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Ringwoodite (γ-Mg2SiO4) is the stable polymorph of olivine in the transition zone between 525–660 km depth, and can incorporate weight percent amounts
of H2O as hydroxyl, with charge compensated mainly by Mg vacancies (Mg2+ = 2H+), but also possibly as (Si4+ = 4H+ and Mg2+ + 2H+ = Si4+). We synthesized pure Mg ringwoodite containing 2.5(3) wt% H2O,...
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... 3 . Raman and FTIR spectra of sample SZ0820T are shown in Figure 1 for characterization purposes. Whereas the water content of ringwoodite SZ0820T from FTIR using the cali- bration of Libowitzky and Rossman (1997) Koch-Müller and Rhede (2010), we obtain a value of 2.3 wt% H 2 O, which is in good agreement with estimates of 2.5(6) wt% H 2 O from the lattice parameter ( Smyth et al. 2003). ...
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... volumes as a function of temperature for measure- ments at ambient pressures are plotted in Figure 10, with second- order polynomial fitting of the measured data up to 586 K as in Equation 1. From 638 K and above, the measured unit-cell volumes are significantly above the extrapolation of the fitting curve, consistent with irreversible expansion starting at about 606 K for this sample ( Ye et al. 2009). The mean coefficient F 0 is 40(4) w10 ...
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... polyhedral volumes at various temperatures are plotted in Figure 11, normalized to the values at lowest experimental temperature of 143 K. V(Si) is found to expand significantly and abruptly above the onset of irreversible ex- pansion at 586 K. The F 0 for V(Si) is 20(3) w10 6 /K (143-586 K), while a much larger value of 132(4) w10 6 /K (586-736 K). (3) 100 529.5(5) 159(7) 6.7 (7) 29.4 ...
Citations
... Several high-temperature XRD measurements have been conducted on wet forsterite (Mg-pure olivine, Fo 100 ), wadsleyite and ringwoodite (Ye et al., 2012(Ye et al., , 2011(Ye et al., , 2010Inoue et al., 2004), as well as the DHMS phases on the brucite (Bru)olivine (Ol) join: brucite (Fukui et al., 2003;Fei and Mao, 1993;Redfern and Wood, 1992), Phase A (Pawley et al., 1995), chondrodite (Ye et al., 2015) and clinohumite (Qin et al., 2017;Ye et al., 2015). In these experiments, the thermal expansion coefficients at ambient pressure are typically simplified as a linear function of temperature (α = a 0 + a 1 ×T), and sometimes even used as constant α 0 values (averaged), which would cause serious deviation from reality at high temperatures (above 1 000°C). ...
... Dehydration happened in the wet wadsleyite samples (C H2O = 1.6 wt.% and 2.8 wt.% ) around T = 630 K at P = 0 GPa, as indicated by the phenomenon that the unit-cell volumes decrease abruptly (Ye et al., 2011(Ye et al., , 2009. Irreversible expansion was observed in wet ringwoodite samples (C H2O = 0.7 wt.% and 2.4 wt.% ) (Ye et al., 2012(Ye et al., , 2009, which was latterly explained by the Mg 2+ (octahedral) -Si 4+ (tetragonal) cation disordering, as triggered by high temperatures and coupled with protonation in the lattices (Liu et al., 2020). Inoue et al. (2004) conducted high-T powder XRD on wet and dry wadsleyite and ringwoodite, and found that hydration decreases the thermal expansivities for these phases. ...
