2010 Fall AGU Abstract

Chang, Y.-Y., S. D. Jacobsen, S.-M. Thomas, C. R. Bina, J. R. Smyth, D. J. Frost, E. H. Hauri, Y. Meng, and P. K. Dera, Comparative compressibility of hydrous wadsleyite, Eos, Transactions of the American Geophysical Union, 91, Fall Meeting Supplement, in press, 2010.

Abstract

Determining the effects of hydration on the density and elastic properties of wadsleyite, β-Mg₂SiO₄, is critical to constraining Earth's global geochemical water cycle. Whereas previous studies of the bulk modulus (K_T) have studied either hydrous Mg-wadsleyite or anhydrous Fe-bearing wadsleyite, the combined effects of hydration and iron are under investigation. Also, whereas K_T from compressibility studies is relatively well constrained by equation-of-state fitting to P-V data, the pressure derivative of the bulk modulus (K′) is usually not well constrained, either because of poor data resolution, uncertainty in pressure calibrations, or narrow pressure ranges of previous single-crystal studies. Here we report the comparative compressibility of dry versus hydrous wadsleyite with Fo90 composition containing 1.9(2) wt% H₂O, nearly the maximum water storage capacity of this phase. The composition was characterized by EPMA and nanoSIMS. The experiments were carried out using high-pressure, single-crystal diffraction up to 30 GPa at HPCAT, Advanced Photon Source. By loading three crystals each of hydrous and anhydrous wadsleyite together in the same diamond-anvil cell, we achieve good hkl coverage and eliminate the pressure scale as a variable in comparing the relative value of K′ between the dry and hydrous samples. We used MgO as an internal diffraction standard, in addition to recording ruby fluorescence pressures. By using neon as a pressure medium and about 1 GPa pressure steps up to 30 GPa, we obtain high-quality diffraction data for constraining the effect of hydration on the density and K′ of hydrous wadsleyite. Due to hydration, the initial volume of hydrous Fo90 wadsleyite is larger than anhydrous Fo90 wadsleyite; however, the higher compressibility of hydrous wadsleyite leads to a volume crossover at 6 GPa. Hydration to 2 wt% H₂O reduces the bulk modulus of Fo90 wadsleyite from 170(2) to 157(2) GPa, or about 7.6% reduction. In contrast to previous results on hydrated Mg-wadsleyite, the pressure derivatives of K_T for both samples are similar, with K′=3.72(16) and 3.77(14) for anhydrous and hydrous Fo90 wadsleyite, respectively. The results indicate that hydration has a significant influence on the bulk modulus of wadsleyite at transition zone pressures.