The compressibility of a natural composition calcium ferrite-type aluminous phase to 70 GPa

doi:10.1016/S0031-9201(02)00065-1

Physics of the Earth and Planetary Interiors

Volume 131, Issues 3–4, 30 August 2002, Pages 311-318

https://doi.org/10.1016/S0031-9201(02)00065-1 Get rights and content

Abstract

In situ X-ray diffraction (XRD) experiments of a calcium ferrite-type aluminous phase that is a sodium host mineral of subducted oceanic crusts into the Earth’s lower mantle have been carried out using a laser-heated diamond anvil cell (LHDAC), up to a pressure of 70 GPa with synchrotron radiation source at the Photon Factory (PF) in Japan. The sample was heated using a Nd:YAG laser at each pressure increment to relax the deviatoric stress in the sample. XRD measurements were carried out at 300 K using an angle-dispersive technique. The pressure was determined from an internal platinum pressure calibrant. A Birch–Murnaghan equation of state (EOS) was determined from the experimental unit cell parameters: volume V₀=244.07 (±55) Å³, density ρ₀=4.143 g/cm³, bulk modulus K₀=253 (±14) GPa, and K₀′=3.6 (±0.6). When the first pressure derivative of the bulk modulus K₀′ was fixed at 4, the value of K₀=243 (±2) GPa was obtained. The density of the calcium ferrite-type aluminous phase is lower than those of co-existing Mg-, Ca-perovskite, and hexagonal aluminous phase in subducted oceanic crusts.

Introduction

The phase relationship of mid-oceanic ridge basalt (MORB) at high pressures and temperatures is of interest to the geophysical community because MORB is one of the rocks in the oceanic crust that seems to subduct into the lower mantle (e.g.Van der Hilst et al., 1997). An aluminous phase appears in rocks having the MORB composition at pressures corresponding to lower mantle conditions, and this phenomenon has been investigated by several groups using multi-anvil high pressure apparatus (Irifune and Ringwood, 1993, Hirose et al., 1999, Ono et al., 2001). These studies have reported that the aluminous phase has a calcium ferrite-type structure (Pnam) with orthorhombic symmetry. This aluminous phase amounts to some 15–25 wt.% of all minerals having the MORB composition in the lower mantle (Kesson et al., 1994, Ono et al., 2001). The physical properties of any phase present in the subducted slab may affect its dynamics and that of material circulation in the mantle. Therefore, it is important to determine the compressibility of the MORB aluminous phase. Recently, it was reported that the incorporation of Al₂O₃ into Mg-perovskite could significantly vary its bulk modulus (Zhang and Weidner, 1999, Daniel et al., 2001, Andrault et al., 2001). The mineral compositions in the natural mantle are often quite different from those of simple systems, and to understand fully the dynamics of the mantle, it is, therefore, necessary to know the compositional effect on the physical properties.

In this study, we used a laser-heated diamond anvil cell (LHDAC) which made possible to acquire precise data on a sample under high pressure, and we also used intense X-rays from a synchrotron radiation source. We have directly investigated the stability and the pressure–volume equation of state (EOS) of a calcium ferrite-type aluminous phase in the subducted MORB over the pressure range from 0.1 MPa to 70 GPa, equivalent to the lower mantle conditions. In this paper, we compare the density of calcium ferrite-type aluminous phase with that of co-existing minerals in the lower mantle.

Section snippets

Experimental procedure

A polycrystalline specimen of calcium ferrite-type aluminous phase was synthesized in a 1000 t 6–8-type multi-anvil apparatus (SEDI-1000; see Takahashi et al., 1993) at the Magma Factory at the Tokyo Institute of Technology. A synthetic gel was used to produce a reactive and homogeneous starting material (Hamilton and Henderson, 1968). The gel was prepared as follows. Reagent-grade Fe, MgO, CaCO₃, Na₂CO₃, and K₂CO₃ were dissolved in nitric acid and mixed with solutions of (C₂H₅O)₄Si, (C₃H₉O)₄Ti

Results and discussion

The powder XRD data of the calcium ferrite-type aluminous phase at room pressure before compression revealed that this phase has an orthorhombic cell (Pnam) with unit cell dimensions of a=8.660 (2) Å, b=10.128 (2) Å and c=2.786 (1) Å. The observed and calculated XRD patterns of the calcium ferrite-type aluminous phase are compared in Table 2. The calcium ferrite-type aluminous phase in the MORB has a unit cell volume, V₀=244.34 (12) Å³, which is higher than that of MgAl₂O₄ as reported by Irifune

Acknowledgements

We thank E. Takahashi, T. Suto, T. Kurashina, T. Kawamoto, K. Mibe, M. Isshiki, and Y. Ohishi for help during experiments. The synchrotron radiation experiments were performed at the PF with the approval of the High Energy Accelerator Research Organization (KEK) (proposal no. 2001G222).

