富集尘埃系统凝结预测的实验研究
Experimental Investigation of Condensation Predictions for Dust-Enriched Systems
AI总结：
本研究通过23个Pt环平衡实验测试了富集尘埃系统中VAPORS代码对多种矿物凝结相域的预测，发现部分矿物与预测吻合，但尖晶石、钙钛矿和黄长石的稳定域存在显著偏差，为改进MELTS等热力学模型提供了校准数据。
摘要：
凝结模型描述了在封闭化学系统中，共存的矿物固溶体、硅酸盐液体和蒸气之间元素的平衡分布，蒸气相始终存在，使用在固定总压力P（< 1 bar）和温度T下相关相的状态方程。VAPORS代码在高于橄榄石稳定域的T下使用CaO-MgO-Al$_2$O$_3$-SiO$_2$液体模型，在较低T下使用MELTS算法。淬火的高温晶体+液体组合在陨石中以B型富Ca、Al包体（CAIs）和富橄榄石铁镁质球粒的形式保存。对组成区域的实验测试可以阐明存在相的性质、相边界以及微量元素在这些相之间的分配。在特定T和富集尘埃因子下，针对12种整体成分的7个相域进行了23个Pt环平衡实验，测试了从液体中结晶的镁橄榄石（Mg$_2$SiO$_4$）、顽火辉石（MgSiO$_3$）、含Cr尖晶石（MgAl$_2$O$_4$）、钙钛矿（CaTiO$_3$）、黄长石（Ca$_2$Al$_2$SiO$_7$ - Ca$_2$Mg$_2$Si$_2$O$_7$）和/或grossite（CaAl4O7）的预测稳定域。镁橄榄石、顽火辉石和grossite的实验结果在化学和相丰度上均与预测非常吻合。另一方面，尖晶石与橄榄石共存稳定域，以及钙钛矿和钙铝黄长石的稳定域与预测大相径庭。所有实验中均未出现钙钛矿。即使在低氧逸度（IW-3.4）下，最富TiO$_2$的实验也未结晶出富Al、Ti的钙质辉石。尖晶石和橄榄石共存的稳定性仅限于比预测更小的相域。黄长石稳定域远大于预测，表明当前液体或黄长石活度模型存在不足。就此而言，这些实验有助于改进包括MELTS在内的热力学模型校准数据。
英文摘要：
Condensation models describe the equilibrium distribution of elements between coexisting mineral solid solutions, silicate liquid, and vapor in a closed chemical system, vapor phase always present, using equations of state of the phases involved at a fixed total P (< 1 bar) and temperature T. The VAPORS code uses a CaO-MgO-Al$_2$O$_3$-SiO$_2$ liquid model at T above the stability field of olivine and the MELTS algorithm at lower T. Quenched high-T crystal + liquid assemblages are preserved in meteorites as Type B Ca-, Al-rich inclusions (CAIs) and olivine-rich ferromagnesian chondrules. Experimental tests of compositional regions may clarify the nature of the phases present, the phase boundaries, and the partition of trace elements among these phases. Twenty-three Pt-loop equilibrium experiments in seven phase fields on twelve bulk compositions at specific T and dust enrichment factors tested the predicted stability fields of forsteritic olivine (Mg$_2$SiO$_4$), enstatite (MgSiO$_3$), Cr-bearing spinel (MgAl$_2$O$_4$), perovskite (CaTiO$_3$), melilite (Ca$_2$Al$_2$SiO$_7$ - Ca$_2$Mg$_2$Si$_2$O$_7$) and/or grossite (CaAl4O7) crystallizing from liquid. Experimental results for forsterite, enstatite, and grossite are in very good agreement with predictions, both in chemistry and phase abundances. On the other hand, the stability of spinel with olivine, and stability of perovskite and gehlenite are quite different from predictions. Perovskite is absent in all experiments. Even at low oxygen fugacity (IW-3.4), the most TiO$_2$-rich experiments do not crystallize Al-, Ti-bearing calcic pyroxene. The stability of spinel and olivine together is limited to a smaller phase field than is predicted. The melilite stability field is much larger than predicted, indicating a deficiency of current liquid or melilite activity models. In that respect, these experiments contribute to improving the data for calibrating thermodynamic models including MELTS.
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