5:15 PM - 6:30 PM
[SCG19-P05] The stability of Topaz-OH II in the mantle transition zone
Keywords:Topaz, synchrotron X-ray diffraction, mantle transition zone, subducting slab
The starting material was prepared in the MgO-Al2O3-SiO2-H2O system with the bulk composition of 4.27 wt.% MgO, 52.95 wt.% Al2O3, 28.47 wt.% SiO2 and 14.30 wt.% H2O. Experiments were carried out using a 1000-ton Kawai type multi-anvil press at Tohoku University. Semisinterd zirconia and platinum were used as pressure medium and sample container, respectively. The pressure was calibrated by the β-γ phase boundary of Mg2SiO4. In order to identify synthesized phases, X-ray diffraction measurements and Micro-Raman spectroscopy measurements were carried out at the Photon Factory in KEK, Tsukuba, Japan and at Tohoku University, respectively. Compositions of the recovered samples were determined using an electron probe microanalyzer (EPMA) at Tohoku University.
We found that hydrous phases of δ-AlOOH and phase egg were stable in the pressure range of 18-20 GPa and the temperature range of 1000-1300 ℃. Topaz-OH II was stable at 18 GPa and 1400 ℃, which is higher-pressure and -temperature condition than that reported previously. The composition of Topaz-OH II was 2.23(7) wt.% MgO, 55.24(13) wt.% Al2O3 and 31.46(21) wt.% SiO2, (88.93(23) wt.% total) and its Al / Si ratio was 2.50(4). The lattice parameters of Topaz-OH II at ambient condition was a= 4.719(9) Å, b= 8.922(23) Å, c= 2.777(6) Å, and V0=116.7(5) Å3, which is consistent with the parameters reported by Kanzaki (2010).
Our results indicate that Topaz-OH II would be a potential candidate of the water carrier at the middle of the mantle transitions zone.