... Isothermal compression experiments have been reported for the DHMS and NAM phases in the peridotite system: including brucite (Xia et al., 1998;Catti et al., 1995;Duffy et al., 1995;Parise et al., 1994;Fei and Mao, 1993), phase A (Holl et al., 2006;Kuribayashi et al., 2003;Crichton and Ross, 2002;Kudoh et al., 2002;Pawley et al., 1995), chondrodite (Kuribayashi et al., 2004;Friedrich et al., 2002;Ross and Crichton, 2001), clinohumite (Qin et al., 2017;Ross and Crichton, 2001), superhydrous phase B (Litasov et al., 2007a;Inoue et al., 2006;Shieh et al., 2000a;Crichton et al., 1999;Kudoh et al., 1994), phase E (Crichton and Ross, 2000;Shieh et al., 2000b), phase D (Chang et al., 2013;Hushur et al., 2011;Litasov et al., 2008Litasov et al., , 2007bShinmei et al., 2008;Frost and Fei, 1999), as well as the Mg 2 SiO 4 polymorphs of olivine (Finkelstein et al., 2014;Manghnani et al., 2013;Zhang, 1998;Downs et al., 1996;Andrault et al., 1995;Will et al., 1986;Kudoh and Takeuchi, 1985), wadsleyite (Ye et al., 2010;Katsura et al., 2009;Holl et al., 2008;Li et al., 2001;Hazen et al., 2000) and ringwoodite (Ye et al., 2012;Katsura et al., 2004;Smyth et al., 2004;Meng et al., 1994;Hazen, 1993). ...
Water in the deep Earth’s interior has important and profound impacts on the geodynamical properties at high-temperature (T) and high-pressure (P) conditions. A series of dense hydrous Mg-silicate (DHMS) phases are generated from dehydration of serpentines in subduction slabs below the lithosphere, including phase A, chondrodite, clinohumite, phase E, superhydrous phase B and phase D. On the other hand, olivine and its high-P polymorphs of wadsleyite and ringwoodite are dominant nominally anhydrous minerals (NAMs) in the upper mantle and transition zone, which could contain significant amount of water in the forms of hydroxyl group (OH−) defects. The water solubilities in wadsleyite and ringwoodite are up to about 3 weight percent (wt.%), making the transition zone a most important layer for water storage in the mantle. Hydration can significantly affect the pressure-volume-temperature equations of state (P-V-T EOSs) for the DHMS and NAM phases, including the thermal expansivities and isothermal bulk moduli. In this work, we collected the reported datasets for the DHMS and NAM phases, and reconstruct internally consistent EOSs. Next, we further evaluated the thermodynamic Grüneisen parameters, which are fundamental for constraining the temperature distribution in an isentropic process, such as mantle convection. The adiabatic temperature profiles are computed for these minerals in the geological settings of normal mantle and subduction zone, and our calculation indicates that temperature is the dominant factor in determining the gradient of a geotherm, rather than the mineralogical composition.
... From each batch, one representative crystal was selected, and double-side polished to a thickness of for the FTIR measurements. The integrated absorbance of the spectral range was converted into water concentration by employing the Lambert-Beer law (4.4.8), with the molar absorption coefficient reported in Thomas et al. (2015) , and the density reported by Ye et al. (2012) . For cubic minerals, the integrated absorbance area measured in one crystallographic direction has to be multiplied by three (Thomas et al., 2015). ...
... The integrated absorbance was computed in the spectral range. The Lambert-Beer law was applied by considering (Thomas et al. 2015), (Ye et al. 2012), and the data reported in Table 5.2. Ringwoodite is a cubic mineral, therefore was measured in one crystallographic direction and then multiplied by three. ...
... Pressure-and waterdependence of were based on the empirical equation derived in section 5.5.3. In order to properly study thermal evolution of the slab crossing the it is fundamental to use a -dependent (Xu et al., 2004), (Ye et al., 2012) and (Dorogokupets et al., 2015) to calculate thermal diffusivity (see section 5.8.3). As an approximation, two end-member parameters settings were used in our simulations: one given by the lowest temperature in the slab core ( ), and the other given by highest temperature achievable by the slab, i.e. the ambient mantle temperature ( ). ...
