1984 Zha et al., 1997
11 V P and V S calculated for three olivine polymorphs at ambient conditions Sawamoto et al., 1984; Zha et al., 1996 Inoue et al., 1998; Jackson et al., 2000 Li et al., 2003; Wang et al., 2003; Sinogeikin et al., 2003 See also: Fe-bearing hydrous ringwoodite Jacobsen et al., 2004.
12 2. High-Pressure measurements Photo of hydrous wadsleyite crystal at 12 GPa ruby
13 Single-crystal elasticity of wadsleyite with 0.84 wt% H 2 O as a function of pressure
14 Aggregate bulk and shear moduli as a function of pressure Anhydrous wadsleyite (Zha et al., 1997) Wadsleyite with 0.84 wt% H 2 O (this study)
15 Application to the Earth’s mantle 1.Velocity jump at 410-km depth 2.Velocity gradient in the transition zone
16 Hydrous olivine Hydrous wadsleyite (GPa/K) -0.0164(5) -0.0175(3) 4.2(2) 4.2(1) Table. Thermal elastic parameters Liu et al., 2005; Mayama et al., 2004;Zha et al., 1996;Inoue et al., 2004
24 Litasov and Ohtani 2003 Demouchy et al. 2005 ~0.9 wt% ~0.3 wt%
25 AK 135 : 0.24 wt% njpb: 0.4 wt% PA5: 0.7 wt% TNA-GCA: 0.5 wt% SNA-S25: 0.8 wt% Estimates of the reduction of water content: Grand and Helmberger 1984; Walck 1984; Lefevre and Helmberger 1989 Kennett et al., 1994; Kennett et al., 1995; Gaherty et al., 1996
26 AK 135 : 0.25 wt% njpb: 0.4 wt% PA5: 0.7 wt% TNA-GCA: 0.5 wt% SNA-S25: 0.8 wt% Estimates of the reduction of water content: Grand and Helmberger 1984; Walck 1984; Lefevre and Helmberger 1989 Kennett et al., 1994; Kennett et al., 1995; Gaherty et al., 1996
27 Conclusions Aggregate bulk and shear moduli of hydrous wadsleyite vary linearly as function of water content: For iron-free olivine polymorphs, water has a greater (or similar) effect on the elasticity of wadsleyite than the other two polymorphs. The high pressure measurements of hydrous wadsleyite show pressure derivative of bulk modulus, K S ’ and shear modulus, G’ is similar to its anhydrous phase: K S ’ = 4.2(1), G’ = 1.4(1).
28 Conclusions For a pyrolite upper mantle (60 vol% olivine), 0.8 wt% H 2 O in wadsleyite is required to match the velocity contrast given by seismic model AK135. Transition zone seismic velocity gradient (AK135) can be matched by ~0.3 wt% H 2 O reduction in wadsleyite from 410 to 520 km. Regional model needs more water reduction to match the velocity gradient than AK135.
36 Elasticity of hydrous olivine Jacobsen et al., 2006. Elasticity of hydrous ringwoodite Inoue et al., 1998 and Wang et al., 2003 Assuming a linear relationship between aggregate moduli and water content for three olivine polymorphs. Effect of water on the elasticity of iron free olivine polymorphs Thus, water has a greater (or similar) effect on the elastic moduli of wadsleyite than the other two polymorphs.