Dense Hydrous Magnesium Silicate Phase Research Articles

Nature and origin of an undetected seismic phase in waveforms from Southern Tyrrhenian (Italy) intermediate-depth and deep earthquakes: first evidence for the phase-A in the subducted uppermost lithospheric mantle?

We have found a previously unreported later seismic phase in seismograms of European seismic stations from intermediate-depth and deep earthquakes of the Southern Tyrrhenian subduction zone. We observe this phase at stations from 6 to 9° from the epicentre, towards north. Only seismograms of earthquakes located in a well-defined region of the slab, in the depth range of 215–320 km, show the later x-phase. In this work, we describe the nature and possible origin of this phase, and we provide a simple 2D model to explain the observed arrival times. Our analyses reveal that the x-phase propagates downward in a high velocity layer, possibly located within the deepest part of the slab. We suggest that this layer reveals the presence of the dense hydrous magnesium silicate phase A, introduced from petrological laboratory experiments, inferred to carry water in the upper mantle and predicted to be found in cold subduction zones.

Effect of Al on the stability of dense hydrous magnesium silicate phases to the uppermost lower mantle: implications for water transportation into the deep mantle

Effect of Al on the stability of dense hydrous magnesium silicate phases to the uppermost lower mantle: implications for water transportation into the deep mantle

Crystal chemistry and high-temperature vibrational spectra of humite and norbergite: Fluorine and titanium in humite-group minerals

AbstractThe humite-group minerals along the brucite-olivine join may be important dense hydrous magnesium silicate (DHMS) phases in the subducting slab. Fluorine and titanium can be incorporated into their structures through the substitutions (OH)– = F– and Mg2+ + 2(OH)– = Ti4+ + 2O2–. These substitutions have significant effects on the hydrogen bonding behavior in the structures. Structure refinements and in situ high-temperature Raman and Fourier transform infrared (FTIR) measurements were conducted on natural humite and norbergite crystals. Both minerals crystallize in space group Pbnm, and their isobaric Grüneisen parameters for the lattice and SiO4 internal vibrations are compared with those of chondrodite, clinohumite, brucite, and forsterite. For the humite-group minerals, the OH-stretching modes above 3450 cm–1 are affected by local H-H repulsion, whereas the behavior of those below 3450 cm–1 can be explained by F– and Ti4+ substitutions, either of which may relieve the H-H repulsion effect. The Raman-active OH bands below 3450 cm–1 are affected by Ti4+ substitution, while the IR-active bands can be affected by either F– or Ti4+ substitutions. Based on an analysis of the high-T Raman and FTIR spectra, the OH vibrations above and below 3450 cm–1 behave differently as a function of temperature, and similar behavior has also been observed for other dense hydrous silicate phases in the hydrous peridotite system. Hence, the lengths of the oxygen-oxygen edges in MgO6 octahedra where protonation can occur become similar to each other at elevated temperatures. This may provide an atomistic explanation for the electrical conductivity properties of DHMS phases at high temperatures.

Release of CO2 in Cold Subduction Zones by Hydrous Carbonatitic Liquids

AbstractThe release of CO2 from subducted slabs is an important process of the global carbon cycle. Previous studies have demonstrated that metamorphic reactions, carbonatitic melts, carbonate dissolutions, and hydrous carbonatitic liquids are effective CO2 recycling mechanisms in hot and warm thermal regimes, whereas the fate of subducted carbonates in cold subduction zones remains poorly understood. Here, we investigate the phase relations of subducted ophicarbonate at pressures beyond sub‐arc depths. The presence of complex quench textures indicates that hydrous carbonatitic liquids formed at temperatures as low as ∼850°C at depths of ∼300–440 km, which corresponds to cold thermal regimes. Water released by the decomposition of dense hydrous magnesium silicate phases can substantially affect the stability of subducted carbonates, which facilitates the formation of hydrous carbonatitic liquids. This provides a mechanism for scavenging CO2 from cold slabs and may promote the formation of metasomatized domains in the deep mantle.