A Combined Study on Earth's Deep Water Cycle using Numerical Modelling and Laboratory Experiments. The distinctive feature of Earth’s surface compared to other known planets is the abundance of liquid water. This water was delivered during the accretion stage of the planet by rocky asteroids, with minor contributions from comets and protosolar nebular gas. Experimental evidence shows that water can be incorporated into many of the minerals that make up Earth’s interior. When water is hosted in the crystalline structures, it alters the physical properties of minerals, thereby enhancing deformation processes. Therefore, water-bearing rocks are less dense and weaker compared to their dry counterparts. Geophysical observations and natural samples reveal that water is indeed present in Earth’s mantle, mostly concentrated in the mantle transition zone. The region is bounded by two seismic discontinuities, at 410 km and 660 km depth, and is characterized by the presence of two minerals with high water solubility: wadsleyite and ringwoodite. Water is carried into Earth’s interior by the subduction of oceanic lithosphere, i.e. slabs. This water-delivery mechanism is also known as the ‘deep water cycle,’ and might represent the key element for the onset of plate tectonics on Earth. One essential tool to explore the role of water in plate tectonics is numerical modelling. With this technique, it is possible to reproduce many physical phenomena occurring on Earth by self-consistently simulating mantle convection. This is achieved by solving the governing equations of mass, momentum and energy conservation, also known as Stokes equations. The complexity of these equations requires the use of approximation methods, like Finite Difference (FD), to solve the derivatives over time and space. The feedback loop between mineral physics constraints, numerical modelling and geophysical observations represents the best strategy to unravel Earth’s interior. However, despite the efforts of geoscientists, many questions regarding the deep Earth water cycle remain so far unanswered. This thesis focuses on the hydration state of the MTZ, with three aims addressing different aspects of the topic: (1) provide mineral physics measurements on the effect of water on ringwoodite thermal conductivity; (2) produce a model featuring an Earth-like mobile lid while minimizing the effects of numerical parameters; and (3) analyse the parameters that allow for the stagnation of a slab in the MTZ, which may lead to the water enrichment in this region In project (1), hydrous ringwoodite crystals were synthesized with multi-anvil experiments and characterized by X-ray diffraction, electron-microprobe analysis, and infrared spectroscopy. The samples were loaded into a diamond anvil cell to perform measurements at the high-pressure conditions of the MTZ. The thermal conductivity of ringwoodite, Λ(Rw) was measured with the time-domain thermo-reflectance method. It was found that the presence of 1.73 wt% water reduces Λ(Rw) by 40%. From this analysis, it was possible to derive a parameterized equation to extrapolate Λ(Rw) as a function of pressure and water content. With this tool, the large-scale thermal evolution of a slab was studied. The calculations were performed by assuming a slab stagnating in the MTZ, then being progressively heated by the warm ambient mantle. A 1D FD numerical code was designed to solve the heat diffusion equation, and the derived equation for Λ(Rw) was included into the physical model of the slab. The results reveal that hydrous ringwoodite hinders the heating of the slab, thus promoting the survival of water-bearing minerals. In project (2), a global-scale model to reproduce self-consistently plate-like behaviour was designed. The models were computed with StagYY in a 2D spherical annulus geometry. The tectonic regime of a planet is controlled by the yield strength of the lithosphere τy. Four main tectonic regimes can be identified in nature: micro-plate dripping, plate-like, episodic resurfacing, and stagnant lid. It was found that the tectonic regime in the models is heavily influenced by the grid resolution used for the discretization. The modelled lithosphere is weakened by reducing the horizontal grid spacing ∆w, while it becomes stronger when reducing the vertical grid spacing ∆r. These effects are numerical in nature, and are the consequence of the interpolation of the Stokes equations. It was found that the best results are achieved by accurately resolving the lithosphere, i.e. ∆w ≤40 km and ∆r ≤15 km. In project (3), the interactions between the cold descending slab and the 660 km discontinuity were analysed. The models were computed with StagYY in a 2D spherical annulus geometry. From the models it can be inferred that the stagnation of a slab in the MTZ is controlled by three parameters: (i) the density jump ∆ρ, which enhances the slab pull, and affects the latent heat absorbed by the post-spinel reaction; (ii) the negative Clapeyron slope Υ, which causes a downward deflection of the phase transition; and (iii) the viscosity jump ∆η, which decelerates the descent of the slab.
