Stability of anhydrous phase B: Experimental studies and implications for phase relations in subducting slab and the X discontinuity in the mantle

Jibamitra Ganguly, Daniel J. Frost

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Abstract

We have experimentally determined the equilibrium boundary of the reaction forsterite (Fo) + periclase (Per) = anhydrous phase B (Anh-13) at 900°-1600°C, and we used the retrieved Gibbs free energy of formation of Anh-B to calculate the stability field of Anh-B + stishovite (Sti) with respect to the Mg2SiO4 polymorphs. The results suggest the possibility of the reaction sequence wadsleytite → Anh-B + Sti→ ringwoodite with increasing pressure at temperature below ∼600°C. This might lead to an eye-shaped splitting of the 410 km discontinuity within the interior of a sufficiently cold slab, such as would prevail in a 140 Myr old slab subducting at an angle of ∼60° with a rate of ≥ 14 cm/yr (e.g., Tonga). The equilibrium boundary of Anh-B + Sti with respect to the wadsleyite and ringwoodite has, respectively, positive and negative slope in the P-T space. The reaction of Fo + Per to Anh-B seems to offer a viable explanation of the so-called X discontinuity that has been observed at 275-345 km depth in several subcontinental and subduction zone environments. It is suggested that periclase could form locally by the dissolution of silica in mantle fluid or precipitate from a hydrous melt that became enriched in MgO due to the incongruent melting of olivine at pressures of 6-8 GPa.

Original languageEnglish (US)
Article numberB06203
JournalJournal of Geophysical Research: Space Physics
Volume111
Issue number6
DOIs
StatePublished - Jun 4 2006

Fingerprint

periclase
stishovite
ringwoodite
slab
discontinuity
Earth mantle
slabs
forsterite
experimental study
mantle
wadsleyite
Gibbs free energy
energy of formation
Polymorphism
olivine
Silicon Dioxide
subduction zone
Precipitates
precipitates
dissolving

ASJC Scopus subject areas

  • Geochemistry and Petrology
  • Geophysics
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science
  • Atmospheric Science
  • Astronomy and Astrophysics
  • Oceanography

Cite this

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abstract = "We have experimentally determined the equilibrium boundary of the reaction forsterite (Fo) + periclase (Per) = anhydrous phase B (Anh-13) at 900°-1600°C, and we used the retrieved Gibbs free energy of formation of Anh-B to calculate the stability field of Anh-B + stishovite (Sti) with respect to the Mg2SiO4 polymorphs. The results suggest the possibility of the reaction sequence wadsleytite → Anh-B + Sti→ ringwoodite with increasing pressure at temperature below ∼600°C. This might lead to an eye-shaped splitting of the 410 km discontinuity within the interior of a sufficiently cold slab, such as would prevail in a 140 Myr old slab subducting at an angle of ∼60° with a rate of ≥ 14 cm/yr (e.g., Tonga). The equilibrium boundary of Anh-B + Sti with respect to the wadsleyite and ringwoodite has, respectively, positive and negative slope in the P-T space. The reaction of Fo + Per to Anh-B seems to offer a viable explanation of the so-called X discontinuity that has been observed at 275-345 km depth in several subcontinental and subduction zone environments. It is suggested that periclase could form locally by the dissolution of silica in mantle fluid or precipitate from a hydrous melt that became enriched in MgO due to the incongruent melting of olivine at pressures of 6-8 GPa.",
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N2 - We have experimentally determined the equilibrium boundary of the reaction forsterite (Fo) + periclase (Per) = anhydrous phase B (Anh-13) at 900°-1600°C, and we used the retrieved Gibbs free energy of formation of Anh-B to calculate the stability field of Anh-B + stishovite (Sti) with respect to the Mg2SiO4 polymorphs. The results suggest the possibility of the reaction sequence wadsleytite → Anh-B + Sti→ ringwoodite with increasing pressure at temperature below ∼600°C. This might lead to an eye-shaped splitting of the 410 km discontinuity within the interior of a sufficiently cold slab, such as would prevail in a 140 Myr old slab subducting at an angle of ∼60° with a rate of ≥ 14 cm/yr (e.g., Tonga). The equilibrium boundary of Anh-B + Sti with respect to the wadsleyite and ringwoodite has, respectively, positive and negative slope in the P-T space. The reaction of Fo + Per to Anh-B seems to offer a viable explanation of the so-called X discontinuity that has been observed at 275-345 km depth in several subcontinental and subduction zone environments. It is suggested that periclase could form locally by the dissolution of silica in mantle fluid or precipitate from a hydrous melt that became enriched in MgO due to the incongruent melting of olivine at pressures of 6-8 GPa.

AB - We have experimentally determined the equilibrium boundary of the reaction forsterite (Fo) + periclase (Per) = anhydrous phase B (Anh-13) at 900°-1600°C, and we used the retrieved Gibbs free energy of formation of Anh-B to calculate the stability field of Anh-B + stishovite (Sti) with respect to the Mg2SiO4 polymorphs. The results suggest the possibility of the reaction sequence wadsleytite → Anh-B + Sti→ ringwoodite with increasing pressure at temperature below ∼600°C. This might lead to an eye-shaped splitting of the 410 km discontinuity within the interior of a sufficiently cold slab, such as would prevail in a 140 Myr old slab subducting at an angle of ∼60° with a rate of ≥ 14 cm/yr (e.g., Tonga). The equilibrium boundary of Anh-B + Sti with respect to the wadsleyite and ringwoodite has, respectively, positive and negative slope in the P-T space. The reaction of Fo + Per to Anh-B seems to offer a viable explanation of the so-called X discontinuity that has been observed at 275-345 km depth in several subcontinental and subduction zone environments. It is suggested that periclase could form locally by the dissolution of silica in mantle fluid or precipitate from a hydrous melt that became enriched in MgO due to the incongruent melting of olivine at pressures of 6-8 GPa.

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