Melting of Al‐Rich Phase D up to the Uppermost Lower Mantle and Transportation of H2O to the Deep Earth

AbstractWe investigated the stability of the Al‐rich dense hydrous magnesium silicate Phase D (PhD) in a MgO‐Al2O3‐SiO2‐H2O system between 14 and 25 GPa at 900–1,500 °C. Al‐rich PhD has a very wide stability region from 900 °C and 14 GPa to at least 1,500 °C and 25 GPa, showing strong temperature stability with increasing pressure. Al‐rich PhD decomposes to Phase Egg at pressure of the mantle transition zone, whereas it decomposes to δ‐AlOOH phase with a temperature increase at pressure of the uppermost lower mantle. X‐ray diffraction and Raman spectroscopy measurements of Al‐rich PhD show that the unit‐cell volume is slightly larger, but the Raman spectra resemble that of Al‐free PhD. The wide stability region of Al‐bearing PhD would contribute an important storage site for water in the mantle transition zone, suggesting that it can deliver a certain amount of water into the lower mantle along hot subduction and even at the normal mantle geothermal P‐T condition. Furthermore, the dehydration of Al‐bearing PhD might be responsible for a series of observed seismic discontinuities from the transition zone to the uppermost lower mantle and even for deep earthquakes in some typical locations.

Plasticity of the dense hydrous magnesium silicate phase A at subduction zones conditions

The plasticity of the dense hydrous magnesium silicate (DHMS) phase A, a key hydrous mineral within cold subduction zones, was investigated by two complementary approaches: high-pressure deformation experiments and computational methods. The deformation experiments were carried out at 11GPa, 400 and 580°C, with in situ measurements of stress, strain and lattice preferred orientations (LPO). Based on viscoplastic self-consistent modeling (VPSC) of the observed LPO, the deformation mechanisms at 580°C are consistent with glide on the (0001) basal and (011¯0) prismatic planes. At 400°C the deformation mechanisms involve glide on (2¯110) prismatic, (0001) basal and {112¯1} pyramidal planes. Both give flow stresses of 2.5–3GPa at strain rates of 2–4×10−5s−1. We use the Peierls–Nabarro–Galerkin (PNG) approach, relying on first-principles calculations of generalized stacking fault (γ-surface), and model the core structure of potential dislocations in basal and prismatic planes. The computations show multiple dissociations of the 13[21¯1¯0] and [011¯0] dislocations (〈a〉 and 〈b〉 dislocations) in the basal plane, which is compatible with the ubiquity of basal slip in the experiments. The γ-surface calculations also suggest 13[21¯1¯3] and [01¯11] dislocations (〈a+c〉 or 〈c−b〉 directions) in prismatic and pyramidal planes, which is also consistent with the experimental data. Phase A has a higher flow strength than olivine. When forming at depths from the dehydration of weak and highly anisotropic hydrated ultramafic rocks, phase A may not maintain the mechanical softening antigorite can provide. The seismic properties calculated for moderately deformed aggregates suggest that S-wave seismic anisotropy of phase A-bearing rocks is lower than hydrous subduction zone lithologies such as serpentinites and blueschists.

