Effects of Al content on water partitioning between orthopyroxene and olivine: Implications for lithosphere-asthenosphere boundary

Moe Sakurai, Noriyoshi Tsujino, Hiroshi Sakuma, Katsuyuki Kawamura, Eiichi Takahashi

Research output: Contribution to journalArticle

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Abstract

To investigate the partitioning coefficient of water between orthopyroxene (Opx) and olivine (Ol) (D(Opx/Ol)) under low-water concentrations (3 ~ 387wt ppm) similar to the Earth's mantle conditions, high-pressure experiments have been conducted at pressures of 1.5-6 GPa and a temperature of 1573 K. The experiments were performed with Kawai-type multi-anvil and piston-cylinder apparatus by using starting materials of natural Ol and synthetic Opx with various Al contents. The water contents were obtained with a vacuum type Fourier transform infrared spectrometer (FT-IR6100, IRT5000). IR-spectra of Ol and Al-bearing Opx in this study are similar to those obtained by high-pressure experiments (Bai and Kohlstedt, 1993) and natural rocks (Grant et al., 2007), respectively. It is believed that broad bands in IR spectra of natural Opx are due to effect of crystal distortion by large Al substitution. On the contrary, IR-spectra of Al-free Opx are not consistent with those reported by Rauch and Keppler (2002) likely because of the large difference of water fugacity. D(Al-free Opx/Ol) is ~1 at all pressure conditions or decreases with increasing pressure. However, the water contents of Al-bearing Opx are significantly larger than those of Ol at the same conditions. In addition, the effect of Al concentration in Opx on D(Opx/Ol) becomes larger with increasing pressure. The high Al content in Opx significantly increases D(Opx/Ol) and the trend increases with increasing pressure. D(Opx/Ol) drops sharply at the pressure at which the Al concentration of Opx becomes nearly 0 in the Earth's mantle conditions. This conclusion indicates that viscosity of the upper mantle decreases sharply at depths larger than those in which orthopyroxene contains no Al. The dramatic change of D(Opx/Ol) may explain the lithosphere-asthenosphere boundary beneath oceans and continents.

Original languageEnglish
Pages (from-to)284-291
Number of pages8
JournalEarth and Planetary Science Letters
Volume400
DOIs
Publication statusPublished - Aug 15 2014

Fingerprint

asthenosphere
orthopyroxene
olivine
lithosphere
partitioning
Water
water
Bearings (structural)
Earth mantle
Water content
moisture content
Earth (planet)
effect
Infrared spectrometers
Experiments
infrared spectrometers
continents
anvils
pistons
Pistons

Keywords

  • FT-IR
  • Lithosphere-asthenosphere boundary
  • Olivine
  • Orthopyroxene
  • Viscosity
  • Water partitioning coefficient

ASJC Scopus subject areas

  • Geochemistry and Petrology
  • Geophysics
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science

Cite this

Effects of Al content on water partitioning between orthopyroxene and olivine : Implications for lithosphere-asthenosphere boundary. / Sakurai, Moe; Tsujino, Noriyoshi; Sakuma, Hiroshi; Kawamura, Katsuyuki; Takahashi, Eiichi.

In: Earth and Planetary Science Letters, Vol. 400, 15.08.2014, p. 284-291.

Research output: Contribution to journalArticle

Sakurai, Moe ; Tsujino, Noriyoshi ; Sakuma, Hiroshi ; Kawamura, Katsuyuki ; Takahashi, Eiichi. / Effects of Al content on water partitioning between orthopyroxene and olivine : Implications for lithosphere-asthenosphere boundary. In: Earth and Planetary Science Letters. 2014 ; Vol. 400. pp. 284-291.
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T2 - Implications for lithosphere-asthenosphere boundary

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AU - Sakuma, Hiroshi

AU - Kawamura, Katsuyuki

AU - Takahashi, Eiichi

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AB - To investigate the partitioning coefficient of water between orthopyroxene (Opx) and olivine (Ol) (D(Opx/Ol)) under low-water concentrations (3 ~ 387wt ppm) similar to the Earth's mantle conditions, high-pressure experiments have been conducted at pressures of 1.5-6 GPa and a temperature of 1573 K. The experiments were performed with Kawai-type multi-anvil and piston-cylinder apparatus by using starting materials of natural Ol and synthetic Opx with various Al contents. The water contents were obtained with a vacuum type Fourier transform infrared spectrometer (FT-IR6100, IRT5000). IR-spectra of Ol and Al-bearing Opx in this study are similar to those obtained by high-pressure experiments (Bai and Kohlstedt, 1993) and natural rocks (Grant et al., 2007), respectively. It is believed that broad bands in IR spectra of natural Opx are due to effect of crystal distortion by large Al substitution. On the contrary, IR-spectra of Al-free Opx are not consistent with those reported by Rauch and Keppler (2002) likely because of the large difference of water fugacity. D(Al-free Opx/Ol) is ~1 at all pressure conditions or decreases with increasing pressure. However, the water contents of Al-bearing Opx are significantly larger than those of Ol at the same conditions. In addition, the effect of Al concentration in Opx on D(Opx/Ol) becomes larger with increasing pressure. The high Al content in Opx significantly increases D(Opx/Ol) and the trend increases with increasing pressure. D(Opx/Ol) drops sharply at the pressure at which the Al concentration of Opx becomes nearly 0 in the Earth's mantle conditions. This conclusion indicates that viscosity of the upper mantle decreases sharply at depths larger than those in which orthopyroxene contains no Al. The dramatic change of D(Opx/Ol) may explain the lithosphere-asthenosphere boundary beneath oceans and continents.

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KW - Viscosity

KW - Water partitioning coefficient

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