Superhydrous Phase Research Articles

Effect of Al‐Incorporation on the Sound Velocities of Superhydrous Phase B at High Pressure and High Temperature

AbstractSound velocities and densities of Al‐bearing superhydrous phase B (Al‐bearing SuB) were investigated up to 24 GPa and 1300 K by synchrotron X‐rays combined with ultrasonic interferometry techniques in a multi‐anvil apparatus. We found that Al + H incorporation decreases the adiabatic bulk modulus and shear modulus of SuB. Our results however suggest that this effect is less important than that of temperature. The presence of hydrous pyrolite with ∼10 wt.% Al‐bearing SuB in cold subducting slabs could explain up to ∼2% high velocity anomalies at the bottom of the mantle transition zone (500–660 km) while it turns into ∼7.7% low velocity anomalies below 660 km. Al‐bearing SuB is reportedly stable at mantle temperatures, where it could account for ∼12.2% low velocity anomalies beneath subduction zones in the uppermost lower mantle.

On the Dynamics of Water Transportation and Magmatism in the Mid‐Mantle

AbstractThe distribution of water within the Earth's mantle has significant implications for the Earth's dynamics and evolution. Recent mineral physics experiments indicate that dense hydrous magnesium silicates can contain large amounts of water stable up to 60 GPa or even beyond along slab geotherms. Here we perform petrological‐thermomechanical numerical simulations of water transportation by deep slab subduction and related magmatism in the mid‐mantle. Key parameters including those defining the slab thermal parameter and the water storage capacity in the oceanic lithosphere and surrounding mantle are explored. The results show two major dehydration events of ultramafic rocks at around 150 and 750 km by dehydration of serpentine at 600°C and superhydrous phase B in the entrained wet upper mantle, respectively. Large amounts of water, ∼1.5 wt% at least locally, are carried down to the mantle transition zone and lower mantle. We estimate an upper limit of slab water flux into the mid‐mantle of 0.1–0.28 × 1012 kg/yr, which is ∼13%–37% of the input water from the serpentinized mantle. Moreover, a substantial fraction of the water released by the slab is absorbed by the entrained mantle and overlying mid‐mantle portions, such that ∼30%–70% of the water injected at the trench could be delivered to the lower mantle. The deepest magmatism is observed at ∼1,500 km in case of phase H breakdown (MgO‐SiO2‐H2O system), coinciding with the depth of strong seismic attenuation. Overall, these simulations suggest that up to 0.2 ocean mass per billion years could be transported down to the mid‐mantle and beyond.

Phase Stability of Al‐Bearing Dense Hydrous Magnesium Silicates at Topmost Lower Mantle Conditions: Implication for Water Transport in the Mantle

AbstractIn this study, we investigated the phase stability of Al‐free and Al‐bearing superhydrous phase B (shy‐B) up to 55 GPa and 2500 K. In comparison with Al‐free shy‐B, the incorporation of 11.7 wt.% Al2O3 in shy‐B expands the stability by ∼400–800 K at 20–30 GPa. The determined dehydration boundary for Al‐bearing phase D indicates that it could be present even at normal mantle geotherm conditions at 30–40 GPa. Up to 23.8 mol.% Al2O3 can be dissolved into the structures of akimotoite and bridgmanite as a result of the decomposition reactions of Al‐bearing shy‐B and phase D between 20 and 40 GPa. Results of further experiments indicate that δ‐AlOOH is the stable hydrous phase coexisting with Al‐depleted bridgmanite at pressures above 52 GPa. This study shows that the incorporation of Al in dense hydrous magnesium silicates can have a profound impact on our picture of the water cycle in the deep Earth.

Sound Velocities of Superhydrous Phase B up to 21 GPa and 900 K

AbstractSound velocities and density of Mg10Si3H4O18 superhydrous phase B (SuB) were investigated by synchrotron X‐rays combined with ultrasonic interferometry techniques in a multi‐anvil apparatus, up to 21 GPa and 900 K. Our data show that SuB has compressional (VP) and shear velocities (VS) lower than major anhydrous minerals but higher than majorite garnet. Our data combined with those of other mantle minerals allowed estimation of VP and VS of hydrated mantle regions at depths of the mantle transition zone and upper lower mantle. The local hydration of pyrolitic mantle with SuB as the main water host could explain low VP and low VS anomalies near cold subduction zones while it would be associated with low VP and high VS anomalies in hot mantle regions. The anomalies may be particularly strong in the ∼30 km beneath the 660 km discontinuity, due to the discrepancy of post‐spinel transition between hydrous and dry pyrolite lithologies.