Elasticity of superhydrous phase, B, Mg10Si3O14(OH)4

We have used first principles simulation based on density functional theory to calculate the equation of state and elasticity of superhydrous phase B, Mg10Si3O14(OH)4. The pressure–volume results for superhydrous phase B is well represented by a third order Birch–Murnaghan formulation, with K0=161.8 (±0.2)GPa and K0′=4.4 (±0.01). The calculated full elastic tensor at 0GPa is in good agreement with Brillouin scattering results, with the compressional elastic constants: c11=329.5GPa, c22=294.9GPa, c33=306.8GPa, the shear elastic constants – c44=99.8GPa, c55=98GPa, and c66=99GPa; the off-diagonal elastic constants c12=82.5GPa, c13=84.6GPa, and c23=98.7GPa. At the depths corresponding to the mantle transition zone, the aggregate sound wave velocities for superhydrous phase B is slower compared to dry ringwoodite which is the dominant mineral phase. However, hydrous ringwoodite bulk sound velocities are comparable to that of superhydrous phase B. Majoritic garnet, the second most abundant mineral in the transition zone, has bulk sound wave velocities slower than superhydrous phase B. An assemblage consisting of hydrous ringwoodite, superhydrous phase B, and majorite garnet could account for the low velocities observed in certain subduction zone settings at depths corresponding to the base of the transition zone and upper mantle. Superhydrous phase B exhibits moderate single-crystal elastic anisotropy with AVP∼3% and AVS∼5% at the base of the transition zone. Single-crystal elastic anisotropy of other dense hydrous magnesium silicate phases phase such as hydrous phase D is significantly larger at these conditions and might play a major role in explaining the observed mid mantle seismic anisotropy.

Crystal chemistry of dense hydrous magnesium silicates: The structure of phase H, MgSiH2O4, synthesized at 45 GPa and 1000 C

The crystal structure of the dense hydrous magnesium silicate phase H, MgSiH 2 O 4 , synthesized at 45 GPa and 1000 °C, was investigated by single-crystal X-ray diffraction. Although showing a deterioration process under the X-ray beam, the compound was found to be orthorhombic, space group Pnnm (CaCl 2 -type structure), with lattice parameters a = 4.733(2), b = 4.3250(10), c = 2.8420(10) A, V = 58.18(3) A 3 , and Z = 1. The structure was refined to R 1 = 0.0387 using 53 observed reflections [2σ( I ) level]. Magnesium and silicon were found to be disordered at the same octahedral site (with a mean bond distance of 1.957 A). Hydrogen was not located in the difference Fourier maps, but it is very likely disordered at a half-occupied 4 g position. The centrosymmetric nature of the structure of phase H is examined in relation to that reported for pure δ-AlOOH at ambient conditions (non-centrosymmetric, P 2 1 nm ), and the possibility that these two compounds can form a solid solution at least at high pressure is discussed.

Melting of phase D in the lower mantle and implications for recycling and storage of H2O in the deep mantle

We determined the melting phase relations and conditions of the dense hydrous magnesium silicate phase D (nominally MgSi2O4(OH)2) on composition in MgO–SiO2–H2O (MSH), MgO–Al2O3–SiO2–H2O (MASH), and FeO–MgO–Al2O3–SiO2–H2O (FMASH), and on a mixture of phase D+olivine+enstatite (MSH) at 22–32GPa and 1000–1800°C. Contrasting to previous studies, we performed H2O-undersaturated experiments. Bulk compositions were synthetic mixtures of brucite+silica or brucite+olivine+enstatite on the silica-rich side of the tie-line perovskite–H2O. At 22–24GPa, the maximum thermal stability of phase D is between 1350 and 1400°C in MSH and FMASH, but 1600°C at 24GPa in the Fe-free, Al-bearing bulk composition (MASH). Apparently, addition of Al2O3 increases the stability field of phase D by 200°C, an effect that is counter balanced by addition of FeO. At 32GPa, the stability of phase D (MSH and FMASH) is between 1350 and 1400°C. At 22GPa, phase D melts to a Mg-rich melt coexisting with MgSi-ilmenite+stishovite, whereas at 24–32GPa melt coexists with perovskite and stishovite. Even melts from bulk compositions in the silica-rich part of the MSH system (molar bulk Mg/Si<0.5) are magnesian-rich (Mg/Si molar ratio of 2–5) and are distinct from aqueous fluids and hydrous melts at lower pressures.The temperature stabilities determined in this study indicate that slabs that thermally relax when stagnating on top of the 660-km discontinuity or penetrating into the lower mantle will have their last dense hydrous magnesium silicate phase, i.e., phase D, melting and producing a magnesian and hydrous melt that will rise through the transition zone. Such a melt could be responsible for observed low velocity zones, and may be neutrally buoyant at the 410-km discontinuity and will affect the structure and dynamics of the mantle.

