Calculation of the energetics of water incorporation in majorite garnet

Jeffrey S. Pigott, Kate Wright, Julian D. Gale, Wendy R. Panero

Research output: Contribution to journalArticle

4 Citations (Scopus)

Abstract

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 MgSiO3 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 (□) as the hydrogarnet defect, [□Si+4OHO]X, at the Si2 tetrahedral site or as the [□Mg+2OHO]X defect at the octahedral Mg3 site. The hydrogarnet defect is more favorable than the [□Mg+2OHO]X defect by 0.8-1.4 eV/H at 20 GPa. The presence of 0.4 wt% Al2O3 substituted into the octahedral sites further increases the likelihood of the hydrogarnet defect by 2.2-2.4 eV/H relative to the [□Mg+2OHO]X defect at the Mg3 site. OH defects affect the seismic ratio, R = dlnvs/dlnvp, in MgSiO3 majorite (ΔR = 0.9-1.2 at 20 GPa for 1400 ppm wt H2O) differently than ringwoodite at high pressure, yet may be indistinguishable from the thermal dlnvs/dlnvp for ringwoodite. The incorporation of 3.2 wt% Al2O3 also decreases R(H2O) 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.

Original languageEnglish
Pages (from-to)1065-1075
Number of pages11
JournalAmerican Mineralogist
Volume100
Issue number5-6
DOIs
Publication statusPublished - 1 May 2015
Externally publishedYes

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majorite
Garnets
garnets
defect
garnet
energetics
Defects
Water
defects
water
transition zone
ringwoodite
seismic wave
Earth mantle
anomalies
Seismic waves
body wave
seismic waves
incorporation
calculation

Cite this

Pigott, Jeffrey S. ; Wright, Kate ; Gale, Julian D. ; Panero, Wendy R. / Calculation of the energetics of water incorporation in majorite garnet. In: American Mineralogist. 2015 ; Vol. 100, No. 5-6. pp. 1065-1075.
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abstract = "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 MgSiO3 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 (□) as the hydrogarnet defect, [□Si+4OHO]X, at the Si2 tetrahedral site or as the [□Mg+2OHO]X defect at the octahedral Mg3 site. The hydrogarnet defect is more favorable than the [□Mg+2OHO]X defect by 0.8-1.4 eV/H at 20 GPa. The presence of 0.4 wt{\%} Al2O3 substituted into the octahedral sites further increases the likelihood of the hydrogarnet defect by 2.2-2.4 eV/H relative to the [□Mg+2OHO]X defect at the Mg3 site. OH defects affect the seismic ratio, R = dlnvs/dlnvp, in MgSiO3 majorite (ΔR = 0.9-1.2 at 20 GPa for 1400 ppm wt H2O) differently than ringwoodite at high pressure, yet may be indistinguishable from the thermal dlnvs/dlnvp for ringwoodite. The incorporation of 3.2 wt{\%} Al2O3 also decreases R(H2O) 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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Calculation of the energetics of water incorporation in majorite garnet. / Pigott, Jeffrey S.; Wright, Kate; Gale, Julian D.; Panero, Wendy R.

In: American Mineralogist, Vol. 100, No. 5-6, 01.05.2015, p. 1065-1075.

Research output: Contribution to journalArticle

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AU - Pigott, Jeffrey S.

AU - Wright, Kate

AU - Gale, Julian D.

AU - Panero, Wendy R.

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AB - 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 MgSiO3 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 (□) as the hydrogarnet defect, [□Si+4OHO]X, at the Si2 tetrahedral site or as the [□Mg+2OHO]X defect at the octahedral Mg3 site. The hydrogarnet defect is more favorable than the [□Mg+2OHO]X defect by 0.8-1.4 eV/H at 20 GPa. The presence of 0.4 wt% Al2O3 substituted into the octahedral sites further increases the likelihood of the hydrogarnet defect by 2.2-2.4 eV/H relative to the [□Mg+2OHO]X defect at the Mg3 site. OH defects affect the seismic ratio, R = dlnvs/dlnvp, in MgSiO3 majorite (ΔR = 0.9-1.2 at 20 GPa for 1400 ppm wt H2O) differently than ringwoodite at high pressure, yet may be indistinguishable from the thermal dlnvs/dlnvp for ringwoodite. The incorporation of 3.2 wt% Al2O3 also decreases R(H2O) 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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KW - defect mechanisms

KW - density functional theory

KW - force field

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DO - 10.2138/am-2015-5063

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JO - American Mineralogist

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SN - 0003-004X

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