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

More documents

Recommendations

Info

244 K. Hirose et al. / Earth and Planetary Science Letters 237 (2005) 239–251 disappeared and the sharp single (200) peak was observed upon heating, indicating that Ca-perovskite adopts cubic structure at high temperature. NALphase, which is a K-bearing Al-phase, was not found in X-ray diffraction patterns and TEM observations in the present study [5,31]. It is possibly present as a host of potassium, but the amount should be very minor. In runs #3 and #5, (101) peak of stishovite changed to doublet both at 300 K and high temperatures, showing a second-order structural phase transition from tetragonal to orthorhombic. Representative Xray diffraction spectrum is presented in Fig. 2a. A mineral assemblage of Mg-perovskite +CaCl2-type SiO2 phase + Ca-perovskite+CaFe2O4-type Al-phase was obtained from 67 GPa and 1760 K to 100 GPa and 2060 K (Fig. 1). Observed and calculated X-ray diffraction pattern is presented in Table 3. Phase transition boundary between Al-bearing stishovite to CaCl2-type phase in MORB composition is located at ~62 GPa and 2000 K. At higher pressures above 113 GPa at 2240 K (runs #4, #6, and #7), X-ray diffraction pattern drastically changed (Fig. 2b). These X-ray peaks can be explained by the coexistence of MgSiO 3-rich postperovskite phase +a-PbO 2-type SiO 2 +Ca-perovskite+CaFe 2O 4-type Al-phase. This mineral assemblage was confirmed to 134 GPa and 2300 K, corresponding to the condition at the base of the mantle. Recently Ono et al. [7] reported CaTi2O4type Al-phase with a unit-cell volume of 150.2 A˚ 3 in MORB composition at 143 GPa. This volume is much smaller than that of CaFe 2O 4-type phase observed in this study at 132 GPa (173.8 A˚ 3 )(Table 1). It contrasts with the fact that volume change is minimal at the phase transition between CaFe2O4type and CaTi2O4-type structures in Mg-end-member MgAl2O4 [15]. Phase transition from (Al,Fe)-bearing MgSiO3 perovskite to a post-perovskite phase occurred between 100 and 113 GPa at 2060–2240 K. This is consistent with the result on a natural pyrolite composition [32], in which the post-perovskite phase transition was observed between 103 and 115 GPa at 2060–2550 K on the basis of Tsuchiya’s gold scale same as in this study. Pressures of post-perovskite phase transition in pyrolite and MORB compositions are apparently lower by about 10 GPa than that in pure MgSiO3 composition determined by using platinum pressure standard. It could be primarily due to the difference in the pressure scale used in the experiments. In addition, there should be compositional effects on post-perovskite phase transition, but they are not yet well understood. MORB contains high Al2O3, FeO, Fe2O3, and Na2O. Presence of Al2O3 stabilizes perovskite relative to post-perovskite phase [33]. Post-perovskite phase includes much more Na 2O than perovskite (Table 2), indicating that Na 2O expands the stability P–T field of the post-perovskite phase. Partitioning of iron is under hot debate [32,34]. It depends on the valence state and spin state, both of which significantly affect the ionic radius of iron. It is noted that Mg 2+ site is smaller in post-perovskite phase than in perovskite. Phase transition from CaCl 2-type to a-PbO 2-type SiO 2 phase occurred also between 100 and 113 GPa. a-PbO 2-type phase includes large amount of Al 2O 3 (Table 2), which should remarkably expands its stability P–T field. Murakami et al. [12] demonstrated that phase transition between CaCl2-type and a-PbO2type structures occurs above 121 GPa and 2400 K in pure SiO2 based on platinum pressure scale. 4. Mineral chemistry Chemical compositions of coexisting phases were determined for both low-pressure (perovskite-dominant) and high-pressure (post-perovskite phase-dominant) assemblies, recovered respectively from 60 and 113 GPa (Table 2). A typical TEM image is presented in Fig. 3. Analyses were made only for the part coexisting with gold grains. The coexistence of gold indicates that this portion was heated to high temperatures. The TEM analyses confirmed the mineral assemblages same as those observed in X-ray diffraction patterns. Both MgSiO3-rich post-perovskite phase and Ca-perovskite converted to amorphous on release of pressure. Chemical analyses showed that the heated area lost 10–15% iron, which is usually observed in the LHDAC experiments due to relatively large thermal gradient [32]. Post-perovskite phase is similar in composition to Mg-perovskite, except that the former has remarkably high Na2O content. Similar observation was made in a natural pyrolite composition [32]. Here we consider