Crystal defects in dense hydrous magnesium silicate phase A deformed at high pressure: characterization by transmission electron microscopy

Dense hydrous magnesium silicate (DHMS) Phase A has been deformed at 11 GPa, 700 and 400 °C in the multianvil apparatus. Transmission electron microscopy (TEM) characterizations, using the weak-beam dark-field (WBDF) technique, have shown that at 700 °C, two different types of perfect dislocations are activated with ⅓〈2110〉 and ⅓〈2113〉 Burgers vectors. The ⅓〈2110〉 dislocations glide in the basal plane, but for ⅓〈2113〉 dislocations, no glide plane could be identified. At 400 °C, dissociation of ⅓〈2110〉 dislocations is observed in the basal plane. Furthermore, prismatic and pyramidal glide planes were identified, with dissociations in pyramidal planes, at this temperature. Also a growth reticular merohedral twin has been observed; the twin law results from a rotation of 180° around the [1540] axis.

The structure of a super-aluminous version of the dense hydrous-magnesium silicate phase D

The dense hydrous-magnesium silicate phase D, which has the ideal formula MgSi2H2O6, may be an important link in a chain of hydrous phases that carry H2O in the ultramafic portions of subducting lithosphere, into the Earth’s lower mantle. We have synthesized a new Al-rich form of phase D, containing up to 50 wt% Al2O3, using a multi-anvil device at ~1300 °C and 25 GPa. The phase, with the formula Mg0.2Fe0.15Al1.8H1.8SiO6, was initially produced in a bulk composition designed to synthesize Al- and Fe-rich magnesium silicate perovskite with a composition similar to that produced in experiments on mid-ocean ridge basalt bulk compositions at lower mantle conditions. Further experiments using a starting mixture based on the composition of this Al-rich phase resulted in the synthesis of 60–70 μm long single crystals at similar conditions. The recovered crystals were slightly richer in H2O (Mg0.2Fe0.12Al1.5Si0.92H3.1O6) and their unit-cell parameters were similar to those of MgSi2H2O6 phase D. A refinement of the crystal structure was carried out in the P 31 m space group and revealed a more disordered cation distribution than magnesium silicate phase D. All cation-oxygen distances are similar, suggesting a high degree of Si/Al disorder. Although the stability field of this new variant of phase D is yet to be determined, this phase may be an important host for H2O within portions of subducted oceanic crust in the lower mantle.

Sound velocity measurement in liquid water up to 25 GPa and 900 K: Implications for densities of water at lower mantle conditions

We extended the pressure range of sound velocity measurements for liquid water to 25 GPa and 900 K along the melting curve using a laser heated diamond anvil cell with a combined system of Brillouin scattering and synchrotron X-ray diffraction. Experimental pressure and temperature were obtained by solving simultaneous equations: the melting curve of ice and the equation of state for gold. The sound velocities obtained in liquid water at high pressures and melting temperatures were converted to density using Murnaghan's equation of state by fitting a parameter of the pressure derivative of bulk modulus at 1 GPa. The results are in good agreement with the values predicted by a previously reported equation of state for water based on sound velocity measurements. The equation of state for water obtained in this study could be applicable to water released by dehydration reactions of dense hydrous magnesium silicate phases in cold subducting slabs at lower mantle conditions, although the validity of Murnaghan's equation of state for water should be evaluated in a wider pressure and temperature ranges. The present velocity data provides the basis for future improvement of the accurate thermodynamic model for water at high pressures.

