In situ determination of the phase boundary between Wadsleyite and Ringwoodite in Mg₂SiO₄

Abstract

Abstract. The phase boundary between wadsleyite and ringwoodite in Mg2SiO 4 composition was determined by in situ observation using synchrotron X-ray and multi anvil apparatus in KEK, Tsukuba, Japan. An energy dispersive method was employed using the Ge solid state detector and the white X-ray beam from the synchrotron radiation source. The pressure was determined by the equation of state of NaCl. The stability field was identified by the change in intensities of diffraction lines of each phases. As a result, the phase boundary is expressed as a linear equation P=I0.32(28)+0.00691(9)xT, where P is pressure in gigapascals and T is temperature in degrees Celsius. IntroductionOlivine is the major constituent in the Earth's upper mantle, and the significant seismic discontinuities, which locate 400 km and 670 km deep, are considered to be caused by the phase transition of olivine to wadsleyite (modified spinel structure) and the decomposition of ringwoodite (spinel structure) to magnesiowustite and There are a number of quench experiments so far on the phase boundary between wadsleyite and ringwoodite [e.g., Kawada, 1977; $uito, 1977;Katsura and Ito, 1989], however, the in situ X-ray determination have not been conducted yet.In the quench experiments, pressure was estimated from the calibration curve, which is based on the fixed point at room and/or high temperatures, and the considerable uncertainty remains in pressure (e.g., +I.5GPa above 14 GPa

Reaction boundary between akimotoite and ringwoodite + stishovite in MgSiO3

“…The phase boundary between wadsleyite and ringwoodite has been investigated repeatedly in the previous studies (e.g., Katsura and Ito, 1898;Akaogi et al 1989;Inoue et al 2006). If these steeper Clapeyron slopes, which might be less accurate than that of Suzuki et al (2000), are used to estimate the triple point, this triple point would move to a higher temperature region.…”

“…The oxide mixtures, including a flux that was expected to enhance the chemical reaction, were used as the starting material by and previous studies at 12-25 GPa and 600-2200 K. The thick lines are estimates in our study. The boundaries between wadsleyite and ringwoodite and between akimotoite and bridgmanite are based on the works of Suzuki et al (2000) and Ono et al (2001), respectively, revised using the EOS of gold (Dorogokupets and Dewaele 2007). The boundary between wadsleyite + stishovite and akimotoite was not constrained based solely on our data.…”

“…If the temperature of the triple point is lower than the geotherm (i.e., Gasparik 1990), the wadsleyite + shishovite mixture changes directly into akimotoite as the pressure increases. When the reaction boundary determined in our study is combined with the phase boundary between wadsleyite and ringwoodite determined by Suzuki et al (2000) using the in situ XRD method, the estimated triple point locates at 1700 ± 100 K and 20 ± 1 GPa, which is 2 GPa and 300 K higher than that reported by Gasparik (1990). The phase boundary estimated in our study is in agreement with that reported by Sawamoto (1987); however, the temperature of the triple point of our study is 200 K lower than that determined by Sawamoto (1987).…”

An experimental study of phase transformations in olivine under pressure and temperature conditions corresponding to the mantle transition zone

“…At the end of the experiment, heating was terminated first by shutting Figure 1 a, The 10/5 multianvil cell assembly used in the present study; b, the high-temperature pressure calibration curve for the assembly. The three points in b are coesite-stishovite at 9.6 GPa (Coe-St, 1200°C) [15], olivine-wadsleyite in forsterite at 14.6 GPa and 1400°C [16], and wadsleyite-ringwoodite at 20 GPa and 1400°C [17]. For abbreviations of different minerals, see Table 1. off the power supply.…”

“…The truncated edge length (TEL) used in this study was 5 mm and the edge-length of the octahedral pressure medium (made of MgO+Al 2 O 3 ) was 10 mm (referred to as the 10/5 assembly). Figure 1a is We calibrated the pressure for the 10/5 assembly using the phase transitions of coesite-stishovite in SiO 2 at 1200°C [15], forsterite-wadsleyite and wadsleyite-ringwoodite in Mg 2 SiO 4 ( Figure 1b) at 1400°C [16,17]. Clearly, the 10/5 assembly can be used for experiments up to 21 GPa.…”