... Initial speculation of a transition zone water reservoir by Smyth in 1987 was later met with experimental evidence that ringwoodite (and wadsleyite) could host up to 2.7-3.1 wt% H 2 O (Inoue, 1994;Kohlstedt et al., 1996). Several studies have focused on the synthesis and investigation of hydrous ringwoodite to monitor the effect of water on its physical properties and extract larger-scale geophysical implications (e.g., Fei et al., 2017;Jacobsen & Smyth, 2006;Mao et al., 2012;Schulze et al., 2018;Ye et al., 2012). ...
Some rare diamonds originate below the lithosphere, from depths of 300–800 km and perhaps deeper. Ongoing sublithospheric or super‐deep diamond research is providing new insight into the mantle and the hidden consequences of plate tectonics. Here we highlight several advances in the past decade, stemming from the discovery of inclusions from oceanic crust at lower mantle depths; inclusions having geochemical imprints of low‐degree carbonatitic melt, possibly from subducted slabs; hydrous ringwoodite and other signs of deep water; major mantle minerals preserved in their original crystal structure, including ringwoodite and CaSiO3‐perovskite; additional diamond varieties with a super‐deep origin (CLIPPIR and type IIb diamonds), greatly increasing the known prevalence and diversity of super‐deep diamonds; and consistent, recurring Fe‐Ni‐C‐S metallic melt inclusions from depths of 360–750 km. Redox freezing of oxidized, slab‐derived fluid/melt upon interaction with ambient metal‐saturated mantle appears to be a phenomenon broadly recorded by many super‐deep diamonds. Melting of carbonate, as well as dehydration reactions, from subducted slabs are relevant mechanisms that may generate fluid/melt contributing to diamond growth. Fe‐Ni metal, with dissolved carbon, sulfur, and other elements is also indicated as a possible diamond‐forming melt. These mobile and dynamic phases are agents of chemical mass‐transfer in the deep mantle.
... At ambient conditions, C 11 , C 12 , C 44 , K S , and G of Mg 15 H 2 Si 8 O 32 ringwoodite are 8-10% lower than those of Mg 2 SiO 4 ringwoodite (Núñez Valdez et al., 2012). Elastic moduli show noticeable nonlinear dependences on pressure, which (Chang et al., 2015;Jacobsen et al., 2004;Jacobsen and Smyth, 2006;Mao et al., 2012;Schulze et al., 2018;Wang et al., 2006;Ye et al., 2012) (scattered points). Solid lines represent the density of hydrous ringwoodite with different water and Fe contents using linear interpolation from calculated results of Mg 15 Si 8 O 30 (OH) 2 ringwoodite in this study and anhydrous ringwoodite (Fo100 and Fo87.5) in previous work (Núñez Valdez et al., 2012). ...
... The calculated density and elastic properties of hydrous ringwoodite with variable iron and water contents are compared with experimental results in Fig. 2 and Fig. 3, respectively. Our results for the density of hydrous ringwoodite show excellent agreement with previous experimental data (Chang et al., 2015;Jacobsen et al., 2004;Jacobsen and Smyth, 2006;Mao et al., 2012;Schulze et al., 2018;Wang et al., 2006;Ye et al., 2012), with a discrepancy between theoretical and experimental results of less than 1.3% (Fig. 2). This small difference is likely caused by two factors: the mild underestimated volume from LDA calculations (Núñez- Valdez et al., 2013;Núñez Valdez et al., 2012;Wang et al., 2019) and the uncertainties of iron and water contents in experimental samples (Chang et al., 2015). ...
The mantle transition zone (MTZ) is potentially a geochemical water reservoir because of the high H2O solubility in its dominant minerals, wadsleyite and ringwoodite. Whether the MTZ is wet or dry fundamentally impacts our understanding of the deep-water distribution, geochemical recycling, and the pattern of mantle convection. However, the water content in the MTZ inferred from previous studies remains disputed. Seismic observations such as velocity anomalies were used to evaluate the water content in the MTZ, but the hydration effect on the velocities of MTZ minerals under appropriate pressure (P) and temperature (T) conditions is poorly constrained. Here we investigated the elastic properties and velocities of hydrous ringwoodite at high P-T conditions using first-principles calculations. Our results show that the hydration effects on elastic moduli and velocities of ringwoodite are significantly reduced by pressure but strongly enhanced by temperature. The incorporation of 1.0 wt% water into ringwoodite decreases the compressional and shear velocities of the pyrolitic mantle by −1.0% and −1.4% at the conditions of MTZ, respectively. Using results from seismic tomography and together with the topography of the 660-km discontinuity, we evaluate the global distribution of water in the lower MTZ. We find that about 80% of the MTZ can be explained by varying water content and temperature, however, the remaining 20% requires the presence of high-velocity heterogeneities such as harzburgite. Our models suggest an average water concentration of ∼0.2 wt% in the lower MTZ, with an interregional variation from 0 to 0.9 wt%. Together with our previous work, we conclude that the water concentration in the MTZ likely decreases with depth globally and the whole MTZ contains the equivalent of about one ocean mass of water.