Partial dehydration of brucite and its implications for water distribution in the subducting oceanic slab

Hydrous minerals within the subducting oceanic slab are important hosts for water. Clarification of the stability field of hydrous minerals helps to understand transport and distribution of water from the surface to the Earth’s interior. We investigated the stability of brucite, a prototype of hydrous minerals, by means of electrical conductivity measurements in both open and closed systems at 3 GPa and temperatures up to 1300 K. Dramatic increase of conductivity in association with characteristic impedance spectra suggests that partial dehydration of single-crystal brucite in the open system with a low water fugacity occurs at 950 K, which is about 300 K lower than those previously defined by phase equilibrium experiments in the closed system. By contrast, brucite completely dehydrates at 1300 K in the closed system, consistent with previous studies. Partial dehydration may generate a highly defective structure but does not lead to the breakdown of brucite to periclase and water immediately. Water activity plays a key role in the stability of hydrous minerals. Low water activity (aH2O) caused by the high wetting behavior of the subducted oceanic slab at the transition zone depth may cause the partial dehydration of the dense hydrous magnesium silicates (DHMSs), which significantly reduces the temperature stability of DHMS (this mechanism has been confirmed by previous study on super hydrous phase B). As a result, the transition zone may serve as a ‘dead zone’ for DHMSs, and most water will be stored in wadsleyite and ringwoodite in the transition zone.

Thermal equation of state of F-bearing superhydrous phase B (Mg10Si3O14(OH,F)4): Implications for the transportation of fluorine and water into the lower mantle

Superhydrous phase B (ShyB), an important hydrous magnesium silicate, plays a key role in the transportation of water from upper to lower mantle via subduction slabs. Moreover, it may also be a potential carrier for another important volatile element: fluorine (F). To explore the influence of F on mantle minerals and its behaviors during subducting, we investigated the compressibility of F-bearing Shy-B using synchrotron-based single-crystal X-ray diffraction combined with diamond anvil cells up to 27 GPa and 750 K. Our results show that the substitution of OH by F can enhance the incompressibility of ShyB. Based on the obtained thermal elastic parameters, density and velocity profiles are evaluated along cold and warm slabs. Our results demonstrate that the addition of F enhances the bulk velocity (~1.0–2.4%) of Shy-B relative to the OH end-member at uppermost lower mantle conditions. The decomposition of F-bearing Shy-B into bridgmanite and periclase would lead to a small increase in bulk velocity (~0.7–1.8%). Thus, the accumulation and decomposition of F-bearing Shy-B is hard to explain the velocity anomaly at the uppermost lower mantle. Our results provide constraints for modeling the geodynamic process related to subduction and transportation of F and H2O into the lower mantle.

Synthesis, structure, and single-crystal elasticity of Al-bearing superhydrous phase B

Abstract Dense hydrous magnesium silicates (DHMSs) with large water contents and wide stability fields are a potential H2O reservoir in the deep Earth. Al-bearing superhydrous phase B (shy-B) with a wider stability field than the Al-free counterpart can play an important role in understanding H2O transport in the Earth’s transition zone and topmost lower mantle. In this study, a nominally Al-free and two different Al-bearing shy-B samples with 0.47(2) and 1.35(4) Al atoms per formula unit (pfu), were synthesized using a rotating multi-anvil press. Their single-crystal structures were investigated by X-ray diffraction (XRD) complemented by Raman spectroscopy and Fourier-transform infrared spectroscopy (FTIR). Single-crystal XRD shows that the cell parameters decrease with increasing Al-content. By combining X-ray diffraction and spectroscopy results, we conclude that the Al-poor shy-B crystallizes in the Pnn2 space group with hydrogen in two different general positions. Based on the results of the single-crystal X-ray diffraction refinements combined with FTIR spectroscopy, three substitutions mechanisms are proposed: 2Al3+ = Mg2+ + Si4+; Mg2+ = ☐Mg2+ + 2H+ (☐Mg2+ means vacancy in Mg site); Si4+ = Al3+ + H+. Thus, in addition to the two general H positions, hydrogen is incorporated into the hydrous mineral via point defects. The elastic stiffness coefficients were measured for the Al-shy-B with 1.35 pfu Al by Brillouin scattering (BS). Al-bearing shy-B shows lower C11, higher C22, and similar C33 when compared to Al-free shy-B. The elastic anisotropy of Al-bearing shy-B is also higher than that of the Al-free composition. Such different elastic properties are due to the effect of lattice contraction as a whole and the specific chemical substitution mechanism that affect bonds strength. Al-bearing shy-B with lower velocity, higher anisotropy, and wider thermodynamic stability can help understand the low-velocity zone and the high-anisotropy region in the subducted slab located in Tonga.

