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The postspinel phase boundary in Mg2SiO4 determined by in situ X-ray diffraction
1T. Irifune, N. Nishiyama, K. Kuroda, T. Inoue, M. Isshiki, Department of Earth Sciences, Ehime University, Matsuyama 790, Japan. W. Utsumi, Japan Atomic Energy Research Institute, Kamigori, Ako-gun, Hyogo 678-12, Japan. K. Funakoshi, Ja.
Science (New York, N.Y.)
|March 26, 1998
Summary
The postspinel phase boundary in Mg2SiO4 was determined using high-pressure experiments. This key geological boundary is located at lower pressures than previously estimated, impacting models of Earth's mantle.
Area of Science:
- Mineral Physics
- High-Pressure Geochemistry
- Geodynamics
Background:
- Understanding the phase transitions of magnesium silicate (Mg2SiO4) is crucial for interpreting seismic data and modeling Earth's deep mantle.
- The transition from the spinel (gamma) phase to the MgSiO3 perovskite + MgO periclase assemblage marks a significant change in mineral properties within the mantle transition zone.
Purpose of the Study:
- To precisely determine the phase boundary between the spinel and postspinel phases in Mg2SiO4 at high temperatures and pressures.
- To refine estimates of the Clapeyron slope for this critical phase transition.
Main Methods:
- In situ X-ray diffraction measurements were conducted using synchrotron radiation at the SPring-8 facility.
- A large multi-anvil high-pressure apparatus was employed to achieve mantle-relevant pressure and temperature conditions.
- Experiments were performed across a temperature range of 1400 to 1800 degrees Celsius.
Main Results:
- The postspinel phase boundary was located at 21.1 (+/-0.2) gigapascals at 1600 degrees Celsius.
- The determined boundary exhibits a negative Clapeyron slope, consistent with thermodynamic predictions.
- This pressure is approximately 2 gigapascals lower than previously reported estimates.
Conclusions:
- The revised phase boundary provides a more accurate constraint for geophysical models of the Earth's mantle transition zone.
- The negative Clapeyron slope indicates that the postspinel transition becomes stable at lower pressures with increasing temperature.
- These findings refine our understanding of deep Earth mineralogy and mantle dynamics.