We present a structural update to the thermodynamic model for calculating peridotite phase relations and melt compositions at 0·01 to 60 kbar and from 600 °C to the peridotite liquidus in the system K2O–Na2O–CaO–FeO–MgO–Al2O3–SiO2–TiO2–Fe2O3–Cr2O3 (KNCFMASTOCr), based on the model of Holland et al., 2018 [Melting of Peridotites through to Granites: A Simple Thermodynamic Model in the System KNCFMASHTOCr. Journal of Petrology 59, 881–900]. The new model is better able to predict the phase relations and melting of ultramafic rocks, in particular the abundance of orthopyroxene in the residue and the concentration of silica in the melt. In addition, improvements in modelling Cr-spinels mean that the model is now able to reproduce Cr-content of garnet and spinel above and below the solidus without modification to the knorringite free energy. Model calculations indicate that, for peridotite composition KR4003, the spinel to garnet transition intersects the solidus at 22·1–24·8 kbar and orthopyroxene disappears from the solidus at 29·1 kbar. Below the solidus, the model is able to reproduce the abundances and compositions of phases in experimental studies and natural samples spanning a range of compositions, allowing it to be used for investigating subsolidus equilibration during mantle cooling and pressurisation/decompression. The liquid model provides a good fit to experimental data and is able to replicate the position of the solidus and the composition of both melt and residue at and above the solidus for a range of peridotite compositions. The model may, therefore, be used to investigate fractional mantle melting and basalt generation in modern geodynamic regimes, and also to explore equilibrium mantle melting in the early Earth. The model can also be used to explore liquid and residue compositions for melting of non-pyrolitic mantle, for which there is a paucity of experimental data. We demonstrate the scope of the model using two case studies investigating the subsolidus evolution and melting of a silica-rich cratonic peridotite from the Kaapvaal craton.

中文翻译：

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橄榄岩亚固相线演化和熔融的热力学模型

我们对热力学模型进行了结构更新，用于计算 0·01 至 60 kbar 和从 600 °C 到 K2O–Na2O–CaO–FeO–MgO–Al2O3–SiO2– 体系中橄榄岩液相线的橄榄岩相关系和熔体成分TiO2–Fe2O3–Cr2O3 (KNCFMASTOCr)，基于 Holland 等人的模型，2018 [将橄榄岩熔化到花岗岩：系统中的简单热力学模型 KNCFMASHTOCr。岩石学杂志 59, 881–900]。新模型能够更好地预测超镁铁质岩的相关系和熔融，特别是残渣中斜方辉石的丰度和熔体中二氧化硅的浓度。此外，Cr-尖晶石建模的改进意味着该模型现在能够在固相线上方和下方重现石榴石和尖晶石的 Cr 含量，而无需修改 knorringite 自由能。模型计算表明，对于橄榄岩成分 KR4003，尖晶石到石榴石的转变在 22·1–24·8 kbar 时与固相线相交，而斜方辉石在 29·1 kbar 时从固相线消失。在固相线下方，该模型能够重现实验研究和自然样品中跨越一系列成分的相的丰度和组成，使其可用于研究地幔冷却和加压/减压期间的亚固相线平衡。液体模型提供了与实验数据的良好拟合，并且能够复制固相线的位置以及固相线及其上方的熔体和残余物的组成，用于一系列橄榄岩组成。因此，该模型可用于研究现代地球动力学条件下的部分地幔熔融和玄武岩生成，以及探索地球早期的平衡地幔融化。该模型还可用于探索非热解地幔熔融的液体和残渣成分，但缺乏实验数据。我们使用两个案例研究来展示模型的范围，这些案例研究调查了来自 Kaapvaal 克拉通的富含二氧化硅的克拉通橄榄岩的亚固相线演化和熔融。