Melting of Bridgmanite Under Hydrous Shallow Lower Mantle Conditions

AbstractHigh pressure and temperature experiments were carried out on the oxide mixtures corresponding to the bridgmanite stoichiometry under the hydrous shallow lower mantle conditions (24–25 GPa and 1673–1873 K with 5–10 wt. % of water in the starting material). Oxide mixtures investigated correspond to MgSiO3, (Mg, Fe)SiO3, (Mg, Al, Si)O3, and (Mg, Fe, Al, Si)O3. Melting was observed in all runs. Partitioning of various elements, including Mg, Fe, Si, and H is investigated. Melting under hydrous lower mantle conditions leads to increased (Mg + Fe)O/SiO2in the melt compared to the residual solids. The residual solids often contain a large amount of stishovite, and the melt contains higher (Mg,Fe)O/SiO2ratio than the initial material. (Mg + Fe)O‐rich hydrous melt could explain the low‐velocity anomalies observed in the shallow lower mantle and a large amount of stishovite in the residual solid may be responsible for the scattering of seismic waves in the mid‐lower mantle and may explain the “stishovite paradox." Since stishovite‐rich materials are formed only when silica‐rich source rock (MORB) is melted (not a typical peridotitic rock [bulk silicate Earth]), seismic scattering in the lower mantle provides a clue on the circulation of subducted MORB materials. To estimate hydrogen content, we use a new method of estimating the water content of unquenchable melts, and also propose a new interpretation of the significance of superhydrous phase B inclusions in bridgmanite. The results provide revised values of water partitioning between solid minerals and hydrous melts that are substantially higher than previous estimates.

Thermal equation of state of F-bearing superhydrous phase B (Mg10Si3O14(OH,F)4): Implications for the transportation of fluorine and water into Earth's lower mantle

Thermal equation of state of F-bearing superhydrous phase B (Mg10Si3O14(OH,F)4): Implications for the transportation of fluorine and water into Earth's lower mantle

Incorporation of Ti into the crystal structures of the high-pressure dense silicates anhydrous phase B and superhydrous phase B

The structures of Ti-bearing anhydrous phase B and superhydrous phase B synthesized at 12 GPa/1600 °C and 21 GPa/1600 °C, respectively, have been determined by single-crystal X-ray diffraction. Anhydrous phase B has space group Pmcb. The structure of superhydrous phase B was refined in space groups Pnnm and Pnn2; crystallographic arguments unambiguously indicated that the centrosymmetric space group Pnnm is the correct choice for the crystal studied here. Ti orders at the octahedrally coordinated Si(1) site in both phases. The refined site occupancies at the Si(1) sites of anhydrous phase B and superhydrous phase B are 0.848(3) Si + 0.152 Ti and 0.92(1) + 0.08 Ti, respectively. These Ti occupancy levels correlate with a 4% expansion of the Si(1)O6 octahedron relative to values typically observed for end-member anhydrous phase B, Fe2+-bearing anhydrous phase B, and end-member superhydrous phase B. Ordering of Ti at the Si(1) is explained in terms of a homovalent substitution VISi → VITi4+, being the only option for these structures. There is no evidence for a deprotonation substitution of the kind Mg + 2OH− → Ti4+ + 2O2−, as occurs in humites. Synthesis of Ti-bearing superhydrous phase B indicates that it is stable at typical P–T conditions of the mantle transition zone and the lower mantle. Hence, water can be transported by Ti-bearing superhydrous B into the lower mantle even at temperatures of the normal mantle geotherm.

Stability of Al-bearing superhydrous phase B at the mantle transition zone and the uppermost lower mantle

Stability of Al-bearing superhydrous phase B at the mantle transition zone and the uppermost lower mantle

Elasticity of superhydrous phase B at the mantle temperatures and pressures: Implications for 800 km discontinuity and water flow into the lower mantle

AbstractThe thermodynamic properties and elasticity of superhydrous phase B (ShyB) at high pressure and temperature are calculated using first‐principles calculations based on density functional theory. The velocities and densities of ShyB are significantly lower than those of bridgmanite and periclase, the major minerals in the lower mantle. The anisotropy of ShyB is unremarkable at relevant mantle conditions. The decomposition of ShyB into bridgmanite, periclase, and water, which can occur at a depth around 800 km at a cold slab, will cause an increase of 7.5%, 15.0%, and 12% on shear velocity, compressional velocity, and density, respectively. Thus, the decomposition of a small amount of ShyB can produce a local discontinuity at the depth of ~800 km. The water released from the decomposition of ShyB may rise upward and promote the partial melt, which can further explain the low‐velocity zones just above 800 km discontinuity in Western‐Pacific Subduction Zones.

