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Effects of hydration on the structure and compressibility of wadsleyite

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American Mineralogist, Volume 93, pages 598–607, 20080003-004X/08/0004–598$05.00/DOI: 10.2138/am.2008.2620 598 Effects of hydration on the structure and compressibility of wadsleyite, β-(Mg2SiO4)Ch r i s t o p h e r M. ho l l ,1,* Jo s e p h r. sM y t h ,1 st e v e n D. Ja C o b s e n ,2 a n D Da n i e l J. Fr o s t31Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309, U.S.A.2Department of Earth and Planetary Sciences, Northwestern University, Evanston, Illinois 60208, U.S.A.3Bayerisches Geoinstitut, Universität Bayreuth, Bayreuth 95440, Germanyab s t r a C tA suite of pure magnesian wadsleyite (β-Mg2SiO4) samples containing 0.005, 0.38, 1.18, and 1.66 wt% H2O was studied by single-crystal X-ray diffraction to determine the effects of hydration on cation ordering and crystal symmetry. Separate compressibility experiments were carried out to 9.6 GPa to determine the effects of hydration on isothermal equations of state. Crystal-structure refinements at ambient conditions show cation vacancies order onto the M3 site. The most hydrous sample (1.6 wt% H2O) displayed monoclinic symmetry with β = 90.090(7)°, whereas the samples with lower content were statistically orthorhombic. The density of wadsleyite decreases with increasing water content at STP according to the empirical relation, ρ = 3.470(2) – 0.046(2) CH2O g/cm3 (with CH2O in wt% H2O). Bulk moduli and pressure derivatives of wadsleyite are KT0 = 173(5) GPa, K0' = 4.1(15) for 0.005 wt% H2O; KT0 = 161(4) GPa, K0' = 5.4(11) for 0.38 wt% H2O; KT0 = 158(4) GPa, K0' = 4.2(9) for 1.18 wt% H2O; and KT0 = 154(4) GPa, K0' = 4.9(11) for 1.66 wt% H2O. Variation of the bulk modulus of wadsleyite with water content is non-linear, which may be attributable to softening of the structure by ordering of vacancies onto two non-equivalent M3 sites (M3a and M3b) and an accompanying dilution of orthorhombic symmetry.Keywords: Wadsleyite, bulk modulus, equation of state, nominally anhydrous minerals, mantle Transition Zonein t r o D u C t i o nWadsleyite (β-Mg2SiO4) is the stable polymorph of olivine in the mantle Transition Zone from 410 to 525 km depth. The olivine-wadsleyite (α-β) transition of Mg2SiO4 is thought to produce the seismic discontinuity at 410 km (e.g., Ringwood 1975; Jeanloz and Thompson 1983; Bina and Wood 1987). Wadsleyite is a sorosilicate, isostructural with spinelloid III in the Ni-aluminosilicate system (Moore and Smith 1970; Akaogi and Navrotsky 1984). Because of the presence of an underbonded oxygen site (O1) not bonded to Si, Smyth (1987) predicted that wadsleyite could incorporate significant amounts of water as hydroxyl. Further theoretical studies based on crystal-chemical models (Smyth 1994) predicted that up to 3.3 wt% H2O could be accommodated by fully protonating the non-silicate oxygen. The presence of variable amounts of hydroxyl in laboratory-synthe-sized wadsleyite samples was confirmed by Raman and infrared spectroscopy (McMillan et al. 1991; Young et al. 1993). Inoue et al. (1995) synthesized hydrous wadsleyite at 15.5 GPa and 1200 °C containing 3.1 wt% H2O (determined by sec-ondary ion mass spectrometry, SIMS), close to the theoretical limit of 3.3 wt% H2O. The water storage capacity of wadsleyite coexisting with hydrous melt decreases above ~1300 °C, but re-mains as high as ~1 wt% H2O at 1400 °C and 15 GPa (Demouchy et al. 2005). Because of the unusually high water storage capacity of β-Mg2SiO4, knowledge of the elastic properties of hydrous wadsleyite is needed to constrain the potential hydration state of the mantle Transition Zone from geophysical observation (e.g., van der Lee and Wiens 2006). The incorporation of water as hydroxyl into wadsleyite and other nominally anhydrous minerals (NAMs) generally requires divalent cation vacancies, which can strongly influence physical properties such as elasticity (e.g., Jacobsen 2006) and rheology (e.g., Karato 2006). Water affects melting and phase relations (e.g., Inoue 1994; Hirschmann 2006; Komabayashi 2006), the depth and pressure interval of phase transitions (e.g., Wood 1995; Smyth and Frost 2002; Chen et al. 2002; Frost and Dolejš 2007), electrical conductivity (e.g., Karato 1990; Huang et al. 2005), strain rates and viscosity (e.g., Hirth and Kohlstedt 1996; Mei and Kohlstedt 2000), and shear strength (e.g., Jung and Karato 2001; Kavner 2003).Because the mass of liquid-water equivalent that may be stored or recycled through the solid mantle could amount to several oceans, the incorporation of water into mantle minerals has implications for understanding the crust-mantle evolution and regulation of ocean levels (Drake and Righter 2002; Bercovici and Karato 2003). Hydrogen has an indefinite atomic radius resulting in geochemical properties that are strongly pressure and temperature dependent. Smyth et al. (2006) and Mosenfelder et al. (2006) showed that the solubility of OH in olivine increases with * Present address: Department of Earth and Planetary Sciences, Northwestern University, Evanston, Illinois 60208, U.S.A. E-mail: [email protected] ET AL.: EFFECTS OF HYDRATION ON PROPERTIES OF WADSLEYITE599pressure and temperature, reaching a maximum of about one weight percent H2O at 12 GPa and 1250 °C, suggesting that hydrogen is more geochemically compatible at conditions near the 410 km discontinuity. Therefore, H may be exchanged between the upper mantle and Transition Zone without severe limitations imposed by dehydration melting, maintaining a deep water cycle between the Transition Zone and the surface (e.g., Smyth and Jacobsen 2006). Hydrogen is probably the least well-constrained compositional variable in current geochemical models of the mantle.Published compressibility studies of hydrous wadsleyite are limited to one composition with 2.5 wt% H2O (SIMS) using powder X-ray diffraction to 8.5 GPa (Yusa and Inoue 1997), who reported an isothermal bulk modulus (KT0) as 155(2) GPa, with pressure derivative (K') fixed at 4.3; significantly lower than KT0 = 170–173 GPa, determined in studies of anhydrous wadsleyite by Zha et al. (1997), Li et al. (1998), Hazen et al. (2000), and the cur-rent study. Smyth et al. (1997) described a hydrous (2.3 wt% H2O) iron-bearing (Fo95) wadsleyite with monoclinic I2/m symmetry, but did not report its equation of state. Smyth et al. (1997) attributed the dilution of symmetry from orthorhombic to monoclinic


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