Masami Kanzaki

Stability of hydrous magnesium silicates in the mantle transition zone

Phase relations in the Mg(OH)2SiO2 system have been studied up to 18 GPa. A new hydrous phase (phase E: Mg2.3Si1.3H2.4O6) was encountered at 13–17 GPa and 1000°C. Phase F (Mg1.2Si1.8H2.4O6), another new hydrous phase, was observed at 15.5 GPa and 800°C. The stability field of phase A was also briefly studied. These new hydrous phases might be stabilized in the mantle transition zone, especially in subducting slabs at those depths.

Silicon Coordination and Speciation Changes in a Silicate Liquid at High Pressures

Coordination and local geometry around Si cations in silicate liquids are of primary importance in controlling the chemical and physical properties of magmas. Pressure-induced changes from fourfold to sixfold coordination of Si in silicate glass samples quenched from liquids has been detected with 29Si magic-angle spinning nuclear magnetic resonance spectrometry. Samples of Na2Si2O5 glass quenched from 8 gigapascals and 1500�C contained about 1.5 percent octahedral Si, which was demonstrably part of a homogeneous, amorphous phase. The dominant tetrahedral Si speciation in these glasses became disproportionated to a more random distribution of bridging and nonbridging oxygens with increasing pressure.

In situ Observation of ilmenite‐perovskite phase transition in MgSiO3 using synchrotron radiation

In situ observations of the ilmenite-perovskite transition in MgSiO3 were carried out in a multianvil high-pressure apparatus interfaced with synchrotron radiation. The phase boundary between ilmenite and perovskite in the temperature range of 1300–1600 K was determined to be P (GPa) = 28.4(±0.4) - 0.0029(± 0.0020)T (K) based on Jamiesons equation of state of gold [Jamieson et al., 1982] and P (GPa) = 27.3(±0.4) - 0.0035(±0.0024)T (K) based on Andersons equation of state of gold [Anderson et al., 1989]. The consistency of our results, using Jamiesons equation of state, with previous studies obtained by quench methods leads us to conclude that the 660 km seismic discontinuity in the mantle can be attributed a phase transition to perovskite phase. However, the phase boundary based on the Andersons equation of state implies that the depth of the 660-km seismic discontinuity does not match the pressure of this transition.

Characterization of quenched high pressure phases in CaSiO3 system by XRD and 29Si NMR

We have studied quenched high pressure phases in the CaSiO3 system by x-ray diffraction (XRD) and 29Si MAS NMR, XRD study of the previously reported “e-CaSiO3 phase” synthesized at 12 GPa and 1500 °C reveals that it is actually a mixture of β-Ca2SiO4 (larnite) and a previously unknown CaSi2O5 phase. This result is supported by the 29Si NMR spectra. Furthermore, both the XRD and the NMR data suggest that the CaSi2O5 phase may have a titanite (CaTiSiO5) structure in which Ti is replaced by an octahedral Si. Samples quenched from 15 GPa and 1500°C consist mostly of an amorphous phase, but a small amount of CaSiO3-perovskite was identified by both XRD and NMR. The 29si NMR spectrum of the amorphous phase suggests that its local structure is similar to that of a glass quenched from melt at 1 bar.

Ultrahigh-pressure phase relations in the system Mg4Si4O12Mg3Al2Si3O12

High-pressure phase relations in the system Mg4Si4O12Mg3Al2Si3O12 were investigated at pressures from 10 to 22 GPa and at a temperature of 1000°C. The experimental results indicate that the maximum solubility of enstatite (Mg4Si4O12) in pyrope (Mg3Al2Si3O12) is 80 mol% enstatite at 15 GPa. At pressures between 15 and 19 GPa, the solubility is insensitive to pressure. At 19 GPa, garnet-ilmenite transformation first occurs in enstatite composition. At pressures above 19 GPa, the compositional range of the ilmenite solid solution expands toward pyrope composition with increasing pressure. Based on the present experiments, if pyrolite mantle is assumed, the pyroxene-garnet transformation appears at 350–400 km depth, and garnet + modified spinel and garnet + spinel phase assemblages become stable successively up to ∼ 580 km depth. At greater depths, garnet decomposes to ilmenite + garnet and hence the pyrolite mantle consists of ilmenite + garnet + spinel assemblage.

Local Structure and Chemical Shifts for Six-Coordinated Silicon in High-Pressure Mantle Phases

Most of the earths mantle is made up of high-pressure silicate minerals that contain octahedrally coordinated silicon (SiVI), but many thermodynamically important details of cation site ordering remain unknown. Silicon-29 nuclear magnetic resonance (NMR) spectroscopy is potentially very useful for determining short-range structure. A systematic study of silicon-29 chemical shifts for SiVI has revealed empirical correlations between shift and structure that are useful in understanding several new calcium silicates. The observed ordering state of a number of high-pressure magnesium silicates is consistent with the results of previous x-ray diffraction studies

Phase E: A high pressure hydrous silicate with unique crystal chemistry

AbstractThe unique cation-disordered crystal structures of two samples of phase E, a non-stoichiometric, hydrous silicate synthesized in a uniaxial, split-sphere, multi-anvil apparatus at conditions above 13 GPa and 1000° C, have been solved and refined in space group

Separation of supercritical slab-fluids to form aqueous fluid and melt components in subduction zone magmatism

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