... Pressure and water dependence of Λ Rw were based on the empirical equation derived in this study. In order to properly study thermal evolution of the slab crossing the MTZ it is fundamental to use a P-T-dependent Λ Rw (Xu et al., 2004), ρ Rw (Ye et al., 2012) and C P Rw (Dorogokupets et al., 2015;Saxena, 1996) to calculate thermal diffusivity κ (Text S2). As an approximation, two end-member parameters settings were used in our simulations: one given by the lowest temperature in the slab core (1021 K), and the other given by the ambient mantle temperature (1600 K). ...
Plain Language Summary
The physical properties of minerals are determined by the interaction of atoms in the crystal lattice. Water can be incorporated into the crystal structure and alter its behavior. Ringwoodite is a high‐pressure mineral that can host large quantities of water and is expected to be abundant in the lower part of Earth's mantle transition zone, a region ranging from 520 to 660‐km depth. Here we studied ringwoodite thermal conductivity, describing how effectively heat is transported through solids. Based on our measurements we determined that water in ringwoodite significantly slows down heat propagation. We performed computer simulations to investigate the large‐scale implications of our findings. For this purpose, we modeled a cold oceanic plate, entirely made of ringwoodite, which is surrounded by warm mantle. The delayed heat transport is sufficient to maintain low temperatures in the inner part of the oceanic plate and potentially preserve the hydrous minerals for an extended period of time.
... Unfortunately, a large number of variables, such as the chemical composition of Rw [5][6][7], the order-disorder state of its cations [8][9][10] and the incorporation mechanism and content of its water [11,12], can all affect these properties. As to water in Rw, experimental studies have not only indicated that Rw coexisting with a hydrous fluid/melt phase at the P-T conditions of the lower part of the mantle transition zone can host large amounts of water [13][14][15], but also demonstrated that water in Rw can strongly affect the phase relationship [16,17], the melting behavior [18,19], the thermal expansivity [20][21][22][23], the compressibility [15,24,25], the strength and rheology behavior [26][27][28], the seismic velocity [29][30][31], the electrical conductivity [32][33][34], the thermal conductivity [35,36], etc. Since field observation on the Rw inclusions in some diamonds with deep origin has demonstrated significant amounts of water in the Rw structure [37,38], a good understanding about the water incorporation mechanism and solubility bears on important geological implications. ...
... A large number of experimental investigations and theoretical studies have been conducted to explore the issues relevant to the water incorporation mechanism and solubility in Rw, and much knowledge has been obtained [39][40][41][42]. Down the road of the experimental investigations, different techniques, such as single-crystal X-ray diffraction [22,23,43,44], neutron diffraction [45], Raman spectroscopy [46][47][48], infrared spectroscopy [12,[49][50][51][52][53][54], 29 Si NMR-CPMAS spectroscopy [55], protonproton scattering [38], elastic recoil detection analysis [56] and secondary ion mass spectroscopy [11,14,57], have been employed. Among these experimental techniques, infrared spectroscopy plays a key role due to some special advantages. ...