Sound velocities and elasticity of DHMS phase A to high pressure and implications for seismic velocities and anisotropy in subducted slabs

Dense hydrous magnesium silicate (DHMS) phase A forms in cold subducted slabs after the breakdown of antigorite serpentine, and may play an important role in the transport of water within the upper mantle. In this paper we present acoustic velocities and the single-crystal elastic properties of Fe-bearing phase A, (Mg 0.981Fe 0.019) 7Si 2O 8(OH) 6, measured by Brillouin spectroscopy on a sample compressed to 12.4(2) GPa in a diamond anvil cell. A fit to the acoustic data using a 3rd order finite-strain EOS yields the following adiabatic bulk ( K S) and shear ( μ) moduli and their pressure derivatives: K S = 106(1) GPa, (∂ K S/∂ P) T0 = 5.8(3), μ = 61(1) GPa, (∂ μ/∂ P) T0 = 1.8(1). Within the experimental resolution, the pure longitudinal elastic constants, C 11 and C 33, and the off-diagonal C 12 constants exhibit positive linear pressure dependence, whereas C 44, C 66 and C 13 increase with a quadratic dependence on pressure. The axial compressibility of phase A remains highly anisotropic in the investigated pressure range, with the a-axis being 15% more compressible than the c-axis at 12.4(2) GPa. Compared to forsterite, the aggregate compressional ( V P) and shear ( V S) acoustic velocities of phase A are 7% slower at room pressure. Although the velocity contrast diminishes to 3.5% for V P, it is maintained for V S over the investigated pressure range. Phase A has high shear wave anisotropy ( A S) and shear-wave polarization anisotropy ( A S P 0 ) of A S = 20% and A S P 0 = 18 % , and a more moderate compressional wave anisotropy A P = 12% at room pressure. The A P of phase A decreases to 8% at 12.4(2) GPa, remaining significantly lower than that of forsterite, whereas the shear anisotropy is nearly constant at ∼20% over the same pressure range and exceeds that of forsterite by 12% at 12.4(2) GPa. At upper mantle pressures, the shear wave splitting in phase A ( A S P 0 ) is 20% higher than in forsterite. The results of this study were used with thermoelastic data for other relevant minerals to compute the density, seismic velocities and V P/ V S ratios of subducted garnet–harzbugite and moderately depleted harzburgite assemblages with various degrees of hydration as a function of pressure along a slab isotherm at 1073 K. The results suggest that the seismic velocities of dry and water-saturated harzburgites (44.5 vol% phase A) may be indistinguishable at upper mantle P–T conditions because of the increasing concentration of high-pressure orthopyroxene upon hydration. This phase displays high seismic velocities that offset the decrease in velocities due to phase A, rendering hydration difficult to detect (anelastic attenuation is not considered). Combined observations from the analysis of seismic parameters indicate that significant shear wave anisotropy, accompanied by high V P/ V S and Poisson's ratios and pronounced shear wave splitting, could be major diagnostic features for identifying phase A-bearing assemblages at depth (180–350 km) in cold subducted slabs.

Crystal structure and compression of an iron-bearing Phase A to 33 GPa

Fe-bearing dense hydrous magnesium silicate Phase A, Mg6.85Fe0.14Si2.00O8(OH)6 has been studied by single-crystal X-ray diffraction at ambient conditions and by high-pressure powder diffraction using synchrotron radiation to 33 GPa. Unit cell parameters at room temperature and pressure from single crystal diffraction are a=7.8678 (4) A, c=9.5771 (5) A, and V=513.43 (4) A3. Fitting of the P–V data to a third-order Birch-Murnaghan isothermal equation of state yields V 0=512.3 (3) A3, K T,0=102.9 (28) GPa and K′=6.4 (3). Compression is strongly anisotropic with the a-axes, which lie in the plane of the distorted close-packed layers, approximately 26% more compressible than the c-axis, which is normal to the plane. Structure refinement from single-crystal X-ray intensity data reveals expansion of the structure with Fe substitution, mainly by expansion of M-site octahedra. The short Si2–O6 distance becomes nearly 1% shorter with ~2% Fe substitution for Mg, possibly providing additional rigidity in the c-direction over the Mg end member. K T obtained for the Fe-bearing sample is ~5.5% greater than reported previously for Fe-free Phase A, despite the larger unit cell volume. This study represents a direct comparison of structure and K T–ρ relations between two compositions of a F-free dense hydrous magnesium silicate (DHMS) phase, and may help to characterize the effect of Fe substitution on the properties of other DHMS phases from studies of the Fe-free end-members.

