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Comparison of Fe‐Mg interdiffusion results with data for other mantle minerals. Data for ringwoodite: (a) and (b) represent dry (d = 20 μm, ~50 wt. ppm H2O) and wet (60 μm, ~3,000 wt. ppm H2O) cases reported in this study and (c) sample with 10‐μm grain size but no water content reported by Farber et al. (2000). Wadsleyite normalized to 16 GPa: (d) from Holzapfel et al. (2009; d = 25–60 μm, 35 wt. ppm H2O), (e) from Kubo et al. (2004; d = 20 μm, 50–90 wt. ppm H2O) and the diamond point is also from Kubo et al. (2004) for hydrous wadsleyite (d = 21–37 μm, 300–2,100 wt. ppm H2O), and (f) from Chakraborty et al. (1999; d = 60–220 μm). Olivine: (g) from Hier‐Majumder et al. (2005), (h) from Dohmen et al. (2007), and (i) from Chakraborty (1997).
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We determined the kinetics of Fe‐Mg interdiffusion in ringwoodite aggregates as a function of water content (up to ~6,000 wt. ppm H2O) at 20 GPa and 1,373–1,673 K by the diffusion couple method. The dependence of Fe‐Mg interdiffusivity (DFe‐Mg) on Fe concentration was determined using the Boltzmann‐Matano method. The experimentally reported DFe‐Mg...
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Citations
... Trace amounts of water have significant effects on the physical and chemical properties of minerals and rocks. In particular, water can promote the deformation of minerals and rocks (Mackwell et al. 1985), reduce seismic wave velocity and viscosity (Dixon et al. 2004, Karato andJung 1998), enhance conductivity (Karato 1990, Yoshino et al. 2006, Yang 2012, change the energy efficiency of light and heat radiation (Hofmeister 2004, Zhang et al. 2019a, affect the diffusion rate of ions (Zhang et al. 2019b), reduce the melting temperature (Hui andZhang 2007, Grove et al. 2012), deepen the initial depth of melting (Mei and Kohlstedt 2000a, b), and affect the melting degree (Hui andZhang 2007, Schmandt et al. 2014). Thus, water plays an important role in affecting processes related to subduction zones, such as magmatism (Hirschmann 2006, Kelley et al. 2010, Grove et al. 2012, intraplate earthquakes (Hacker et al. 2003, Zhang et al. 2004, mantle convection (Hirth and Kohlstedt 2003), and geochemical recycling (Zack andJohn 2007, Shaw et al. 2008), and thus affects the long-term thermal and chemical evolution of the Earth (Zheng et al. 2016). ...
An accurate determination of water content in garnet is critical to quantify the transport of water to the deep mantle by the subducted oceanic crust beyond the breakdown of hydrous phases. Fourier transform infrared spectroscopy (FTIR) is the most widely used approach to determine the species and contents of water in garnet. Accurate quantification of OH in garnet requires independent calibration using an external method, as OH absorbance is mineral and composition specific. To obtain the infrared absorption coefficients of structural hydroxyl in garnet, a combined study of spectrometric analyses by FTIR and a method combining a thermal conversion elemental analyser with isotope ratio mass spectrometry (TC/EA‐MS) was carried out for fourteen gem‐quality natural garnet crystals with variable compositions. The obtained molar absorption coefficients were 9322 ± 338 and 240 ± 26 l mol‐1 cm‐2 for grossular‐ and spessartine‐rich garnet and pyrope‐almandine garnet, respectively. These results are within the range of previous studies. A new molar absorption coefficient of 689 ± 177 l mol‐1 cm‐2 was obtained for pyrope‐spessartine garnet. The large variation in the absorption coefficient indicates it is controlled by both garnet composition and OH‐absorption bands. The obtained absorption coefficients are only appropriate for certain types of eclogitic garnet, and more studies should be carried out on eclogitic garnets.
