Phase transition and density of subducted MORB crust in the lower ...

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246 strated that the volume decreased by about 1% at the post-perovskite phase transition [8]. These P–V data in MORB composition, however, do not indicate such a large volume decrease. It should be due to a change in the chemical composition from perovskite to post-perovskite phase in MORB bulk composition. The P–V data of stishovite and CaCl2-type SiO2 were fitted together to the Birch–Murnaghan equation of state (Fig. 4b). Fitting result shows K0=279(F13) GPa and V0=47.3(F0.3) A˚ 3 when KV=4, which are quite consistent with the previous results on Al-bearing stishovite [40]. a-PbO2-type phase in MORB composition contains 12.6 wt.% Al2O3 that significantly expands volumes. The unitcell volume of a-PbO2-type phase synthesized in MORB composition is 73.79 A˚ 3 at 132 GPa (Table 1), which is larger by about 4% than that of pure SiO2 phase obtained at 136 GPa [12]. It is noted that the unit-cell volume of a-PbO2-type phase [Z =4] is remarkably larger than double unit-cell volume of CaCl2-type phase [Z =2] at equivalent pressure (Fig. 4b). The volumes of Ca-perovskite were determined on the basis of tetragonal structure (space group; P4/ mmm) proposed by Shim et al. [13] at 300 K and cubic structure at high temperatures. The P–V data of Ca-perovskite and CaFe2O4-type Al-phase from 40 to 132 GPa respectively lie on a single compression curve (Fig. 4c and d). Fitting to the Birch–Murnaghan equation of state show K0=245(F6) GPa and V0=45.6(F0.2) A˚ 3 K. Hirose et al. / Earth and Planetary Science Letters 237 (2005) 239–251 for Al-bearing Ca-perovskite and K 0=214(F8) GPa and V 0=239.7(F1.5) A˚ 3 for CaFe 2O 4-type Al-phase, when KV is fixed at 4. Previous measurements of the compressibility of CaFe 2O 4type Al-phase showed relatively wide variation in the value of K0. Present result is higher than that measured by Guignot and Andrault [36] (~190 GPa) but is lower than that by Ono et al. [41] (243 GPa). Fig. 4. Change in the unit-cell volumes as a function pressure for (a) MgSiO 3-rich perovskite (circles) and post-perovskite phase (squares), (b) stishovite (circles), CaCl2-type SiO2 (triangles), and a-PbO2-type SiO2 (squares), (c) CaSiO3 perovskite, and (d) CaFe2O4-type Al-phase. Half unit-cell volumes are plotted for a- PbO 2-type SiO 2 phase. Closed and open symbols indicate room temperature and high temperature (1750–2290 K) data, respectively. Solid lines show apparent compression curves at 300 K. See text for details. unit-cell volume (Å 3 unit-cell volume (Å ) 3 ) unit-cell volume (Å 3 unit-cell volume (Å ) 3 ) 155 150 145 140 135 130 125 43 42 41 40 39 38 37 40 38 36 34 210 200 190 180 170 a b c d MgPv+MgPP SiO 2 -phases CaPv CF 40 60 80 100 120 140 Pressure (GPa)
Density (g/cm 3 ) Density (g/cm 3 ) Density (g/cm 3 ) Density (g/cm 3 ) 6.5 6.0 5.5 5.0 4.5 5.6 5.4 5.2 5.0 4.8 4.6 5.6 5.4 5.2 5.0 4.8 4.6 5.6 5.4 5.2 5.0 4.8 4.6 4.4 a b c d K. Hirose et al. / Earth and Planetary Science Letters 237 (2005) 239–251 247 PREM MgPv+MgPP SiO 2 -phases CaPv CF 40 60 80 100 120 140 Pressure (GPa) 5.2. Density profile of MORB On the basis of measured crystal chemistry and volume data, the density of each constituent mineral was calculated for both 300 K and high temperatures. The chemical composition of each phase is assumed to be constant in perovskite-dominant (below 100 GPa) and post-perovskite phase-dominant assemblies (above 104 GPa), respectively. Phase transformation between stishovite and CaCl 2-type structure is second-order, and therefore both phases likely have similar chemical compositions. The mineral densities were plotted in Fig. 5, together with the PREM density profile [42] in order to illustrate which phase contributes to the buoyancy of the subducted MORB crust in the lower mantle. Mg-perovskite is the densest phase below 100 GPa and is much denser than the mean lower mantle (Fig. 5a). The post-perovskite phase is denser than perovskite by about 3% at the phase transition. SiO2 phases are least compressible and are the lightest mineral in MORB composition in a pressure range studied here (Fig. 5b). Al-bearing stishovite and CaCl2-type SiO2 phase are less dense than the PREM density at high temperatures. The density a-PbO 2-type SiO 2 phase strongly depends on the substitution mechanism of Al 2O 3. Two types of mechanism, (1) oxygen vacancytype and (2) octahedral vacancy-occupied-type, are considered. In both the cases, Al-bearing a-PbO2type SiO2 phase is remarkably less dense than CaCl2-type phase at equivalent pressure. The density of SiO2 phase in MORB composition decreases at the phase transition to a-PbO 2-type phase due to the incorporation of much higher Al 2O 3 content. a- PbO 2-type SiO 2 phase significantly contributes to the buoyancy of subducted MORB crust in the lowermost mantle. Ca-perovskite is marginally denser than the mean lower mantle (Fig. 5c). The density Fig. 5. Density profile of each constituent mineral in MORB composition. Two types of Al substitution mechanisms in a- PbO2-type SiO2 phase, oxygen vacancy-type (reversed triangles) and octahedral vacancy-occupied-type (squares), are considered here. Other symbols are same as those in Fig. 4. The PREM density is shown for comparison by broken lines [42]. High-temperature (2070–2410 K) data by Ono et al. [7] are also plotted by pluses. The main difference is seen in the density profile of CaFe2O4-type Al phase, which is primarily due to the difference in chemical composition, especially in FeO content.
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Page 3 and 4: Table 1 Volumes of coexisting miner
Page 5 and 6: CaMgAl-rich perovskite with interme
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