Stability of dense hydrous magnesium silicate phases and water storage capacity in the transition zone and lower mantle

We evaluated the water storage capacity in the mantle based on the phase relations of silicate systems containing water. We conducted high pressure and temperature synthesis experiments on some silicate systems, such as the superhydrous phase B (=phase C) and phase G (=phase D and F) compositions in the MgO–SiO 2–H 2O system and the CMAS pyrolite–2 wt.% H 2O system up to the uppermost part of the lower mantle. Superhydrous phase B is stable at temperatures below 1200°C at 18 GPa and below 1300°C at 20 GPa. The following reaction sequences were observed with increasing temperature from 1200 to 1600°C in the pressure range from 18 to 20 GPa: superhydrous phase B→ phase B + liquid→ wadsleyite+ periclase+ liquid→ anhydrous phase B+periclase+liquid. Phase G is stable at temperatures below 1000°C at 18 GPa, below 1100°C at 20–22 GPa, and below 1200°C at 25 GPa. Wadsleyite+stishovite+liquid are stable above 1000°C at 18 GPa, ringwoodite ( or phase E )+ stishovite+ liquid above 1100°C at 22 GPa, ilmenite+stishovite+liquid above 1100°C at 23.5 GPa, and perovskite+stishovite+liquid above 1200°C at 25 GPa. Superhydrous phase B is stable at 1200°C at 18.5 and 25 GPa in CMAS pyrolite–2 wt.% H 2O composition, whereas it decomposes at around 1400°C at 25 GPa in the composition. There may be a layered structure in the mantle in its water storage potential (solubility of water per unit mass); i.e. the upper and lower mantles have relatively small water storage potentials, whereas the transition zone has a larger potential. The water content in the transition zone might be large, because the primordial water trapped in the lower mantle and the recycled water transported by the slab subduction might have been released upwards and stored in the transition zone during geological time.

Stability of dense hydrous magnesium silicate phases in the systems Mg 2 SiO 4 -H 2 O and MgSiO 3 -H 2 O at pressures up to 27 GPa

We conducted high-pressure phase equilibrium experiments in the systems MgSiO3 with 15 wt% H2O and Mg2SiO4 with 5 wt% and 11 wt% H2O at 20 ∼ 27 GPa. Based on the phase relations in these systems, together with the previous works on the related systems, we have clarified the stability relations of dense hydrous magnesium silicates in the system MgO-SiO2-H2O in the pressure range from 10 to 27 GPa. The results show that the stability field of phase G, which is identical to phase D and phase F, expands with increasing water contents. Water stored in serpentine in the descending cold slabs is transported into depths greater than 200 km, where serpentine decomposes to a mixture of phase A, enstatite, and fluid. Reaction sequences of the hydrous phases which appear at higher pressures vary with water content. In the slabs with a water content less than about 2 wt%, phase A carries water to a depth of 450 km. Hydrous wadsleyite, hydrous ringwoodite, and ilmenite are the main water reservoirs in the transition zone from 450 to 660 km. Superhydrous phase B is the water reservoir in the uppermost part of the lower mantle from 670 to 800 km, whereas phase G appears in the lower mantle only at depths greater than 800 km. In cold slabs with local water enrichment greater than 2 wt%, the following hydrous phases appear with increasing depths; phase A to 450 km, phase A and phase G from 450 km to 550 km, brucite, superhydrous phase B, and phase G from 550 km to 800 km, and phase G at depths greater than 800 km.

