Sound velocities in MgSiO<sub>3</sub>‐garnet to 8 GPa

Gwanmesia, Gabriel D.; Chen, Ganglin; Liebermann, Robert C.

doi:10.1029/1998gl900189

Cited by 43 publications

(25 citation statements)

References 30 publications

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“… Conrad et al , 1999; Chai et al , 1997] on garnets and Mj‐Py solid solutions. The reasons for the much higher values of K S ′ and μ′ obtained by high‐pressure ultrasonic measurements [ Gwanmesia et al , 1998; Chen et al , 1999; Liu et al , 2000] and the compression measurements of Kavner et al [2000] are not clear. It is likely that the state of stress (that is, degree of non‐hydrostaticity) differs among these experiments, which might explain the spread in reported pressure derivatives.…”

Section: Resultsmentioning

confidence: 99%

Elasticity of Majorite and a Majorite‐Pyrope solid solution to high pressure: Implications for the Transition Zone

Sinogeikin

2002

Geophysical Research Letters

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[1 ] High seismic velocity gradients in the transition zone have been attributed to large pressure derivatives of the elastic moduli of transition zone minerals, such as majorite-garnet. Here we present Brillouin scattering measurements of the elasticity of polycrystalline Mg-majorite (Mj 100 , Mg 4 Si 4 O 12 ) and a Mj 50 Py 50 majorite -pyrope solid solution to pressures similar to those at the top of the transition zone (15.1(1) GPa and 12.8(1) GPa, respectively). The pressure derivatives of the adiabatic bulk (K S ) and shear (m ) moduli of both Mj 50 Py 50 and Mj 100 are 4.2(3) and 1.4(2), respectively, and are equal to those of pyrope within the experimental uncertainties. We conclude that neither the adiabatic compression of majorite-garnet nor any other likely transition zone phase is able to produce the high velocity gradients observed seismically. High velocity gradients are consistent with gradual transformations between minerals with very different elastic properties. The transformation of $ 25% or more of clinopyroxene into majorite above 520 km, and the formation of Ca silicate perovskite below 520 km could produce the observed seismic gradients.

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Section: Resultsmentioning

confidence: 99%

Elasticity of Majorite and a Majorite‐Pyrope solid solution to high pressure: Implications for the Transition Zone

Sinogeikin

2002

Geophysical Research Letters

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“…The only V-P data, which constrain the static lattice properties in our analysis, were measured by Yagi et al (1992). Therefore, apart from the sound velocity measurements by Gwanmesia et al (1998) and phonon frequency data by Chopelas (1999), we constrained thermophysical properties by using two triple points in the phase diagram measured by Gasparik (2003). The first triple point for the phase assemblages clino-enstatite, b-Mg 2 SiO 4 + stishovite and majorite is located at 1873 K and 16.9 GPa consistent with that reported by Yusa et al (1993).…”

Section: Ortho-enstatitementioning

confidence: 99%

Placing constraints on phase equilibria and thermophysical properties in the system MgO–SiO2 by a thermodynamically consistent vibrational method

Jong

2007

Geochimica et Cosmochimica Acta

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“…b Vinograd et al (2006). c Mj 100 (Gwamnesia et al 1998). d Pyrolite minus olivine composition (Irifune et al 2008).…”

Section: Protonation Mechanismunclassified

Calculation of the energetics of water incorporation in majorite garnet

Pigott

Wright

Gale

et al. 2015

American Mineralogist

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Interpretation of lateral variations in upper mantle seismic wave speeds requires constraints on the relationship between elasticity and water concentration at high pressure for all major mantle minerals, including the garnet component. We have calculated the structure and energetics of charge-balanced hydrogen substitution into tetragonal MgSiO 3 majorite up to P = 25 GPa using both classical atomistic simulations and complementary first-principles calculations. At the pressure conditions of Earth's transition zone, hydroxyl groups are predicted to be bound to Si vacancies (o) as the hydrogarnet defect, [o Si +4OH O ] X , at the Si2 tetrahedral site or as the [o Mg +2OH O ] X defect at the octahedral Mg3 site. The hydrogarnet defect is more favorable than the [o Mg +2OH O ] X defect by 0.8-1.4 eV/H at 20 GPa. The presence of 0.4 wt% Al 2 O 3 substituted into the octahedral sites further increases the likelihood of the hydrogarnet defect by 2.2-2.4 eV/H relative to the [o Mg +2OH O ] X defect at the Mg3 site. OH defects affect the seismic ratio, R = dlnv s /dlnv p , in MgSiO 3 majorite (ΔR = 0.9-1.2 at 20 GPa for 1400 ppm wt H 2 O) differently than ringwoodite at high pressure, yet may be indistinguishable from the thermal dlnv s /dlnv p for ringwoodite. The incorporation of 3.2 wt% Al 2 O 3 also decreases R(H 2 O) by ~0.2-0.4. Therefore, to accurately estimate transition zone compositional and thermal anomalies, hydrous majorite needs to be considered when interpreting seismic body wave anomalies in the transition zone.

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Sound velocities in MgSiO₃‐garnet to 8 GPa

Cited by 43 publications

References 30 publications

Elasticity of Majorite and a Majorite‐Pyrope solid solution to high pressure: Implications for the Transition Zone

Elasticity of Majorite and a Majorite‐Pyrope solid solution to high pressure: Implications for the Transition Zone

Placing constraints on phase equilibria and thermophysical properties in the system MgO–SiO2 by a thermodynamically consistent vibrational method

Calculation of the energetics of water incorporation in majorite garnet

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Sound velocities in MgSiO3‐garnet to 8 GPa

Cited by 43 publications

References 30 publications

Elasticity of Majorite and a Majorite‐Pyrope solid solution to high pressure: Implications for the Transition Zone

Elasticity of Majorite and a Majorite‐Pyrope solid solution to high pressure: Implications for the Transition Zone

Placing constraints on phase equilibria and thermophysical properties in the system MgO–SiO2 by a thermodynamically consistent vibrational method

Calculation of the energetics of water incorporation in majorite garnet

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Sound velocities in MgSiO₃‐garnet to 8 GPa