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Phase transition and density of subducted MORB crust in the lower mantle
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Phase relations, mineral chemistry, and density of a natural mid-oceanic ridge basalt (MORB) composition were investigated up to 134 GPa and 2300 K by a combination of in-situ X-ray diffraction measurements and chemical analyses using transmission electron microscope (TEM). Results demonstrate that the MORB composition consists of MgSiO₃-rich perovskite, stishovite, CaSiO₃ perovskite, and CaFe₂O₄-type Al-phase in the upper part of the lower mantle. The most abundant mineral of MgSiO₃-rich perovskite undergoes phase transition to a CaIrO₃-type post-perovskite phase above 110 GPa and 2500 K. Post-perovskite phase is similar in composition to perovskite except considerably high Na₂O content. Stishovite transforms to CaCl₂-type SiO₂ phase above 62 GPa and 2000 K and further to α-PbO₂-type phase above 110 GPa. α-PbO₂-type SiO₂ phase includes large amount of Al₂O₃, which significantly expands its stability relative to CaCl₂-type phase. Phase transition of CaSiO₃ perovskite from tetragonal to cubic was also observed with increasing temperature. CaFe₂O₄-type Al-phase is stable to the bottom of the mantle. The density of MORB crust was calculated using volume data, combining with measured chemical compositions and calculated mineral proportions. The former MORB crust is denser than the average lower mantle at all depths greater than ∼ 720 km, contrary to earlier predictions. The subducted basaltic crust may have accumulated at the base of the mantle.
Compressibility of the calcium aluminosilicate, CAS, phase to 44 GPa
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In situ X-ray diffraction measurements on a calcium aluminosilicate (CAS) phase have been carried out using a laser-heated diamond anvil cell up to a pressure of 44 GPa, employing a synchrotron radiation source. CAS is the major mineral formed from sediments subducted into the Earth's mantle. The sample was heated using a YAG laser after each pressure increment to relax the deviatoric stress in the sample. X-ray diffraction measurements were carried out at T = 300 K using an angle-dispersive technique. The pressure was calculated using an internal platinum metal pressure calibrant. The Birch–Murnaghan equation of state for the CAS phase obtained from the experimental unit cell parameters showed a density of ρ₀ = 3.888 g/cm³ and a bulk modulus of K₀ = 229 ± 9 GPa for ${K^{'}}_{0}$ = 4.7 ± 0.7. When the first pressure derivative of the bulk modulus was fixed at ${K^{'}}_{0}$ = 4, then the value of K₀ = 239 ± 2 GPa. From the experimental compressibility, the density of the CAS phase was observed to be lower than the density of co-existing Al-bearing stishovite, calcium perovskite, calcium ferrite-type phases, and (Fe,Al)-bearing Mg-perovskite in subducted sediments in the lower mantle. Therefore, the density of subducted sediments in the lower mantle decreases with increasing mineral proportion of the CAS phase.
Phase relations in hydrous MORB at 18-28 GPa: Implications for heterogeneity of the lower mantle
2005, Physics of the Earth and Planetary Interiors
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High-pressure synthesis is a powerful method for the preparation of novel materials with high elastic moduli and hardness. Additionally, such materials may exhibit interesting thermal, optoelectronic, semiconducting, magnetic, or superconducting properties. We report on the new high-pressure, high-temperature synthesis of platinum carbide. The experiments were performed in a laser-heated diamond anvil cell and data were collected using the synchrotron X-ray diffraction method at pressures >75 GPa at high-temperatures. The new platinum carbide has a rock-salt type structure, with space group Fm3m and cubic symmetry. It was confirmed to remain stable to at least 120 GPa. This structure is the same as that of other metal carbides reported in previous studies. After decompression, the new high-pressure phase was recoverable at ambient pressure. The Birch–Murnaghan equation of state for this new phase was determined from the experimental unit cell parameters, with K₀=301 (±15) GPa, and K′₀=5.2 (±0.4).

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The compressibility of a natural composition calcium ferrite-type aluminous phase to 70 GPa

Abstract

Introduction

Section snippets

Experimental procedure

Results and discussion

Acknowledgements

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Phys. Earth Plant. Inter.

Pressure-induced Landau-type transition in stishovite

Science

Finite elastic strain of cubic crystals

Phys. Rev.

Equation of state of Al-bearing perovskite to lower mantle pressure conditions

Geophys. Res. Lett.

Thermoelastic properties and crystal structure of MgSiO3 perovskite at lower mantle pressure and temperature conditions

Geophys. Res. Lett.

The preparation of silicate composition by a gelling method

Miner. Mag.

Thermoelastic properties and crystal structure of MgSiO₃ perovskite at lower mantle pressure and temperature conditions