K. Hirose et al. / Earth and Planetary Science Letters 237 (2005) 239–251 245 Fig. 3. TEM image of the sample recovered from 113 GPa and 2240 K. Abbreviations are similar to those in Fig. 2. Na2SiO3 end-member. Post-perovskite phase has a chemical formula on the join [(Mg,Fe 2+ ,Ca)1 X (Al,Fe 3+ ) 2X(Si,Ti) 1 X]O 3–Na 2SiO 3 (Table 2). It suggests a high abundance of Fe 3+ in the post-perovskite phase (Fe 3+ /total Fe =0.81). Stishovite includes 3.4 wt.% Al2O3 at 60 GPa, consistently with the results of previous studies on MORB composition [1–6]. On the other hand, a- PbO2-type SiO2 phase includes significantly high Al2O3 content (12.6 wt.%) at 113 GPa. It has been reported that a-PbO 2-type TiO 2 phase contains large amount of Fe 2O 3 [35]. A trivalent cation of Al 3+ may substitute Si 4+ with oxygen vacancy. Alternatively, Al may be incorporated without oxygen vacancy under such ultra-high pressure conditions. Half of the octahedral sites of oxygen hexagonal close-packed structure are vacant in a-PbO2-type structure. Such vacant site may be partially occupied by Al ions when three Si 4+ ions are substituted by four Al 3+ ions. Both Ca-perovskite and CaFe 2O 4-type Al-phase exhibit similar chemical compositions in perovskitedominant and post-perovskite phase-dominant assemblies. CaFe2O4-type Al-phase has complex chemical formula approximately on the join NaAlSiO4– (Mg,Fe)Al2O4 [36]. Relatively wide chemical variations when synthesized in MORB bulk composition were reported in the literature [1–6,22]. Mineral proportions were estimated by mass-balance calculations using all oxides analyzed except K2O. Results show 35% Mg-perovskite, 25% CaFe2O4-type Al-phase, 23% Ca-perovskite, and 17% stishovite in weight in the low-pressure assembly. The high-pressure assembly consists of 38% MgSiO 3-rich post-perovskite phase, 23% Ca-perovskite, 23% a-PbO2-type SiO2, and 16% CaFe2O4type Al-phase. A remarkable decrease of CaFe2O4type Al-phase is due to the partitioning of high Al2O3 into a-PbO2-type SiO2 phase and high Na2O into post-perovskite phase (Table 2). 5. Density of MORB crust in the lower mantle 5.1. Volume of each constituent mineral The unit-cell volumes of coexisting phases were determined from the X-ray diffraction patterns both at high temperature (1750 to 2290 K) and at 300 K (Table 1). These room temperature P–V data were fitted to the Birch–Murnaghan equation of state in order to obtain the bapparentQ compressibility of each constituent mineral in MORB composition. Previous experimental studies suggest that chemical composition of each phase changes little with increasing pressure to 100 GPa [1,3,4]. The isothermal bulk modulus (K 0) was obtained to be 217(F2) GPa for Mg-perovskite, assuming that pressure derivative of the isothermal bulk modulus, KV, is 4 and the volume at ambient condition (V0) is 169.5 A˚ 3 [2,4,6] (Fig. 4a). This is much lower than the values of ~261 GPa previously determined for Mgend-member composition (e.g., [37]). Mg-perovskite in MORB composition includes large amount of Al 2O 3 and Fe 2O 3. These results suggest that solution of Al 2O 3 and Fe 2O 3 significantly reduces the bulk modulus, consistently with theoretical predictions on the effect of Al2O3 [38,39]. It is, however, noted that chemical composition of Mg-perovskite synthesized in MORB bulk composition may be different at each pressure, which could cause apparent increase in the compressibility. The volumes of post-perovskite phase are plotted in Fig. 4a, together with those of Mg-perovskite. The unit-cell parameters of post-perovskite phase are a =2.472(1) A˚ , b =8.096(2) A˚ , and c =6.134(1) A˚ at 132 GPa and 300 K. Each unit-cell length is larger by about 6% than that of pure MgSiO3 postperovskite phase measured at 121 GPa and 300 K [8]. Previous experiments on pure MgSiO3 demon-
Page 1 and 2: Phase transition and density of sub
Page 3 and 4: Table 1 Volumes of coexisting miner
Page 5: CaMgAl-rich perovskite with interme
Page 9 and 10: Density (g/cm 3 ) Density (g/cm 3 )
Page 11 and 12: mal mantle. The density crossover i
Page 13: K. Hirose et al. / Earth and Planet

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

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?