HR: 0800h
AN: MR11A-0898    [Abstracts]
TI: $\it In$ $\it situ$ X-ray diffraction study of post-spinel transformation in peridotite mantle: Implication to 660 km discontinuity
AU: * Litasov, K
EM: klitasov@ganko.tohoku.ac.jp
AF: Inst. Mineral. Petrol. Econ. Geol., Tohoku University, Aoba-ku, Sendai, 980-8578 Japan
AU: Ohtani, E
EM: ohtani@mail.tains.tohoku.ac.jp
AF: Inst. Mineral. Petrol. Econ. Geol., Tohoku University, Aoba-ku, Sendai, 980-8578 Japan
AU: Suzuki, A
EM: a-suzuki@mail.tains.tohoku.ac.jp
AF: Inst. Mineral. Petrol. Econ. Geol., Tohoku University, Aoba-ku, Sendai, 980-8578 Japan
AU: Funakoshi, K
EM: funakosi@spring8.or.jp
AF: Spring-8, Japan Synchrotron Radiation Research Insitute, Kouto, Hyogo, 678-5198 Japan
AB: The 660-km seismic discontinuity in the Earth's mantle is identified with the transformation of ringwoodite (spinel (Mg,Fe)$_2$SiO$_4$-phase) to (Mg,Fe)SiO$_3$-perovskite and (Mg,Fe)O-ferropericlase. It was suggested using quench experiments that the transformation boundary has significant negative Clapeyron slope (-3 MPa/K, Ito and Takahashi, 1989) responsible for depressions and elevations of the 660-km discontinuity in subduction zones and hot spots of mantle plumes. Recent $\it in$ $\it situ$ x-ray diffraction studies in Mg$_2$SiO$_4$ system indicate that negative slope of the boundary is much gentler (-1.3 MPa/K) (Fei et al., 2004). Therefore there must be another factors resulting in significant depth variations of the 660-km discontinuity. In this study, we present the phase relations in anhydrous pyrolite by $\it in$ $\it situ$ X-ray diffraction measurements to examine the influence of additional components and Mg/Si-ratio on post-spinel phase transformation. Experiments were carried out using Speed-1500 multianvil apparatus installed at BL04B1 at synchrotron radiation facility `Spring-8' (Hyogo, Japan). Starting materials were synthetic glass representing SiO$_2$-Al$_2$O$_3$-FeO-MgO-CaO-pyrolite. Graphite capsule were used as a sample container. Co-doped MgO was used as the pressure medium and a cylindrical LaCrO$_3$ heater was used as the heating element. Temperature was measured with a WRe thermocouple. Different equation of states for Au and MgO was used for pressure calibration. The phase relations were determined at 20-25 GPa and temperature up to 2300 K. We observed easy nucleation of Mg-perovskite and ferropericlase from ringwoodite-bearing assembly in the temperature range of 1600-2200 K. The obtained post-spinel phase boundary can be expressed as P (GPa) = - 0.0004 T (K) + 22.26 using pressures calibrated by Au scale (Anderson et al., 1989). The choice of pressure scale does not have significant influence on the slope of phase transformation. Our experiments demonstrated that variations of chemical composition of pyrolite relative to Mg$_2$SiO$_4$ do not affect on post-spinel phase transformation. Therefore, we should carefully account some other factors, like effect of water on phase transformation or experimental problems like effect of pressure on emf of thermocouples.
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 3630 Experimental mineralogy and petrology
DE: 3924 High-pressure behavior
DE: 3954 X ray, neutron, and electron spectroscopy and diffraction
DE: 1025 Composition of the mantle
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting