Phase relations and equation-of-state of aluminous Mg-silicate perovskite and implications for Earth's lower mantle

M. J. Walter, A. Kubo, Takashi Yoshino, J. Brodholt, K. T. Koga, Y. Ohishi

Research output: Contribution to journalArticle

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Abstract

We have investigated the effect of Al3+ on the room-temperature compressibility of perovskite for stoichiometric compositions along the MgSiO3-AlO1.5 join with up to 25 mol% AlO1.5. Aluminous Mg-perovskite was synthesized from glass starting materials, and was observed to remain a stable phase in the range of ∼30-100 GPa at temperatures of ∼2000 to 2600 K. Lattice parameters for orthorhombic ( Pbnm ) perovskite were determined using in situ X-ray diffraction at SPring8, Japan. Addition of Al3+ into the perovskite structure increases orthorhombic distortion and unit cell volume at ambient conditions ( V0). Compression causes anisotropic decreases in axial length, with the a axis more compressive than the b and c axes by about 25% and 3%, respectively. The magnitude of orthorhombic distortion increases with pressure, but aluminous perovskite remains stable to pressures of at least 100 GPa. Our results show that substitution of Al3+ causes a mild increase in compressibility, with the bulk modulus ( K0) decreasing at a rate of -67±35 GPa/XAl. This decrease in K0 is consistent with recent theoretical calculations if essentially all Al3+ substitutes equally into the six- and eight-fold sites by charge-coupled substitution with Mg2+ and Si4+. In contrast, the large increase in compressibility reported in some studies with addition of even minor amounts of Al is consistent with substitution of Al3+ into six-fold sites via an oxygen-vacancy forming substitution reaction. Schematic phase relations within the ternary MgSiO3-AlO1.5-SiO2 indicate that a stability field of ternary defect Mg-perovskite should be stable at uppermost lower mantle conditions. Extension of phase relations into the quaternary MgSiO3-AlO1.5-FeO1.5-SiO2 based on recent experimental results indicates the existence of a complex polyhedral volume of Mg-perovskite solid solutions comprised of a mixture of charge-coupled and oxygen-vacancy Al3+ and Fe3+ substitutions. Primitive mantle with about 5 mol% AlO1.5 and an Fe3+/(Fe3++Fe2+) ratio of ∼0.5 is expected to be comprised of ferropericlase coexisiting with Mg-perovskite that has a considerable component of Al3+ and Fe3+ defect substitutions at conditions of the uppermost lower mantle. Increased pressure may favor charge-coupled substitution reactions over vacancy forming reactions, such that a region could exist in the lower mantle with a gradient in substitution mechanisms. In this case, we expect the physical and transport properties of Mg-perovskite to change with depth, with a softer, probably more hydrated, defect dominated Mg-perovskite at the top of the lower mantle, grading into a stiffer, dehydrated, charge-coupled substitution dominated Mg-perovskite at greater depth.

Original languageEnglish
Pages (from-to)501-516
Number of pages16
JournalEarth and Planetary Science Letters
Volume222
Issue number2
DOIs
Publication statusPublished - May 30 2004

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Silicates
perovskite
lower mantle
Equations of state
equation of state
silicates
Earth mantle
equations of state
silicate
Earth (planet)
substitutes
substitution
Substitution reactions
compressibility
Compressibility
defect
Oxygen vacancies
fold
Defects
defects

Keywords

  • Defects
  • Equation-of-state
  • Lower mantle
  • Perovskite
  • Substitution mechanisms

ASJC Scopus subject areas

  • Geochemistry and Petrology
  • Geophysics

Cite this

Phase relations and equation-of-state of aluminous Mg-silicate perovskite and implications for Earth's lower mantle. / Walter, M. J.; Kubo, A.; Yoshino, Takashi; Brodholt, J.; Koga, K. T.; Ohishi, Y.

In: Earth and Planetary Science Letters, Vol. 222, No. 2, 30.05.2004, p. 501-516.

Research output: Contribution to journalArticle

Walter, M. J. ; Kubo, A. ; Yoshino, Takashi ; Brodholt, J. ; Koga, K. T. ; Ohishi, Y. / Phase relations and equation-of-state of aluminous Mg-silicate perovskite and implications for Earth's lower mantle. In: Earth and Planetary Science Letters. 2004 ; Vol. 222, No. 2. pp. 501-516.
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T1 - Phase relations and equation-of-state of aluminous Mg-silicate perovskite and implications for Earth's lower mantle

AU - Walter, M. J.

AU - Kubo, A.

AU - Yoshino, Takashi

AU - Brodholt, J.

AU - Koga, K. T.

AU - Ohishi, Y.

PY - 2004/5/30

Y1 - 2004/5/30

N2 - We have investigated the effect of Al3+ on the room-temperature compressibility of perovskite for stoichiometric compositions along the MgSiO3-AlO1.5 join with up to 25 mol% AlO1.5. Aluminous Mg-perovskite was synthesized from glass starting materials, and was observed to remain a stable phase in the range of ∼30-100 GPa at temperatures of ∼2000 to 2600 K. Lattice parameters for orthorhombic ( Pbnm ) perovskite were determined using in situ X-ray diffraction at SPring8, Japan. Addition of Al3+ into the perovskite structure increases orthorhombic distortion and unit cell volume at ambient conditions ( V0). Compression causes anisotropic decreases in axial length, with the a axis more compressive than the b and c axes by about 25% and 3%, respectively. The magnitude of orthorhombic distortion increases with pressure, but aluminous perovskite remains stable to pressures of at least 100 GPa. Our results show that substitution of Al3+ causes a mild increase in compressibility, with the bulk modulus ( K0) decreasing at a rate of -67±35 GPa/XAl. This decrease in K0 is consistent with recent theoretical calculations if essentially all Al3+ substitutes equally into the six- and eight-fold sites by charge-coupled substitution with Mg2+ and Si4+. In contrast, the large increase in compressibility reported in some studies with addition of even minor amounts of Al is consistent with substitution of Al3+ into six-fold sites via an oxygen-vacancy forming substitution reaction. Schematic phase relations within the ternary MgSiO3-AlO1.5-SiO2 indicate that a stability field of ternary defect Mg-perovskite should be stable at uppermost lower mantle conditions. Extension of phase relations into the quaternary MgSiO3-AlO1.5-FeO1.5-SiO2 based on recent experimental results indicates the existence of a complex polyhedral volume of Mg-perovskite solid solutions comprised of a mixture of charge-coupled and oxygen-vacancy Al3+ and Fe3+ substitutions. Primitive mantle with about 5 mol% AlO1.5 and an Fe3+/(Fe3++Fe2+) ratio of ∼0.5 is expected to be comprised of ferropericlase coexisiting with Mg-perovskite that has a considerable component of Al3+ and Fe3+ defect substitutions at conditions of the uppermost lower mantle. Increased pressure may favor charge-coupled substitution reactions over vacancy forming reactions, such that a region could exist in the lower mantle with a gradient in substitution mechanisms. In this case, we expect the physical and transport properties of Mg-perovskite to change with depth, with a softer, probably more hydrated, defect dominated Mg-perovskite at the top of the lower mantle, grading into a stiffer, dehydrated, charge-coupled substitution dominated Mg-perovskite at greater depth.

AB - We have investigated the effect of Al3+ on the room-temperature compressibility of perovskite for stoichiometric compositions along the MgSiO3-AlO1.5 join with up to 25 mol% AlO1.5. Aluminous Mg-perovskite was synthesized from glass starting materials, and was observed to remain a stable phase in the range of ∼30-100 GPa at temperatures of ∼2000 to 2600 K. Lattice parameters for orthorhombic ( Pbnm ) perovskite were determined using in situ X-ray diffraction at SPring8, Japan. Addition of Al3+ into the perovskite structure increases orthorhombic distortion and unit cell volume at ambient conditions ( V0). Compression causes anisotropic decreases in axial length, with the a axis more compressive than the b and c axes by about 25% and 3%, respectively. The magnitude of orthorhombic distortion increases with pressure, but aluminous perovskite remains stable to pressures of at least 100 GPa. Our results show that substitution of Al3+ causes a mild increase in compressibility, with the bulk modulus ( K0) decreasing at a rate of -67±35 GPa/XAl. This decrease in K0 is consistent with recent theoretical calculations if essentially all Al3+ substitutes equally into the six- and eight-fold sites by charge-coupled substitution with Mg2+ and Si4+. In contrast, the large increase in compressibility reported in some studies with addition of even minor amounts of Al is consistent with substitution of Al3+ into six-fold sites via an oxygen-vacancy forming substitution reaction. Schematic phase relations within the ternary MgSiO3-AlO1.5-SiO2 indicate that a stability field of ternary defect Mg-perovskite should be stable at uppermost lower mantle conditions. Extension of phase relations into the quaternary MgSiO3-AlO1.5-FeO1.5-SiO2 based on recent experimental results indicates the existence of a complex polyhedral volume of Mg-perovskite solid solutions comprised of a mixture of charge-coupled and oxygen-vacancy Al3+ and Fe3+ substitutions. Primitive mantle with about 5 mol% AlO1.5 and an Fe3+/(Fe3++Fe2+) ratio of ∼0.5 is expected to be comprised of ferropericlase coexisiting with Mg-perovskite that has a considerable component of Al3+ and Fe3+ defect substitutions at conditions of the uppermost lower mantle. Increased pressure may favor charge-coupled substitution reactions over vacancy forming reactions, such that a region could exist in the lower mantle with a gradient in substitution mechanisms. In this case, we expect the physical and transport properties of Mg-perovskite to change with depth, with a softer, probably more hydrated, defect dominated Mg-perovskite at the top of the lower mantle, grading into a stiffer, dehydrated, charge-coupled substitution dominated Mg-perovskite at greater depth.

KW - Defects

KW - Equation-of-state

KW - Lower mantle

KW - Perovskite

KW - Substitution mechanisms

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