... For the higher-pressure form, ringwoodite, If the C H2O exponent for D Si in ringwoodite is equal to that of the dislocation mobility (r = 1.1 ± 0.1) (Fei et al., 2017), the C H2O exponent for D Si of wadsleyite is slightly smaller than that of ringwoodite. On the other hand, the D Fe-Mg in ringwoodite has an C H2O exponent of 0.25 ± 0.03 (Zhang et al., 2019), which is comparable with that in wadsleyite (Zhang et al., 2021) and much smaller than that in olivine (Fei et al., 2018). Below we discuss the defect chemistry of the three polymorphs individually to interpret the different C H2O exponents for D Si and D Mg . ...
We investigated the H2O‐content dependence of Si self‐diffusion coefficient in Fe‐free wadsleyite using multi‐anvil experiments at pressures of 19–20 GPa, temperatures of 1573–1873 K, and H2O‐content ranging from ∼10 to 5,300 wt. ppm by the isotopic thin‐film diffusion‐couple method. The ²⁹Si‐doped diffusion profiles were measured by nanoscale secondary ion mass spectrometry in the depth profiling mode. The H2O contents in the samples were analyzed by Fourier transformation infrared spectroscopy. The experimental results show a H2O enhancement of Si diffusion coefficient with a H2O content exponent of 0.8 ± 0.1. The activation enthalpy was found to be 270 ± 40 kJ/mol. The diffusion coefficients in the [100], [010], and [001] directions are indistinguishable. The temperature and H2O‐content dependences of Si diffusion indicate that H2O incorporation dramatically reduces the rheological strength of wadsleyite, whereas temperature has a relatively small effect. The viscosity in the mantle transition zone could be significantly reduced by H2O incorporation in wadsleyite. The viscosity contrast between mantle plumes and surroundings may control the evolution of plume shapes at 410–660 km depths.
... For the higher-pressure form, ringwoodite, If the C H2O exponent for D Si in ringwoodite is equal to that of the dislocation mobility (r = 1.1 ± 0.1) (Fei et al., 2017), the C H2O exponent for D Si of wadsleyite is slightly smaller than that of ringwoodite. On the other hand, the D Fe-Mg in ringwoodite has an C H2O exponent of 0.25 ± 0.03 (Zhang et al., 2019), which is comparable with that in wadsleyite (Zhang et al., 2021) and much smaller than that in olivine (Fei et al., 2018). Below we discuss the defect chemistry of the three polymorphs individually to interpret the different C H2O exponents for D Si and D Mg . ...
... However, the charge carrier responsible for the conduction could vary with composition and temperature. For example, the control of hydrogen on electrical conductivity might be due to its high mobility that leads to the proton conduction, enhancement of mobility of other charged species, and inductive production of charged defects (Dai and Karato, 2014;Fei et al., 2020;Karato, 2019;Zhang et al., 2019). Recently Fei et al. (2020) reported 500 ppm. ...
... Hz) has been used to confirm the validity. This experimental observation was also found consistent with the calculation based on FeeMg interdiffusion in ringwoodite aggregates (Zhang et al., 2019). Huang et al. (2005) also reported the conductivity of an hydrous ringwoodite at 1273 Ke1473 K, which was suggested to be enhanced by proton conduction, thereby being excluded to extrapolate the ionic conductivity at high temperatures. ...
The electrical conductivity of mineral aggregates depends both on the properties of the constitutive minerals and the ways those minerals are assembled. Mixing, or average models combine the conductivity of single phases to give bulk conductivity of rocks, thereby linking experimental measurements to geophysical observations. In order to compare these mixing models and allow an informed choice, several popular approaches, including bounds and average models, have been used to estimate the conductivity of a typical dry upper mantle and transition zone with a pyrolite composition. All the estimations calculated using the various average models lie between the rigorous constraint that is given by the HS bounds. The average models in this study are found to give similar bulk conductivities with the difference of less than 0.5 orders of magnitude, except the geometric mean, implying that the choice of the average models is insignificant. The effective electrical conductivity of pyrolite mantle has been derived from the conductivity of dry mantle minerals using the effective medium theory, and was found consistent with observed conductivity values for some subsurface regions of the Earth which we expect to be relatively dry. This provides us with baseline conductivity for a dry mantle, which is helpful to understand the water distribution in the deep earth.
