P–V–T equation of state of CaAl4Si2O11 CAS phase

Physics and Chemistry of Minerals (Impact Factor: 1.54). 09/2011; 38(8):581-590. DOI: 10.1007/s00269-011-0430-7


The thermoelastic parameters of the CAS phase (CaAl4Si2O11) were examined by in situ high-pressure (up to 23.7GPa) and high-temperature (up to 2,100K) synchrotron X-ray diffraction,
using a Kawai-type multi-anvil press. P–V data at room temperature fitted to a third-order Birch–Murnaghan equation of state (BM EOS) yielded: V
0,300=324.2±0.2Å3 and K
0,300=164±6GPa for K′
0,300=6.2±0.8. With K′
0,300 fixed to 4.0, we obtained: V
0,300=324.0±0.1Å3 and K
0,300=180±1GPa. Fitting our P–V–T data with a modified high-temperature BM EOS, we obtained: V
0,300=324.2±0.1Å3, K
0,300=171±5GPa, K′
0,300=5.1±0.6 (∂K


=−0.023±0.006GPaK−1, and α0,T
=3.09±0.25×10−5K−1. Using the equation of state parameters of the CAS phase determined in the present study, we calculated a density profile
of a hypothetical continental crust that would contain ~10 vol% of CaAl4Si2O11. Because of the higher density compared with the coexisting minerals, the CAS phase is expected to be a plunging agent for
continental crust subducted in the transition zone. On the other hand, because of the lower density compared with lower mantle
minerals, the CAS phase is expected to remain buoyant in the lowermost part of the transition zone.

KeywordsCAS phase–CaAl4Si2O11
–Thermal expansion–Compressibility–High pressure–In situ X-ray diffraction

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    ABSTRACT: Multianvil experiments with long experimental durations have been made with the anorthite composition CaAl2Si2O8at pressure-temperature (P-T) conditions of 14-25 GPa and 1400-2400°C. At subsolidus conditions, these experiments demonstrated three phase assemblages, grossular (Gr) + kyanite (Ky) + stishovite (St) at ˜14 GPa, Gr + calcium-alumino-silicate phase (CAS) + St at ˜18 GPa, and CAS + CaSiO3-perovskite (CaPv) + St at above ˜20 GPa, which are related by the reactions Gr + Ky = CAS + St and Gr + St = CAS + CaPv. Following the method of Schreinemakers, we combined our data with the literature data to deduce aP-Tphase diagram for a portion of the CaO-Al2O3-SiO2system at subsolidus conditions, which subsequently helped to solve some long-lasting discrepancies in the high-Pbehavior of the compositions of anorthite and grossular. The crystal chemistry of the CAS and CaPv solid solutions was examined, and new substitution mechanisms were firmly established. Along the solidus, the melting reaction at ˜14 GPa is peritectic while that at ˜22 GPa is eutectic. For both pressures, St is the first phase to melt out and the melt is generally andesitic. For the An composition, its density starts to be significantly higher than the density of pyrolite at ˜2.5 GPa, a much lower pressure than that for the Or, Ab or Qtz composition (˜7.5-10 GPa), so that the An-enriched continental crust material should readily plunge into the upper mantle.
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    ABSTRACT: We precisely determined detailed phase relations of upper continental crust (UCC) at 20–28 GPa and 1200–1800 °C across the 660-km discontinuity conditions with a high-pressure multi-anvil apparatus. We used multi-sample chambers packed with both of UCC and pressure marker, and they were kept simultaneously at the same high-pressure and high-temperature conditions in each run. The high-pressure experiments were carried out in pressure and temperature intervals of about 1 GPa and 200 °C, respectively. At 22–25 GPa and 1600–1800 °C, UCC transformed from the assemblage of CaAl4Si2O11-rich phase (CAS)+clinopyroxene+garnet+hollandite+stishovite to that of calcium ferrite+calcium perovskite+hollandite+stishovite via the assemblage of CAS+calcium ferrite+calcium perovskite+garnet+hollandite+stishovite. No CAS was observed at 1200 °C. The textures and grain sizes in the run products suggested that hollandite (II) (monoclinic symmetry) was stable above 24–25 GPa and transformed to hollandite (I) (tetragonal symmetry) during decompression. We calculated the density of UCC at high pressure and high temperature from the mineral proportions which were calculated from the mineral compositions. UCC has a higher density than PREM up to 23.5 GPa in the range of 1200–1800 °C. Above 24 GPa, the density of UCC is lower than that of PREM at 1600–1800 °C, but is almost equal to that at 1400 °C and higher than PREM at temperature below 1400 °C. Therefore, we suggest that the subducted UCC may penetrate the 660-km discontinuity into the lower mantle, when its temperature is lower than 1400 °C at around 660 km depth.
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