Hydrogarnet substitution in pyrope: a possible location for “water” in the mantle

Ackermann, L.; Cemič, Ladislav; Langer, Klaus

doi:10.1016/0012-821x(83)90084-5

Cited by 60 publications

(18 citation statements)

References 15 publications

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“…The IR spectra from the synthesized majorites showed hydroxyl absorption around 3350–3650 cm −1 (Figure 3), which are close to those found in the synthesized pyrope [ Ackermann et al , 1983; Geiger et al , 1991; Withers et al , 1998] and natural garnets in mantle xenoliths [ Aines and Rossman , 1984; Bell and Rossman , 1992b]. The hydroxyl absorptions consist of a sharp peak at 3580 cm −1 and a broad peak around 3450 cm −1 .…”

Section: Resultssupporting

confidence: 73%

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Water solubility in majoritic garnet in subducting oceanic crust

Katayama

Hirose

Yurimoto

et al. 2003

Geophysical Research Letters

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[1] Water in majoritic garnet synthesized in the mid-oceanic ridge basalt (MORB) + H 2 O composition at 20 GPa and 1400 -1500°C was measured by secondary ion mass spectrometry (SIMS) and infrared spectroscopy. We found that the majorite contains 1130 to 1250 ppm OH by weight. The infrared absorption band showed that incorporation of the hydroxyl in majorite is most likely due to the hydrogarnet substitution. Our results indicate that water can be transported into the mantle transition zone by nominally anhydrous minerals such as omphacite and majorite in the subducting basaltic crust. Such water may have great influences on the physical properties of slab in the transition zone.

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Section: Resultssupporting

confidence: 73%

“…Previous experimental studies reported limited solubility of hydroxyl (up to 200 ppm) in garnets at mantle depths [ Ackermann et al , 1983; Geiger et al , 1991; Lu and Keppler , 1997; Withers et al , 1998]. Most of these data, however, were restricted to Mg‐end‐member pyrope composition.…”

Section: Introductionmentioning

confidence: 99%

Water solubility in majoritic garnet in subducting oceanic crust

Katayama

Hirose

Yurimoto

et al. 2003

Geophysical Research Letters

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“…Our results show that even at low-water contents, the hydrogarnet defect is likely to form in majorite. Synthesis of majorite in the MORB+H 2 O system results in a sharp peak centered at 3580 cm -1 (Katayama et al 2003), which is only 20 cm -1 lower than the hydrogarnet substitution in synthetic pyrope (Ackermann et al 1983). For the hydrogarnet defect in majorite (P = 0 GPa), our calculated bond lengths using force field methods, d(O-O) = 2.83 and 3.24 Å for the shared and unshared edges, are shorter than those in grossular [d(O-O) = 3.08 and 3.29 Å] also calculated using interatomic potentials .…”

Section: Wave Speeds In Hydrous Majoritementioning

confidence: 98%

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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“…Based on thermodynamic analysis, Mosenfelder (2000) also concluded that the hydrogarnet substitution was the most likely mechanism to incorporate hydrogen into coesite. Up to now, the hydrogarnet substitution has only been observed in orthosilicates such as garnets where the Si tetrahedra are isolated from each other (e.g., Ackermann et al 1983;Geiger et al 1991). Koch Müller et al (2001) suggested that the two weak bands at 3296 and 3210 cm -1 are caused by coupled substitutions of boron and/or aluminum for silicon, according to Al 3+ (B 3+ ) + H + = Si 4+ .…”

Section: Introductionmentioning

confidence: 97%

OH⁻in synthetic and natural coesite

Koch‐Müller

Dera

Fei

et al. 2003

American Mineralogist

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The incorporation of hydrogen into the coesite structure was investigated at pressures ranging from 4.0-9.0 GPa and temperatures from 750-1300 ∞C using Al and B doped SiO 2 starting materials. The spectra show four sharp bands (n 1 , n 2a , n 2b , and n 3 ) in the energy range of 3450-3580 cm -1 , consistent with the hydrogarnet substitution [Si 4+(T2) + 4O 2-= va T2 + 4OH -], two weak sharp bands at 3537 and 3500 cm -1 (n 6a and n 6b ) attributed to B-based point defects, and two weaker and broad bands at 3300 and 3210 cm -1 (n 4 and n 5 ) attributed to substitution of Si 4+ by Al 3+ + H. More than 80% of the dissolved water is incorporated via the hydrogarnet substitution mechanism. The hydrogen solubility in coesite increases with pressure and temperature. At 7.5 GPa and 1100 ∞C, 1335 H/10 6 Si is incorporated into the coesite structure. At 8.5 GPa and 1200 ∞C, the incorporation mechanism changes: in the IR spectra four new sharp bands appear in the energy range of 3380-3460 cm -1 (n 7 -n 10 ) and the n 1 -n 3 bands disappear. Single crystal X-ray diffraction, Raman spectroscopy, polarized single-crystal and in situ high-pressure FTIR spectroscopy confirm that the new bands are due to OH -in coesite. The polarization and high-pressure behavior of the n 7 -n 10 OH bands is quite different from that of the n 1 -n 3 bands, indicating that the H incorporation in coesite changes dramatically at these P and T conditions. Quantitative determination of hydrogen solubility in synthetic coesite as a function of pressure, temperature, and chemical impurity allow us to interpret observations in natural coesite. Hydrogen has not previously been detected in natural coesite samples from ultra high-pressure metamorphic rocks. In this study, we report the first FTIR spectrum of a natural OH-bearing coesite. The dominant substitution mechanism in this sample is the hydrogarnet substitution and the calculated hydrogen content is about 900 z ± 300 H/10 6 Si. The coesite occurs as an inclusion in diamond together with an OH-bearing omphacite. The shift of the OH-bands of coesite and omphacite to lower energies indicates that the minerals are still under confining pressure.

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Hydrogarnet substitution in pyrope: a possible location for “water” in the mantle

Cited by 60 publications

References 15 publications

Water solubility in majoritic garnet in subducting oceanic crust

Water solubility in majoritic garnet in subducting oceanic crust

Calculation of the energetics of water incorporation in majorite garnet

OH⁻in synthetic and natural coesite

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Hydrogarnet substitution in pyrope: a possible location for “water” in the mantle

Cited by 60 publications

References 15 publications

Water solubility in majoritic garnet in subducting oceanic crust

Water solubility in majoritic garnet in subducting oceanic crust

Calculation of the energetics of water incorporation in majorite garnet

OH−in synthetic and natural coesite

Contact Info

Product

Resources

About

OH⁻in synthetic and natural coesite