... where is the concentration of H2O in weight (ppm), Atot is the total integrated absorption, which is calculated as three times the integrated absorption (cm −1 ) in an unpolarized IR spectrum, d is the thickness of the crystal thin section (cm), is the Rw density (g/cm 3 ) and is the total integrated molar absorption coefficient (L·mol −1 ·cm −2 ). For quantifying the water contents of our Mg-Rw, we used = 3.527 g/cm 3 from Ye et al. (for the Mg-Rw with ~0.76 wt% H2O) [23], and = 118,500(5000) L·mol −1 ·cm −2 from Bolfan-Casanova et al. [56]. We integrated the unpolarized IR spectrum in the energy range 3730-2000 cm −1 to derive the integrated absorption, which was in turn multiplied by 3 to obtain the total integrated absorption [56]. ...
Three batches of Mg2SiO4-ringwoodites (Mg-Rw) with different water contents (C-H2O = ~1019(238), 5500(229) and 16,307(1219) ppm) were synthesized by using conventional high-P experimental techniques. Thirteen thin sections with different thicknesses (~14–113 μm) were prepared from them and examined for water-related IR peaks using unpolarized infrared spectra at ambient P-T conditions, leading to the observation of 15 IR peaks at ~3682, 3407, 3348, 3278, 3100, 2849, 2660, 2556, 2448, 1352, 1347, 1307, 1282, 1194 and 1186 cm−1. These IR peaks suggest multiple types of hydrogen defects in hydrous Mg-Rw. We have attributed the IR peaks at ~3680, 3650–3000 and 3000–2000 cm−1, respectively, to the hydrogen defects [VSi(OH)4], [VMg(OH)2MgSiSiMg] and [VMg(OH)2]. Combining these IR features with the chemical characteristics of hydrous Rw, we have revealed that the hydrogen defects [VMg(OH)2MgSiSiMg] are dominant in hydrous Rw at high P-T conditions, and the defects [VSi(OH)4] and [VMg(OH)2] play negligible roles. Extensive IR measurements were performed on seven thin sections annealed for several times at T of 200–600 °C and quickly quenched to room T. They display many significant variations, including an absorption enhancement of the peak at ~3680 cm−1, two new peaks occurring at ~3510 and 3461 cm−1, remarkable intensifications of the peaks at ~3405 and 3345 cm−1 and significant absorption reductions of the peaks at ~2500 cm−1. These phenomena imply significant hydrogen migration among different crystallographic sites and rearrangement of the O-H dipoles in hydrous Mg-Rw at high T. From the IR spectra obtained for hydrous Rw both unannealed and annealed at high T, we further infer that substantial amounts of cation disorder should be present in hydrous Rw at the P-T conditions of the mantle transition zone, as required by the formation of the hydrogen defects [VMg(OH)2MgSiSiMg]. The Mg-Si disorder may have very large effects on the physical and chemical properties of Rw, as exampled by its disproportional effects on the unit-cell volume and thermal expansivity.
... Wadsleyite can incorporate varying amounts of water up to 3.3% as hydroxyl (OH) groups in the structure depending on the pressure and temperature and phase conditions [1][2][3][4]. The hydration of wadsleyite takes place by two H atoms substituting for one octahedral Mg in the structure [1,4,[5][6][7][8][9][10][11] that affects the elasticity when compared to the anhydrous phase, and most notably at high pressure (P) and temperature (T) due to the variability of the H atomic radius. ...
We measured the elastic velocities of a synthetic polycrystalline β-Mg2SiO4 containing 0.73 wt.% H2O to 10 GPa and 600 K using ultrasonic interferometry combined with synchrotron X-radiation. Third-order Eulerian finite strain analysis of the high P and T data set yielded Kso = 161.5(2) GPa, Go = 101.6(1) GPa, and (∂Ks/∂P)T = 4.84(4), (∂G/∂P)T = 1.68(2) indistinguishable from Kso = 161.1(3) GPa, Go = 101.4(1) GPa, and (∂Ks/∂P)T = 4.93(4), (∂G/∂P)T = 1.73(2) from the linear fit. The hydration of the wadsleyite by 0.73 wt.% decreases Ks and G moduli by 5.3% and 8.6%, respectively, but no measurable effect was noted for (∂Ks/∂P)T and (∂G/∂P)T. The temperature derivatives of the Ks and G moduli from the finite strain analysis (∂KS/∂T)P = −0.013(2) GPaK−1, (∂G/∂T)P = −0.015(0.4) GPaK−1, and the linear fit (∂KS/∂T)P = −0.015(1) GPaK−1, (∂G/∂T)P = −0.016(1) GPaK−1 are in agreement, and both data sets indicating the |(∂G/∂T)P| to be greater than |(∂KS/∂T)P|. Calculations yield ∆Vp(α-β) = 9.88% and ∆VS(α-β) = 8.70% for the hydrous β-Mg2SiO4 and hydrous α-Mg2SiO4, implying 46–52% olivine volume content in the Earth's mantle to satisfy the seismic velocity contrast ∆Vs = ∆VP = 4.6% at the 410 km depth.