... ng been proposed to have dramatic influences on many physical and chemical properties, such as thermal conductivity (κ) (Chang et al., 2017;B. H. Zhang et al., 2019a), diffusivity (D) (e.g., Kubo et al., 2004;Hier-Majumder et al., 2005;Costa and Chakraborty, 2008;Fei et al., 2013Fei et al., , 2018Sun et al., 2015Sun et al., , 2018Sun et al., , 2019B. H. Zhang et al., 2019b), electrical conductivity (σ ) (e.g., Karato, 1990;Huang et al., 2005;Yoshino et al., 2006Yoshino et al., , 2008Poe et al., 2010;Yang, 2012;B. H. Zhang et al., 2019c;Liu et al., 2019), sound velocity (V P and V S ) (e.g., Jacobsen and Smyth, 2006;J. Wang et al., 2006;Mao et al., 2010Mao et al., , 2011Buchen et al., 2018;Wang et al., 2019 ...
... Sun et al. (2015) reported a hydrogen self-diffusion coefficient in Fe-bearing ringwood- ; Z12 (Zhang, 2012); S18 (Sun et al., 2018); K06 (Kudo et al., 2006); K04 ; H09 (Holzapfel et al., 2009); S04 ; S09 (Shimojuku et al., 2009); S10 (Shimojuku et al., 2010); Z11 (Zhang et al., 2011); Z15a (Zhang and Shan, 2015a); Z15b (Zhang and Shan, 2015b); Z19b (B. H. Zhang et al., 2019b); Z21 ite through the interdiffusion of hydrogen and deuterium in a pair of synthesized single crystals with 5500-7700 ppm water at 21 GPa and 1000-1300 K. Note that the results obtained by Sun et al. (2015) are 2 orders of magnitude lower than those reported by Kudo et al. (2006) and Zhang (2012), despite the activation energies being comparable to each other (Fig. 2b). ...
... Fe-Mg. To date, only B. H. Zhang et al. (2019b) have determined the kinetics of Fe-Mg interdiffusion in ringwoodite aggregates as a function of water content (40-6000 ppm H 2 O) at 20 GPa and 1373-1673 K by using the diffusion couple method. A relatively small water exponent (r = 0.25) suggests the non-negligible role of water in enhancing Fe-Mg interdiffusion in ringwoodite. ...
The incorporation of water in nominally anhydrous minerals plays a crucial
role in many geodynamic processes and evolution of the Earth and affects the
physical and chemical properties of the main constituents of the Earth's
mantle. Technological advances now allow the transport properties of
minerals to be precisely measured under extreme conditions of pressure and
temperature (P and T) that closely mimic the P–T conditions throughout much of
the Earth's interior. This contribution provides an overview of the recent
progress in the experimental studies on the influence of water on physical
properties (i.e., diffusivity, electrical conductivity, thermal
conductivity, sound velocity, and rheology) of olivine, wadsleyite, and
ringwoodite together with their applications. In particular, consistency
among various experimental data is investigated, discrepancies are
evaluated, and confusions are clarified. With such progress in the
experimental determination of transport properties of major mantle minerals,
we can expect new insights into a broad range of geoscience problems. Many
unresolved issues around water inside Earth require an integrated approach
and concerted efforts from multiple disciplines.
... Previous studies have shown that wadsleyite and ringwoodite are capable of incorporating up to approximately 3 wt.% of H 2 O within their crystal structures (Smyth, 1987;McMillan et al., 1991;Inoue et al., 1995;Kohlstedt et al., 1996;Ohtani et al., 2004), thus enabling them to be probable candidates as water reservoir in the Earth's mantle. The presence of OH-defects in the nominally anhydrous minerals (NAMs) including wadsleyite may affect mantle dynamics globally by influencing melting tempera-ture (Kushiro et al., 1968;Hirschmann, 2006), electrical conductivity (Karato, 1990;Huang et al., 2005;Yoshino et al., 2008), thermal conductivity (Zhang et al., 2019a), atomic diffusivities Zhang, 2017;Zhang et al., 2019b), and rheological properties (Mackwell et al., 1985) of main minerals. ...
