High-pressure phase relations in the system CaAl4Si2O11–NaAl3Si3O11 with implication for Na-rich CAS phase in shocked Martian meteorites
High-pressure phase relations in the system CaAl4Si2O11–NaAl3Si3O11 with implication for Na-rich CAS phase in shocked Martian meteorites
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DOI：
10.1016/j.epsl.2009.11.043
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发表时间：
2010-01
影响因子：
5.3
通讯作者：
M. Akaogi;M. Haraguchi;K. Nakanishi;H. Ajiro;H. Kojitani
中科院分区：
文献类型：
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作者：
M. Akaogi;M. Haraguchi;K. Nakanishi;H. Ajiro;H. Kojitani
在13-23GPa和1600-1900℃条件下，用多砧仪研究了naal3si3o11 - caal4si2o11体系的高压相关系。一种caal4si2o11组成的ca -铝硅酸盐，称为CAS相，在1600℃下稳定在13GPa以上。在NaAl3Si3O11-CaAl4Si2O11体系中，在22GPa以下，溶解naal3si3o11组分的CAS相与硬玉、刚玉和辉石共存，在22GPa以上，CAS相与富钠铁酸钙、刚玉和辉石共存。在1600℃时，naal3si3o11组分在CAS固溶体中的溶解度随压力的增加而增加，在22GPa左右时溶解度可达50mol%左右，超过50mol%后溶解度随压力的增加而降低。在1900℃，22GPa下，naal3si3o11组分在CAS相中的最大溶解度随温度升高而增大，可达70mol%左右。naalsi2o6硬玉在22GPa左右解离为naalsio4铁酸钙+辉石。具有六方ba -铁氧体结构的CAS相晶格参数随着naal3si3o11组分的增加而变化：a轴减小，c轴略有增大，导致摩尔体积减小。采用高温滴液量热法测定了CAS固溶体的焓。结果表明：假设的NaAl3Si3O11CAS相的焓值远高于naalsi2o6硬玉、刚玉和辉石的混合物，与naalsio4钙铁氧体、刚玉和辉石的混合物的焓值接近。当我们采用震荡火星陨石Zagami中天然富Na CAS相的Na:Ca比为75:25时，上述确定的相关系表明，天然CAS相在22GPa左右的压力和接近或高于2000-2200℃的温度下从熔体中结晶。推断的P、T条件与利用冲击陨石中其他高压矿物估计的P、T条件一致。
High-pressure phase relations in the system NaAl3Si3O11–CaAl4Si2O11were examined at 13–23GPa and 1600–1900°C, using a multianvil apparatus. A Ca-aluminosilicate with CaAl4Si2O11composition, designated CAS phase, is stable above about 13GPa at 1600°C. In the system NaAl3Si3O11–CaAl4Si2O11, the CAS phase dissolving NaAl3Si3O11component coexists with jadeite, corundum and stishovite below 22GPa, above which the CAS phase coexists with Na-rich calcium ferrite, corundum and stishovite. At 1600°C, the solubility of NaAl3Si3O11component in the CAS solid solution increases with increasing pressure up to about 50mol% at about 22GPa, above which the solubility decreases with pressure. The maximum solubility of NaAl3Si3O11component in the CAS phase increases with temperature up to around 70mol% at 1900°C at 22GPa. The dissociation of NaAlSi2O6jadeite to NaAlSiO4calcium ferrite plus stishovite occurs at about 22GPa. Lattice parameters of the CAS phase with the hexagonal Ba-ferrite structure change with increase of the NaAl3Si3O11component: a-axis decreases and c-axis slightly increases, resulting in decrease of molar volume. Enthalpies of the CAS solid solutions were measured by high-temperature drop-solution calorimetry techniques. The results show that enthalpy of hypothetical NaAl3Si3O11CAS phase is much higher than the mixture of NaAlSi2O6jadeite, corundum and stishovite and is close to that of the mixture of NaAlSiO4calcium ferrite, corundum and stishovite. When we adopt the Na:Ca ratio of 75:25 of the natural Na-rich CAS phase in a shocked Martian meteorite, Zagami, the phase relations determined above suggest that the natural CAS phase crystallized from melt at pressure around 22GPa and temperature close to or higher than 2000–2200°C. The inferred P, T conditions are consistent with those estimated using other high-pressure minerals in the shocked meteorite.