... Å 3 and density ρ = 3.557(4) g/cm 3 . The unit-cell volume of our hydrous pyrope at ambient conditions is ~0.15% higher than anhydrous pyrope Du et al. 2015), which agrees with previous studies for other mantle minerals (e.g., Smyth et al. 2003;Smyth and Jacobsen 2006;Ye et al. 2010Ye et al. , 2012. ...
The elasticity of single-crystal hydrous pyrope with ~900 ppmw H2O has been derived from sound velocity and density measurements using in situ Brillouin light spectroscopy (BLS) and synchrotron X-ray diffraction (XRD) in the diamond anvil cell (DAC) up to 18.6 GPa at room temperature and up to 700 K at ambient pressure. These experimental results are used to evaluate the effect of hydration on the single-crystal elasticity of pyrope at high pressure and high temperature (P-T) conditions to better understand its velocity profiles and anisotropies in the upper mantle. Analysis of the results shows that all of the elastic moduli increase almost linearly with increasing pressure at room temperature, and decrease linearly with increasing temperature at ambient pressure. At ambient conditions, the aggregate adiabatic bulk and shear moduli (Ks0, G0) are 168.6(4) GPa and 92.0(3) GPa, respectively. Compared to anhydrous pyrope, the presence of ~900 ppmw H2O in pyrope does not significantly affect its Ks0 and G0 within their uncertainties. Using the third-order Eulerian finite-strain equation to model the elasticity data, the pressure derivatives of the bulk [(∂KS/∂P)T] and shear moduli [(∂G/∂P)T] at 300 K are derived as 4.6(1) and 1.3(1), respectively. Compared to previous BLS results of anhydrous pyrope, an addition of ~900 ppmw H2O in pyrope slightly increases the (∂KS/∂P)T, but has a negligible effect on the (∂G/∂P)T within their uncertainties. The temperature derivatives of the bulk and shear moduli at ambient pressure are (∂KS/∂T)P=-0.015(1) GPa/K and (∂G/∂T)P=-0.008(1) GPa/K, which are similar to those of anhydrous pyrope in previous BLS studies within their uncertainties. Meanwhile, our results also indicate that hydrous pyrope remains almost elastically isotropic at relevant high P-T conditions, and may have no significant contribution to seismic anisotropy in the upper mantle. In addition, we evaluated the seismic velocities (Vp and Vs) and the Vp/Vs ratio of hydrous pyrope along the upper mantle geotherm and a cold subducted slabs geotherm. It displays that hydrogen has also no significant effect on the seismic velocities and the Vp/Vs ratio of pyrope at the upper mantle conditions.
... This is in agreement with the predictions of Smyth (1994) based on electrostatic considerations that shed light on the under-bonded character of the O1 oxygen of wadsleyite (the only one not bonded to a silicon atom) and that makes it the best candidate for protonation. This H incorporation mechanism was in fact later confirmed by X-ray single crystal diffraction (Kudoh et al., 1996(Kudoh et al., , 2000Smyth and Kawamoto, 1997;Ye et al., 2012), and more recently by neutron powder diffraction (Sano-Furukawa et al., 2011), showing vacancies mainly in the M3 octahedral site of wadsleyite. Also, most infrared features in wadsleyite have been assigned to hydrogen in Mg vacancies by Jacobsen et al. (2005) and Deon et al. (2010), except for the band around 3,000 cm −1 that was assigned to H replacing Si, based on its pleochroism and frequency (see also Kohn et al., 2002). ...
