Titanium-hydroxyl defect-controlled rheology of the Earth's upper mantle

Faul, U.; Cline, C. J.; David, Emmanuel C.; Berry, Andrew J.; Jackson, Ian

doi:10.1016/j.epsl.2016.07.016

Cited by 54 publications

(39 citation statements)

References 68 publications

(152 reference statements)

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“…Experimental studies have shown clear evidence for coupled substitution of hydrogen with these species in simple systems Berry et al 2007a;Faul et al 2016;Férot and Bolfan-Casanova 2012;Grant et al 2007a, b;Kovacs et al 2010), supported by atomistic simulations (Balan et al 2011;Blanchard et al 2017;Walker et al 2007). Furthermore, the FTIR spectra produced by these studies reproduce well the dominant absorbance bands of natural olivine crystals Schmädicke et al 2013;Tollan et al 2015).…”

Section: +mentioning

confidence: 99%

The responses of the four main substitution mechanisms of H in olivine to H2O activity at 1050 °C and 3 GPa

Tollan

Smith

O’Neill

et al. 2017

Prog. in Earth and Planet. Sci.

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The water solubility in olivine C H 2 O ð Þ has been investigated at 1050°C and 3 GPa as a function of water activity a H 2 O ð Þ at subsolidus conditions in the piston-cylinder apparatus, with a H 2 O varied using H 2 O-NaCl fluids. Four sets of experiments were conducted to constrain the effect of a H 2 O on the four main substitution mechanisms. The experiments were designed to grow olivine in situ and thus achieve global equilibrium (G-type), as opposed to hydroxylating olivine with a pre-existing point-defect structure and impurity content (M-type). Olivine grains from the experiments were analysed with polarised and unpolarised FTIR spectroscopy, and where necessary, the spectra have been deconvoluted to quantify the contribution of each substitution mechanism. Olivine buffered with magnesiowüstite produced absorbance bands at high wavenumbers ranging from 3566 to 3612 cm −1 . About 50% of the total absorbance was found parallel to the a-axis, 30% parallel to the b-axis and 20% parallel to the c-axis. The total absorbance and hence water concentration in olivine follows the relationship of C H 2 O ∝a H 2 O 2 , indicating that the investigated defect must involve four H atoms substituting for one Si atom (labelled as [Si]). Forsterite buffered with enstatite produced an absorbance band exclusively aligned parallel the c-axis at 3160 cm −1 . The band position, polarisation and observed C H 2 O ∝a H 2 O are consistent with two H substituting for one Mg (labelled as [Mg]). Ti-doped, enstatite-buffered olivine displays absorption bands, and polarisation typical of Ti-clinohumite point defects where two H on the Si-site are charge-balanced by one Ti on a Mg-site (labelled as [Ti]). This is further supported by C H 2 O ∝a H 2 O and a 1:1 relationship of molar H 2 O and TiO 2 in these experiments. Sc-doped, enstatitebuffered experiments display a main absorption band at 3355 cm −1 with C H 2 O ∝a H 2 O 0:5 and a positive correlation of Sc and H, indicating the coupled substitution of a trivalent cation plus a H for two Mg (labelled as [triv]). Our data demonstrate that extreme care has to be taken when inferences from experiments conducted at a H 2 O ¼ 1 are applied to the mantle, where in most cases, a low a H 2 O persists. In particular, the higher exponent of the [Si] substitution mechanism means that the contribution of this hydrous defect to total water content will decrease more rapidly with decreasing a H 2 O than the contributions of the other substitution mechanisms. The experiments confirm previous results that the [Mg] mechanism holds an almost negligible amount of water under nearly all T-PfO 2 -fH 2 O conditions that may be anticipated in nature. However, the small amounts of H 2 O we find in substituting by this mechanism are similar in the experiments on forsterite doped with either Sc or Ti to those in the undoped forsterite at equivalent a H 2 O (all buffered by enstatite), confirming the assumption that, thermodynamically, C H 2 O substituting by each mechanism does not depend on the w...

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Section: +mentioning

confidence: 99%

The responses of the four main substitution mechanisms of H in olivine to H2O activity at 1050 °C and 3 GPa

Tollan

Smith

O’Neill

et al. 2017

Prog. in Earth and Planet. Sci.

