Mg lattice diffusion in iron-free olivine and implications to conductivity anomaly in the oceanic asthenosphere

Fei, Hongzhan; Koizumi, Sanae; Sakamoto, Naoya; Hashiguchi, Minako; Yurimoto, H.; Marquardt, Katharina; Miyajima, Nobuyoshi; Katsura, Tomoo

doi:10.1016/j.epsl.2017.12.020

Cited by 27 publications

(74 citation statements)

References 53 publications

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“…Obviously,

D_{Fe ‐ Mg}^{gb}

is estimated to be around four orders faster than

D_{Fe ‐ Mg}^{lat}

. At temperature from 1,373 to 1,873 K,

3 δ D_{Fe ‐ Mg}^{gb} / d

is significantly enhanced with decreasing grain size, which is consistent with the grain size‐dependent phenomena in forsterite when d < 1 μm (Fei et al, , ). Furthermore, a comparison of our calculated grain size‐dependent D Fe‐Mg in ringwoodite with that in wadsleyite estimated using data from Kubo et al () revealed that the effective diffusivity between them is consistent in the whole temperature range investigated (Figure ).…”

Section: Discussionsupporting

confidence: 58%

“…This observation is similar to that reported in wadsleyite (Kubo et al, ) and garnet (Zhang et al, ). However, the water exponent for D Fe‐Mg in ringwoodite ( r = 0.25) is smaller than that in olivine ( r ≈ 0.9, Hier‐Majumder et al, ) and Mg diffusion in forsterite both within the lattice ( r ≈ 1.2, Fei et al, ) and along the grain boundaries ( r ≈ 1.0, Fei et al, ) and in garnet ( r ≈ 1.38, Zhang et al, ).…”

Section: Discussionmentioning

confidence: 99%

“…Diffusion in polycrystalline materials occurs via two mechanisms, volume diffusion and diffusion along grain boundaries. It is well known that grain‐boundary diffusion is generally faster than lattice diffusion by many orders (Farber et al, ; Fei et al, , ; Shimojuku et al, ). According to the classification and criteria of grain‐boundary diffusion (Harrison, ; Joesten, ; Le Claire, ), three kinetic regimes for diffusion denoted as types A, B, and C can be distinguished, which depend upon the magnitude of lattice diffusion coefficients ( D lat ) and grain boundary diffusion ( D gb ), as well as the annealing time ( t ), grain‐boundary width ( δ ), and grain size ( d ).…”

Section: Discussionmentioning

confidence: 99%

“…Therefore, to know the grain‐boundary diffusion contribution to the measured effective bulk diffusivity, indirect evaluation is required. Fei et al (, ) demonstrated that

D_{Mg}^{lat}

and

δ D_{Mg}^{gb}

have almost identical pressure, water content, and oxygen fugacity dependence in forsterite, which gave

D_{Mg}^{lat}

/ δ

D_{Mg}^{gb}

a constant value of ~10 5 regardless of variables mentioned above. For various mantle minerals, the grain‐boundary width has been estimated to be the same of about 1 nm (Farber et al, ; Joesten, ).…”

Section: Discussionmentioning

confidence: 99%

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The Effect of Water on Fe‐Mg Interdiffusion Rates in Ringwoodite and Implications for the Electrical Conductivity in the Mantle Transition Zone

2019

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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 in ringwoodite within 0 ≤ XFe ≤ 0.1 could be fitted by the relation DFe‐Mg()m2/normals=D0XFenCH2normalOrexp[]−()E*+αCH2normalO/italicRT, where E* = (1 − XFe)EMg + XFeEFe (EMg = 140 ± 5 kJ/mol, EFe = 4 ± 2 kJ/mol), D0 = 5.59−1.91+2.90 × 10−10 m2/s, n = −0.21 ± 0.10, r = 0.25 ± 0.03, and α = −24 ± 4. The water content exponent r of 0.25 suggests a nonnegligible role of water in enhancing Fe‐Mg interdiffusion in ringwoodite. The length scale over which the chemical heterogeneities are homogenized by Fe‐Mg interdiffusion in the mantle transition zone is estimated to be only a few hundred meters even assuming the whole Earth age. Comparison between the conductivities predicted from Fe‐Mg interdiffusion and those obtained from magnetotelluric surveys suggests that around 0.1 wt.% water can account for the high conductivity anomalies (~10−0.6–10−1 S/m) observed in the lower part of the mantle transition zone.

