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Effects of hydration on the structure and compressibility of wadsleyite, β-(Mg₂SiO₄)

¹ Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309, U.S.A.
² Department of Earth and Planetary Sciences, Northwestern University, Evanston, Illinois 60208, U.S.A.
³ Bayerisches Geoinstitut, Universität Bayreuth, Bayreuth 95440, Germany

View larger version (10K): [in this window] [in a new window]   FIGURE 1. Unit-cell volumes vs. water content for Fo100 hydrous wadsleyite (a). Solid symbols represent the current study, open symbols are data from Jacobsen et al. (2005). Estimated 10% uncertainty in water contents is shown. The data from Jacobsen et al. (2005) were collected on different instruments than in the current study, so direct comparison of absolute volumes is not warranted. Alternatively, all data were normalized against the near-anhydrous value for each set and plotted in b to directly compare the relative volumetric and axial expansions. Triangles = SS0401, SS0402, and SS0403 from current study; squares = data from Jacobsen et al. (2005); circles = samples BT1 and BT4 from Kohn et al. (2002).

View larger version (9K): [in this window] [in a new window]   FIGURE 2. Density vs. water content for Fo100 hydrous wadsleyite. Solid symbols represent the current study, open symbols are data from Jacobsen et al. (2005). Estimated 10% uncertainty in water contents is shown. The best-fit line is shown for the merged data sets.

View larger version (11K): [in this window] [in a new window]   FIGURE 3. Linear compressibility parallel to crystallographic axes for wadsleyite. Squares = a-axis; triangles = b-axis, circles = c-axis. Solid symbols represent the current study; open symbols near 0 wt% are data from Hazen et al. (2000); open symbols at 2.5 wt% are from Yusa and Inoue (1997). Best-fit lines were added to aid in comparing slopes of the data.

View larger version (10K): [in this window] [in a new window]   FIGURE 4. Bulk modulus of wadsleyite vs. water content, and comparison with other Mg-silicate series. The dashed line represents the trend of the grossular-hydrogarnet series. Wadsleyite appears to have a non-linear relation of bulk modulus to water content, whereas ringwoodite and the forsterite-hydroxylchondrodite series were shown to have linear relations. References: hydrous wadsleyite = this study; anhydrous wadsleyite = Hazen et al. (2000); ringwoodite = Smyth et al. (2004); forsterite = Downs et al. (1992); OH-clinohumite and OH-chondrodite = Ross and Crichton (2001); grossular-hydrogarnet = Wang and Ji (1997), O’Neill et al. (1993), Lager et al. (2002).

View larger version (13K): [in this window] [in a new window]   FIGURE 5. Plots of Birch normalized pressure (FE) vs. Euler finite strain (fE) for hydrous wadsleyites. Bulk moduli from y-intercepts of linear regressions are 160(3), 153(3), and 149(3) GPa for samples with 0.38, 1.18, and 1.66 wt% H2O, respectively. Positive slopes of the linear regressions indicate K' > 4.

View larger version (13K): [in this window] [in a new window]   FIGURE 6. Bulk sound speed plots for anhydrous and hydrous wadsleyite (solid black curves), and a hypothetical curve representing possible behavior of hydrous wadsleyite at Transition Zone pressures (gray). Curves are extrapolated from zero-pressure Birch-Murnaghan equation of state parameters; data points represent pressures at which volume data were collected. The dashed curve represents a refit of the hydrous data with K' = 4.0, consistent with the anhydrous value. The gray curve is calculated from the fitted K0 and K', with K'' selected so that bulk sound speeds for the hydrous composition remain lower than the speeds for the anhydrous composition through the pressure stability range of wadsleyite in the Transition Zone. The vertical line represents the approximate pressure at the 410 km seismic discontinuity.

View larger version (5K): [in this window] [in a new window]   FIGURE 7. Omega scans of the (2 4 4) diffraction peak for two separate wadsleyite crystals from sample SS0402. Black = orthorhombic crystal; gray = monoclinic crystal. Width of scan field is approximately 1.5°. Broadening in the monoclinic sample is interpreted to result from polysynthetic twinning.