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“…Natural dunites deformed in the climb‐controlled dislocation creep regime were significantly weaker under hydrous conditions than under anhydrous conditions [ Carter and Ave'Lallemant , ; Chopra and Paterson , ]. Fine‐grained olivine aggregates deformed in both the climb‐controlled dislocation creep and diffusion creep regimes exhibit significant hydrolytic weakening [ Karato et al , ; Mei and Kohlstedt , , ; Demouchy et al , ; Tasaka et al , ; Faul et al , ]. Olivine aggregates deforming by glide‐controlled dislocation creep [ Katayama and Karato , ] also reveal hydrolytic weakening.…”

Section: Introductionmentioning

confidence: 99%

Hydrolytic weakening in olivine single crystals

2017

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Deformation experiments on single crystals of San Carlos olivine under hydrous conditions were performed to investigate the microphysical processes responsible for hydrolytic weakening during dislocation creep. Hydrogen was supplied to the crystals using either talc or brucite sealed in nickel capsules with the crystal. Deformation experiments were carried out using a gas medium apparatus at temperatures of 1050° to 1250°C, a confining pressure of 300 MPa, differential stresses of 45 to 294 MPa, and resultant strain rates of 1.5 × 10−6 to 4.4 × 10−4 s−1. For talc‐buffered (i.e., water and orthopyroxene‐buffered) samples at high temperatures, the dependence of strain rate on stress follows a power law relationship with a stress exponent (n) of ∼2.5 and an activation energy of ∼490 kJ/mol. Brucite‐buffered samples deformed faster than talc‐buffered samples but contained similar hydrogen concentrations, demonstrating that strain rate is influenced by orthopyroxene activity under hydrous conditions. The values of n and dependence of strain rate on orthopyroxene activity are consistent with hydrolytic weakening occurring in the climb‐controlled dislocation creep regime that is associated with deformation controlled by lattice diffusion under hydrous conditions and by pipe diffusion under anhydrous conditions. Analyses of postdeformation electron‐backscatter diffraction data demonstrate that dislocations with [100] Burgers vectors are dominant in the climb‐controlled regime and dislocations with [001] are dominant in the glide‐controlled regime. Comparison of the experimentally determined constitutive equations demonstrates that under hydrous conditions crystals deform 1 to 2 orders of magnitude faster than under anhydrous conditions.

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“…High oxygen fugacity has been shown to enhance seismic attenuation in olivine (Cline et al 2018). The similarly high oxygen fugacities present in the earlier deformation experiments of Faul et al (2016), produced by the platinum sleeve used to wrap the specimen (Faul et al 2017 In this study we use computational simulations to investigate the possibility that cation vacancies may be able to lubricate dislocation glide in forsterite by reducing the Peierls stress  p ., providing a mechanism by which point defects may influence the rheology of this important mantle mineral in the glide-controlled creep regime. Core structures and Peierls stresses are calculated for important slip systems in forsterite using a semi-discrete Peierls-Nabarro (PN;Peierls 1940;Nabarro 1947) model.…”

Section: Introductionmentioning

confidence: 99%

“…Martin and Donnay 1972;Bai and Kohlstedt 1993;Kohlstedt et al 1996;Lemaire et al 2004), can influence the rheology of olivine under conditions of low-temperature (<1273 K) and high-stress, reducing  p to 1.6-2.9 GPa (Katayama and Karato 2008). Similarly, the incorporation of water as the "titanoclinohumite" defect, consisting of charge-coupled {Ti M }˙˙ and {2H Si }″ defects apparently results in a systematic increase in strain rate (Berry et al 2005;Faul et al 2016). Protonated cation vacancies also change the preferred slip system in olivine.…”

Section: Introductionmentioning

confidence: 99%

Lubrication of dislocation glide in forsterite by Mg vacancies: insights from Peierls-Nabarro modeling

Skelton¹,

Walker²

2018

Preprint

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Dislocation glide is an important contributor to the rheology of olivine under conditions of high stress and low to moderate temperature, such as occur in mantle wedges. Interactions between point defects and dislocation core may alter the Peierls stress,  p , and has been suggested that vacancy-related defects may selectively enhance glide on certain slip systems, changing the olivine deformation fabric.In this study, the Peierls-Nabarro model, parameterized by generalized stacking fault (GSF) energies calculated atomistically using empirical interatomic potentials, is used to determine the effect of bare concentration at the glide plane of 0.125 defects/site is sufficient to reduce cross-over to 0.7 GPa. This may account for the existence of the B-type olivine deformation fabric in the corners of mantle wedges.

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Titanium-hydroxyl defect-controlled rheology of the Earth's upper mantle

Cited by 54 publications

References 68 publications

The responses of the four main substitution mechanisms of H in olivine to H2O activity at 1050 °C and 3 GPa

The responses of the four main substitution mechanisms of H in olivine to H2O activity at 1050 °C and 3 GPa

Hydrolytic weakening in olivine single crystals

Lubrication of dislocation glide in forsterite by Mg vacancies: insights from Peierls-Nabarro modeling

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