Single-crystal elasticity of hydrous wadsleyite and implication for the Earth’s transition zone

1. Single-crystal elasticity of hydrous wadsleyite and implication for the Earth’s transition zone Zhu Mao1, Steven D. Jacobsen1, Fuming Jiang1, Joseph R. Smyth3, Christopher Holl2, Daniel J. Frost4 Thomas Duffy1 1Princeton University, Department of Geosciences, Princeton, NJ, 08540 2Northwestern University, Department of Geological Sciences, Evanston, IL 60208 3University of Colorado, Department of Geological Sciences, Boulder, CO 80309 4Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Germany

2. Estimate of water content in the mantle MORB sourcefrom 0.005 to 0.02 wt% (e.g. Saal et al.2002) OIB sourcefrom 0.03 to 0.1 wt% (e.g. Dixon et al.2002) Shear wave velocity anomaly0.3 wt%from 300-500 km in central and eastern Europe (Nolet and Zielhuis 1994) Width of 410-km discontinuity0.02-0.07 wt% (Wood 1995; van de Meijde et al., 2003) Transition zone electrical conductivity0.1-0.2 wt% (Huang et al., 2005)

3. Model for global water circulation Ohtani 2005

4. Fei and Bertka 1999 ri transition zone Li et al., 2001

5. Wadsleyite ( -Mg2SiO4) • 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. Single-crystal elasticity measurements for hydrous wadsleyite by Brillouin scattering: • Ambient conditions measurements: • 0.37 wt%, 0.84 wt% and 1.67 wt% H2O content • High-pressure measurements to 12 GPa: • 0.84 wt% H2O content

7. 1. Ambient conditions measurements Example of Brillouin spectrum at ambient conditions for sample containing 0.84 wt% H2O.

8. 0.84 wt% water: Measured compressional and shear wave velocities as a function of direction by Brillouin scattering. RMS: 49 m/s

9. Single-crystal elastic moduli of Mg2SiO4 hydrous wadsleyite as a function of water content. Elasticity of anhydrous wadsleyite: Sawamoto et al., 1984 Zha et al., 1997

10. is the water weight percent.

11. VP and VS 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 ruby ruby Photo of hydrous wadsleyite crystal at 12 GPa

13. Single-crystal elasticity of wadsleyite with 0.84 wt% H2O 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% H2O (this study)

15. Application to the Earth’s mantle • Velocity jump at 410-km depth • Velocity gradient in the transition zone

16. Table. Thermal elastic parameters (GPa/K) Hydrous olivine Hydrous wadsleyite -0.0164(5) -0.0175(3) 4.2(2) 4.2(1) Liu et al., 2005; Mayama et al., 2004; Zha et al., 1996; Inoue et al., 2004

18. 1. Velocity jump at 410-km depth Frost and Dolejš 2007

20. Olivine fraction as a function of water in wadsleyite

21. Olivine fraction as a function of water in wadsleyite

22. 2. Velocity gradient in the transition zone Speziale, unpublished

23. Li et al., 2001

24. ~0.3 wt% ~0.9 wt% Litasov and Ohtani 2003 Demouchy et al. 2005

25. Estimates of the reduction of water content: AK135 : 0.24 wt% njpb: 0.4 wt% PA5: 0.7 wt% TNA-GCA: 0.5 wt% SNA-S25: 0.8 wt% Grand and Helmberger 1984; Walck 1984; Lefevre and Helmberger 1989 Kennett et al., 1994; Kennett et al., 1995; Gaherty et al., 1996

26. Estimates of the reduction of water content: AK135 : 0.25 wt% njpb: 0.4 wt% PA5: 0.7 wt% TNA-GCA: 0.5 wt% SNA-S25: 0.8 wt% 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, KS’ and shear modulus, G’ is similar to its anhydrous phase: KS’ = 4.2(1), G’ = 1.4(1).

28. Conclusions • For a pyrolite upper mantle (60 vol% olivine), 0.8 wt% H2O 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% H2O reduction in wadsleyite from 410 to 520 km. Regional model needs more water reduction to match the velocity gradient than AK135.

30. Frost and Dolejš 2007

33. Table. Water storage capacity of olivine polymorphs with pressure and temperature

34. Birch’s plot Effect of composition and structure on the elasticity of olivine (Mg, Fe)2SiO4 polymorphs

35. Effect of hydration

36. Effect of water on the elasticity of iron free olivine polymorphs • 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. Thus, water has a greater (or similar) effect on the elastic moduli of wadsleyite than the other two polymorphs.