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“…Obviously,

D_{Fe ‐ Mg}^{gb}

is estimated to be around four orders faster than

D_{Fe ‐ Mg}^{lat}

. At temperature from 1,373 to 1,873 K,

3 δ D_{Fe ‐ Mg}^{gb} / d

Section: Discussionsupporting

confidence: 58%

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

“…Therefore, to know the grain‐boundary diffusion contribution to the measured effective bulk diffusivity, indirect evaluation is required. Fei et al (, ) demonstrated that

D_{Mg}^{lat}

and

δ D_{Mg}^{gb}

have almost identical pressure, water content, and oxygen fugacity dependence in forsterite, which gave

D_{Mg}^{lat}

/ δ

D_{Mg}^{gb}

Section: Discussionmentioning

confidence: 99%

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The Effect of Water on Fe‐Mg Interdiffusion Rates in Ringwoodite and Implications for the Electrical Conductivity in the Mantle Transition Zone

2019

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“…It has been proposed that olivine has three electrical conduction mechanisms (e.g., Yoshino et al, 2009): (a) proton conduction with charges carried by protons (H i ● ) produced by (2H) Mg × <=> H Mg ′ + H i ● owing to the small amount of H 2 O (protons) incorporated in the crystal structure as point defects; (b) small polaron conduction caused by hopping of electron holes (h ● ) between ferrous and ferric iron, that is, Fe 3+ <=> Fe 2+ + h ● , with h ● as the charge carrier; and (c) ionic conduction controlled by the diffusion of ions between regular sites and vacancies, A A × (site 1) + V A (site 2) <=> V A (site 1) + A A × (site 2), where A A × is the regular site of element A (Mg, Fe, Si, and O) and V A is the corresponding vacant site. Mechanism (c) is dominated by Mg (Fe) diffusion because it is by orders of magnitude faster than Si and O in olivine (Chakraborty et al, 1994; Fei et al, 2013, 2014; Fei et al, 2018a). The charge carrier of ionic conduction is specialized to vacancies on regular sites (also called vacancy conduction; e.g., Gardés et al, 2014), although protons in olivine are also ionic species.…”

Section: Introductionmentioning

confidence: 99%

The Effect of Water on Ionic Conductivity in Olivine

2020

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High‐temperature ionic conductivity in olivine single crystals has been measured in the [100], [010], and [001] crystallographic orientations as a function of pressure from 2 to 10 GPa, temperature from 1450 to 2180 K, and H2O content from 20 to 580 wt. ppm using multianvil presses with in situ impedance analyses. The experimental results yield an activation energy, activation volume, and H2O content exponent of 250–405 kJ/mol, 3.2–5.3 cm3/mol, and 1.3 ± 0.2, respectively, for the high‐temperature ionic conduction regime. Olivine ionic conductivity has negative pressure and positive temperature dependences and is significantly enhanced by H2O incorporation. The [001] direction is more conductive than the [100] and [010] directions. The H2O‐enhanced ionic conductivity may contribute significantly to the electrical conductivity profile in the asthenosphere, especially in the regions under relatively high‐temperature and low‐pressure conditions.

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Explaining the dependence of M-site diffusion in forsterite on silica activity: a density functional theory approach

et al. 2020

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Experimentally, silica activity (aSiO2) has been shown to have an effect on Mg diffusion in forsterite, but no fully satisfactory mechanism has yet been proposed. We calculated the effects of aSiO2 and aluminium content (the main contaminant in some recent experimental studies), and their co-effect, on Mg diffusion in forsterite, using thermodynamic minimisations of defect formation energies [calculated using density functional theory (DFT)] and a Monte-Carlo diffusion model. These two variables, in isolation, do not appreciably change the defect concentrations of forsterite and thus do not affect the diffusivity of Mg. However, when elevated together, they cause large increases in the Mg vacancy content and thus can increase the Mg diffusivity by one to six orders of magnitude depending on temperature, with little pressure dependence. This effect is largely independent of Al2O3 concentration above ~ 1 wt. ppm, and thus, for all practical purposes, should occur wherever forsterite is in the presence of enstatite. It is also largely dependent upon configurational entropy and is thus highly sensitive to the chemistry of the crystal. A low concentration of structurally bound hydroxyl groups at low temperatures (1000 K) suppresses this effect in pure forsterite, but it is likely robust in the presence of water either when alternative water sinks (such as Ti or Fe) are present, or at high temperatures (> 1500 K). This effect is also robust in the presence of ferrous iron (or other substitutional Mg defects) at all temperatures. Fe2O3 can operate like Al2O3 in this reaction and should enhance its effect. These findings explain the experimentally observed dependency of Mg diffusion of aSiO2, and elucidate how chemical activity variations in both experiments and natural settings could affect not only the diffusivity of Mg in forsterite, but of olivine-hosted cations in general.

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Mg lattice diffusion in iron-free olivine and implications to conductivity anomaly in the oceanic asthenosphere

Cited by 27 publications

References 53 publications

The Effect of Water on Fe‐Mg Interdiffusion Rates in Ringwoodite and Implications for the Electrical Conductivity in the Mantle Transition Zone

The Effect of Water on Fe‐Mg Interdiffusion Rates in Ringwoodite and Implications for the Electrical Conductivity in the Mantle Transition Zone

The Effect of Water on Ionic Conductivity in Olivine

Explaining the dependence of M-site diffusion in forsterite on silica activity: a density functional theory approach

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