... Fe-Mg interdiffusion rates have been used to interpret transport and kinetic properties of MTZ such as electrical conductivity, element partitioning, and homogenization of chemical heterogeneities (e.g., Farber et al., 1994Farber et al., , 2000Chakraborty et al., 1999;Zhang et al., 2019b). Because kinetics of phase transformation is to be diffusion-controlled process in deep Earth (Solomatov and Stevenson, 1994), sharpness of mantle discontinuities may be controlled by cation diffusion of certain species during mantle convection. ...
... The truncation edge length of the second-stage tungsten carbide anvil is 5 mm, accommodating 10-mm Cr 2 O 3 -doped MgO octahedra. The pressure assembly consisted of a ZrO 2 thermal insulation sleeve, a LaCrO 3 cylindrical heater, and a MgO sleeve that insulates metal capsule from the furnace (Zhang et al., 2019b for a detailed description). Each assembly was compressed to 16 GPa at room temperature and then heated to the desired temperature (1673 K for water-poor samples and 1373-1473 K for water-rich ones) at a rate of 50 K/min. ...
Fe–Mg interdiffusion rates in polycrystalline wadsleyite aggregates have been determined as a function of water content (up to ∼0.345 wt.% H2O) at 16 GPa and 1373–1773 K in a Kawai-type multi-anvil apparatus. Pre-synthesized water-poor and -rich polycrystalline wadsleyite were used as starting materials. Diffusion profiles were obtained across the interface between Fe-free and -bearing diffusion couples, namely, Mg2SiO4 and (Mg0.9Fe0.1)2SiO4 aggregates by electron microprobe. Fe–Mg interdiffusivities by experiments yield DFe−Mg(m2/s)=D0XFenCH2Orexp[−(E+αXFe+βCH2O)/RT], where D0 = 1.33−0.23+0.20× 10⁻¹¹ m²/s, n = 0.19 ± 0.04, r = 0.29 ± 0.12, E = 92 ± 2 kJ/mol, α = −45 ± 12, and β = −134 ± 2. Our results indicate that water significantly enhances the rates of Fe–Mg interdiffusion in wadsleyite (a factor of 2.4 for fixed temperature and Fe concentration) compared to that in ringwoodite. Although under hydrous condition the transition zone shows the maximum Fe–Mg mixing efficiency as revealed by diffusivity-depth profile in the mantle, homogenization of existing chemical heterogeneity is still very limited at geological time scale only through solid-state diffusion. Combined with the Nernst–Einstein relation, the results suggest that the contribution of water to the electrical conductivity of wadsleyite or ringwoodite may be overestimated from Fe–Mg interdiffusion data at high water content. Further calculation demonstrates that ∼0.1–0.5 wt.% H2O is sufficient to account for the high conductivity values in the upper part (410–520 km) of the mantle transition zone as observed by electromagnetic induction studies.
... 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. ...
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.
... Previous studies have demonstrated that water can strongly enhance element diffusivity in silicate minerals, such as olivine (Hier-Majumder et al. 2005;Wang et al. 2004) and its high pressure polymorphs (Kubo et al. 2004;Shimojuku et al. 2010;Zhang et al. 2019), although the extent has recently been questioned (Fei et al. 2013(Fei et al. , 2014. However, the magnitude for enhancements of different elements diffusivities is different. ...
Water solubility in Mg-endmember silicates is suggested to correlate with cation diffusivity. This study proposes a thermodynamic model by introducing the concept of transient Frenkel defect for Mg ions to quantify the relationship between water solubility and Mg diffusivity in mantle silicates. The formation of the Mg transient Frenkel defect is regarded as the escape of Mg from Mg-O polyhedron, and the concentration of Mg transient vacancy is considered as its statistical probability in silicate minerals. The proposed thermodynamic model reveals that the formation energy for Mg transient vacancy (ΔGtr) is approximately ΔG/6 (ΔG is activation energy for Mg diffusion in silicates) for silicates with Mg-O6 octahedra, such as enstatite, forsterite, wadsleyite, and ringwoodite, and 5ΔG/12 for bridgmanite and post-perovskite. This observation implies a possible diffusion mode of Mg through transient vacancies. Our model connects water solubility and ion diffusivity in silicates.