Elasticity of single‐crystal superhydrous phase B at simultaneous high pressure‐temperature conditions

AbstractWe investigated the combined effect of pressure and temperature on the elasticity of single‐crystal superhydrous phase B (Shy‐B) using Brillouin scattering and X‐ray diffraction up to 12 GPa and 700 K. Using the obtained elasticity, we modeled the anisotropy of Shy‐B along slab geotherms, showing that Shy‐B has a low anisotropy and cannot be the major cause of the observed anisotropy in the region. Modeled velocities of Shy‐B show that Shy‐B will be shown as positive velocity anomalies at the bottom transition zone. Once Shy‐B is transported to the topmost lower mantle, it will exhibit a seismic signature of lower velocities than topmost lower mantle. We speculate that an accumulation of hydrous phases, such as Shy‐B, at the topmost lower mantle with a weight percentage of ~17–26% in the peridotite layer as subduction progresses could help explain the observed 2–3% low shear velocity anomalies in the region.

Experimental study of the electrical conductivity of hydrous minerals in the crust and the mantle under high pressure and high temperature

Abstract Hydrous minerals are important water carriers in the crust and the mantle, especially in the subduction zone. With the recent development of the experimental technique, studies of the electrical conductivity of hydrous silicate minerals under controlled temperature, pressure and oxygen fugacity, have helped to constrain the water distribution in the Earth’s interior. This paper introduces high pressure and temperature experimental study of electrical conductivity measurement of hydrous minerals such as serpentine, talc, brucite, phase A, super hydrous phase B and phase D, and assesses the data quality of the above minerals. The dehydration effect and the pressure effect on the bulk conductivity of the hydrous minerals are specifically emphasized. The conduction mechanism of hydrous minerals and the electrical structure of the subduction zone are discussed based on the available conductivity data. Finally, the potential research fields of the electrical conductivity of hydrous minerals is presented.

Elasticity of superhydrous phase B, seismic anomalies in cold slabs and implications for deep water transport

Seismic anomalies in the vicinity of cold slabs, including low velocity zones and enhanced seismic shear wave splitting have been commonly attributed to the presence of hydrated slab material. The dense hydrous magnesium silicate (DHMS) superhydrous phase B (ShyB, Mg10Si3H4O18) is considered an important water carrier to transition zone depth due to its large pressure stability and abundance (up to 20vol.%) in hydrous peridotites. To interpret the observed seismic anomalies in terms of hydration and to assess the role of ShyB in deep water recycling, we have investigated the sound velocities and single-crystal elasticity of ShyB to pressures of 17.5GPa at ambient temperature by Brillouin scattering spectroscopy in diamond anvil cells. The Voigt–Reuss–Hill averages for the adiabatic bulk moduli KS, shear moduli μ and their pressure derivatives yield KS=150(2)GPa, μ=99(1)GPa, (∂KS/∂P)=4.7(2) and (∂μ/∂P)=1.44(5). The aggregate compressional and shear wave velocities of ShyB at transition zone pressures are comparable to those of hydrous iron-bearing ringwoodite but are significantly lower than anhydrous phases in peridotites. The calculated velocity contrast between dry and ShyB-bearing hydrous peridotite (containing 1.2wt.% of water) indicate that 17vol.% of ShyB decreases both compressional VP and shear VS wave velocities by at most 2.5% at transition zone depths. Our results, combined with data for the deformation mechanisms of ShyB, indicate that ShyB aggregates develop strong textures under down-dip compression regime and yield a maximum shear wave splitting of 1.1% in the plane perpendicular to the compression axis at 17.5GPa. Although ShyB in hydrous subducted peridotite would reduce its seismic velocities, the splitting geometry VSV>VSH generated by ShyB fabrics in peridotite is incompatible with the pattern observed in Tonga and Sangihe, and would require the contribution of other phases to be explained. Textured phase D is a plausible candidate to explain the shear wave splitting in the lower transition zone, while at intermediate transition zone depths, the coexistence of textured phase D and ShyB in hydrous deformed peridotite would generate the observed array polarization. However, the later scenario will require locally very high water concentrations at depth.