Decomposition of phase D in the lower mantle and the fate of dense hydrous silicates in subducting slabs

High pressure, high temperature quench-type experiments were carried out on serpentine to pressures of 53 GPa and temperatures between 800–1800°C. X-ray analyses show that recovered phase assemblages varied considerably for the different high pressure and high temperature studies. The dense hydrous magnesium silicate phases decompose sequentially as the pressure increases and appear to serve as reservoirs for H 2O in the mantle, eventually releasing it at the highest pressure. Superhydrous phase B is stable up to pressure equivalent to the boundary between the transition zone and lower mantle. Phase D decomposes at pressures of 44 GPa, equivalent to about 1250 km depth. This may define the lower depth limit for the presence of dense hydrous magnesium silicates. Beyond this pressure, only nominally anhydrous phases have been recovered in high-pressure experiments. This may indicate a lack of stoichiometric-hydrogen bearing silicate phases at higher pressure.

Stability of phase D at high pressure and high temperature

We have determined the stability of the dense hydrous magnesium silicate phase D in a Mg2SiO4 + 20.5 wt % H2O composition between 16 and 25 GPa at 900°–1400°C. Phase D coexists with superhydrous phase B and a Mg‐rich liquid to temperatures of 1000°C at 17 GPa and 1400°C at 26 GPa. Experiments in an externally heated diamond‐anvil cell confirm that phase D is stable to pressures of at least 50 GPa at 930°C. From static compression measurements, the zero‐pressure bulk modulus of phase D was determined as 200±7 GPa. We also present evidence that phase D is identical to phase F [Kanzaki, 1991] and phase G [Kudoh et al., 1997a; Ohtani et al, 1997]. The high‐pressure and high‐temperature stability of phase D makes it eminently suitable as a hydrous phase which is stable within subducting lithosphere, in the transition zone, and in the lower mantle. The positive pressure‐temperature stability slope of phase D, determined in this study, indicates that it is potentially stable at temperatures much greater than 1400°C in the lower mantle.

Reply [to “Comment on ‘A new hydrous silicate, a water reservoir, in the upper part of the lower mantle’ by Eiji Ohtani, Hiroki Mizobata, Yasuhiro Kudoh, Toshiro Nagase, Haruo Arashi, Hisayoshi Yurimoto, and Isoji Miyagi”

Phase G reported by Ohtani et al. [1997] and Kudoh et al. [1997a] is identical with the new phase D reported by Kuroda and Irifune [1998] and Yang et al. [1997]. However, there is ambiguity on existence of phase D defined by Liu [1987]. There are following possibilities on existence of Liu's phase D; (1) the product was a mixture of phase G and the other contaminant phases, (2) it realy exists stably in different pressure and temperature conditions from those studied by our works, or (3) the phase is a metastable phase formed due to a short run duration and/or no sealing of water in the diamond anvil cell. Therefore, it is not appropriate to use the name phase D for the dense hydrous magnesium silicate phase G reported by us [Ohtani et al., 1997].

Stability of dense hydrous magnesium silicate phase G in the transition zone and the lower mantle.

Re-examination of the run products in the Mg2SiO4–H2O system by Ohtani et al. (1995) revealed stability of dense hydrous magnesium silicate, phase G, and brucite at 20 GPa and 800°C, and stability of the assemblage with superhydrous phase B + phase G + fluid at 20 GPa and 1000°C under the water saturated conditions. We reported the Raman spectrum of phase G, which suggests existence of silicon ions in the six coordination sites. Brucite, superhydrous phase B, and phase G are stable at least up to 22.5 GPa and around 1000°C, and are the candidates for the hydrous phases in a cold slab descending into the base of the transition zone and the uppermost part of the lower mantle. The X-ray diffraction and the Raman spectra data revealed that the dense hydrous magnesium silicate, phase G, and phase D re-defined by Yang et al. (1997) and Kuroda and Irifune (1998) are identical. Phase F by Kanzaki (1991) and Gasparik (1993) is likely to be identical to phase G.