... D. It is suggested that the substitution of silicon by H occurs at elevated water contents (see also Ye et al., 2012). It could be that there is a change in the incorporation mechanism of water in ringwoodite at high water contents. ...
Hydrous wadsleyite, ringwoodite, and phase D have been synthesized in the MgO-SiO2-H2O and MgO-FeO-SiO2-H2O systems at mantle transition zone conditions (15–21 GPa and 1,100–1,700°C) to investigate their water incorporation using Elastic Recoil Detection Analysis (ERDA), which is an absolute quantitative method. Wadsleyite and ringwoodite water contents vary from 0.1 to 3.2 wt% H2O and for ringwoodite containing up to 1.16 wt% H2O we observe that the (Mg+Fe)/Si ratio vs. water content follows the same trend as for wadsleyite, indicating that H is substituting for Mg as for wadsleyite. We also measured, for the first time, the water content of phase D and observed that it varies from 6.7 to 11.2 wt% H2O, up to twice less than estimated from electron microprobe analysis totals. Using these experiments, we were able to determine the absorptivity coefficient for OH infrared absorption bands in four wadsleyite and five ringwoodite samples. The average for the two iron-free wadsleyite samples leads to an absorptivity of 69,000 ± 7,000 L/moles H2O/cm², in very good agreement with previous determinations. The wadsleyite with 8 mole% Fe displays an absorptivity of 67,000 ± 5,000 L/moles H2O/cm². The absorptivity values vary from 118,500 ± 5,000 for Fe-free ringwoodite to ±135,000 ± 9,000 for Fe-bearing ringwoodite (10% mole Fe). Our results show that absorptivity coefficient for OH infrared absorption of ringwoodite do not drastically change with Fe content and that the frequency-based calibration of Paterson (1982) under-estimates its water determinations by 50%. This is very important to know when comparing data from different studies where different extinction coefficients have been used.
... As pointed out by Méducin et al. [45], P has a significant impact on the order-disorder process of the MgAl 2 O 4 -Sp, especially in the T range of 477-1227 • C. Some high-P single-crystal XRD investigations have been conducted up to~28.9 GPa at ambient T, but could not shed light on the Si disorder issue, partially due to the low experimental T potentially unable to trigger the order-disorder reaction, and partially due to the low data resolution caused by the similar X-ray scattering factors of Mg and Si [104,105]. ...
A series of Si-bearing MgAl 2 O 4-spinels were synthesized at 1500-1650 • C and 3-6 GPa. These spinels had SiO 2 contents of up to ~1.03 wt % and showed a substitution mechanism of Si 4+ + Mg 2+ = 2Al 3+. Unpolarized Raman spectra were collected from polished single grains, and displayed a set of well-defined Raman peaks at ~610, 823, 856 and 968 cm −1 that had not been observed before. Aided by the Raman features of natural Si-free MgAl 2 O 4-spinel, synthetic Si-free MgAl 2 O 4-spinel, natural low quartz, synthetic coesite, synthetic stishovite and synthetic forsterite, we infer that these Raman peaks should belong to the SiO 4 groups. The relations between the Raman intensities and SiO 2 contents of the Si-bearing MgAl 2 O 4-spinels suggest that under some P-T conditions, some Si must adopt the M-site. Unlike the SiO 4 groups with very intense Raman signals, the SiO 6 groups are largely Raman-inactive. We further found that the Si cations primarily appear on the T-site at P-T conditions ≤~3-4 GPa and 1500 • C, but attain a random distribution between the T-site and M-site at P-T conditions ≥~5-6 GPa and 1630-1650 • C. This Si-disordering process observed for the Si-bearing MgAl 2 O 4-spinels suggests that similar Si-disordering might happen to the (Mg,Fe) 2 SiO 4-spinels (ringwoodite), the major phase in the lower part of the mantle transition zone of the Earth and the benchmark mineral for the very strong shock stage experienced by extraterrestrial materials. The likely consequences have been explored.