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.

Erratum to: Polymorphic phase transition in Superhydrous Phase B

the original publication of the article reported polarized FtIr spectra taken on oriented superhydrous phase B crystals. unfortunately, the authors mixed up the crystallographic cand a-axes during their measurement. the atomic coordinates for hydrogen H2 should be x = 0.45; y = 0.18; z = 0.27. thus, spectra in Fig. 10a are taken on the (001) plane with E parallel b (0°) and spectra in Fig. 10b are taken on the (010) plane with E parallel c (0°).

Stability and crystal chemistry of iron-bearing dense hydrous magnesium silicates

Dense hydrous magnesium silicates (DHMS) are supposed to be key phases in planetary water cycles because of their ability to carry water to deep mantle regions in subduction slab environments. In order to understand water cycles in iron-enriched planetary systems such as Mars knowledge of the water content and stability of iron-bearing DHMS is required. Iron-bearing DHMS were synthesized based on two starting compositions, MgFeSiO4+9.5wt% H2O system and a simple hydrous Martian mantle composition containing Fe, Mg, Al and Si+12.35wt% H2O (hydrous FMAS system). Compared to literature data on phase D, iron-bearing phase D shows analogous variations in water contents as Mg-phase D but appears to be stable at higher temperatures than Mg-phase D for both starting compositions used in this study. Iron-bearing superhydrous phase B contains up to 7wt% H2O and shows an extended thermal stability in the hydrous FMAS system. The high-temperature stability of iron-bearing DHMS with a Mars-like bulk composition indicates that these hydrous phases could host significant amounts of water at core-mantle boundary conditions (1500°C and 23GPa) in a hydrous Martian mantle.

The determination of hydrogen positions in superhydrous phase B

Nominally hydrous high-pressure silicate phases such as the superhydrous phase B are of considerable importance for the understanding of the water-cycle between the surface and the interior of the Earth. This study tackles the controversial issue of hydrogen positions in superhydrous phase B, a phase believed to be potentially stable in cold subducting ultramafic slabs. To investigate the nature of hydrogen incorporation into the structure of superhydrous phase B, neutron powder diffraction experiments have been performed. A structural model based on Pnn2 symmetry has been used for the analysis of the data, which is consistent with earlier spectroscopic studies. Application of Fourier synthesis with subsequent analyses of difference nuclear density maps and Rietveld fits reveal two distinct positions for deuterium, at 4c (0.194, 0.052, 0.596) and at 4c (0.186, 0.119, 0.388). This unambiguously shows that deuterium lies within large channels, which are formed between the edgeshared octahedra and vertex-linked tetrahedra along the b-axis of the structure. These results contrast with recent polarized single-crystal infrared spectroscopy studies where the position of one of two H atoms was estimated to lie close to the octahedral edge of an MgO6 octahedron, thereby leaving the large structural channel empty.

Electrical conductivity of dense hydrous magnesium silicates with implication for conductivity in the stagnant slab

Electrical conductivities of dense hydrous magnesium silicates (DHMS), phase A, super-hydrous phase B (SuB) and phase D, were measured by means of impedance spectroscopy in the frequency range of 10−1–106Hz at temperatures up to 775, 700 and 700K and pressures of 10, 18 and 22GPa, respectively. For all phases, the increase in electrical conductivity (σ) with temperature follows the Arrhenian formula: σ=σ0exp(−(ΔH/kT)). The pre-exponential factors (σ0) and activation enthalpies (ΔH) of phase A, SuB and phase D yield values of 7.28±0.82S/m and 0.77±0.01eV, 292±48S/m and 0.83±0.01eV and 1342±154 and 0.75±0.01eV, respectively. Higher pressure DHMS phases show higher conductivity values. The electrical conductivities of phase D and super hydrous phase B are about two and one orders of magnitude higher than that of phase A in the same temperature range, respectively. Although the proton conduction is considered to be a dominant mechanism, there is no clear relationship between water content and conductivity. Rather the conductivity of DHMS phase is closely related to the O⋯O distance. The conductivity–depth profiles for a cold subduction zone were constructed based on the phase proportion predicted in the descending slab. The results show distinctly lower conductivity values than those geophysically observed beneath the northeastern China and the Philippine Sea, where the cold slab stagnates in the mantle transition zone. Consequently, the DHMS phases themselves cannot be a main contributor to enhance the conductivity in the stagnant slab. Dehydration of the stagnant slab would strongly enhance the conductivities in the transition zone beneath northeastern China and Philippine Sea.