Phase equilibria modeling is a powerful petrological tool to address both forward and inverse geological problems over a broad range of crustal and upper mantle conditions of pressure (P), temperature (T), composition (X), and redox (fO2). The development of thermodynamic databases, relatively realistic activity−composition (a−X) relations for solids, melts and fluids, pressure-volume-temperature (PVT) equations of state (EOS), and efficient numerical algorithms represent an inflection point in our ability to understand the nexus between tectonics and petrogenesis. While developed—and typically applied in isolation—by either metamorphic or igneous petrologists, some of the published thermodynamic models have overlapping P-T-X calibration ranges, which enables comparisons of model outcomes for similar conditions within the range of applicability. In this paper, we systematically compare the results of two such models that are routinely used for calculating phase equilibria in melt-bearing systems: rhyolite-MELTS (Gualda et al. 2012; Ghiorso and Gualda 2015) and the metabasite set of Green et al. (2016) using the thermodynamic database ds62 (Holland and Powell 2011) (hereafter denoted as “HPx-mb16”). We selected a N-MORB composition and modeled closed system equilibrium phase relations as a function of temperature at 0.25 and 1 GPa for N-MORB with 0.5 and 4 wt% H2O. Our results show that phase relations exhibit some key differences that, in some instances, impact geological inferences. For example, clinopyroxene and plagioclase stabilities are expanded to higher temperatures in HPx-mb16 compared to predictions from rhyolite-MELTS. Orthopyroxene and olivine are stable in greater proportions and at wider temperature ranges in rhyolite-MELTS compared to HPx-mb16. Importantly, HPx-mb16 predicts amphibole in all runs presented here, whereas amphibole is only predicted at high-P–high-H2O (1 GPa and 4 wt% H2O) in rhyolite-MELTS, and in lesser amounts. Garnet stability is systematically expanded at higher temperatures, and the proportion is greater in rhyolite-MELTS. In addition to phase assemblage differences, phase compositions may differ. For example, plagioclase anorthite content is systematically higher in HPx-mb16 (for the same set of conditions), whereas garnet Mg# is higher in rhyolite-MELTS. Calculated amphibole compositions are substantially different between the two models as well. Liquid compositions also show important differences. High-T liquids are generally similar in SiO2 contents but diverge at lower temperatures; in these cases, HPx-mb16 liquids are SiO2-depleted compared to those produced by rhyolite-MELTS. Liquids are also systematically and substantially more mafic in HPx-mb16, and alumina and the alkali concentrations are relatively different and show different trends as a function of temperature at constant pressure. Overall, liquid compositions show the greatest differences near the solidus. Differences in modal abundances of phases and liquid compositions influence liquid trace-element signatures, and these differences can affect geological interpretations. Finally, a comparison between melting experiments of basaltic bulk composition and both thermodynamic models shows that rhyolite-MELTS better reproduces the higher temperature experiments, whereas HPx-mb16 better reproduces the lower temperature experiments. We discuss these and other similarities and differences to highlight the strengths and limitations of each model and to recognize that modeling results have important implications for interpretations of geologic processes. We recognize that our results are informed by a small subset of calculations over a limited range of conditions—but encourage further comparisons over a wider range of conditions and compositions.

中文翻译：

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两相平衡建模工具的比较研究：可变压力和 H2O 浓度下的 MORB 平衡状态

相平衡建模是一种强大的岩石学工具，可解决广泛的地壳和上地幔压力 (P)、温度 (T)、成分 (X) 和氧化还原 (fO2) 条件下的正向和逆向地质问题。热力学数据库的发展、固体、熔体和流体的相对真实的活性-成分 (a-X) 关系、压力-体积-温度 (PVT) 状态方程 (EOS) 以及高效的数值算法代表了我们能力的拐点了解构造和岩石成因之间的关系。虽然由变质岩或火成岩学家开发并通常单独应用，但一些已发表的热力学模型具有重叠的 PTX 校准范围，这使得在适用范围内比较类似条件下的模型结果成为可能。在本文中，我们系统地比较了两种通常用于计算含熔体系统中的相平衡的模型的结果：流纹岩-MELTS (Gualda et al. 2012; Ghiorso and Gualda 2015) 和 Green et al. . （2016 年）使用热力学数据库 ds62（Holland 和 Powell 2011）（以下称为“HPx-mb16”）。对于具有 0.5 和 4 wt% H2O 的 N-MORB，我们选择了 N-MORB 组合物，并将封闭系统平衡相关系建模为 0.25 和 1 GPa 温度的函数。我们的结果表明，相位关系表现出一些关键差异，在某些情况下，这些差异会影响地质推断。例如，与流纹岩-MELTS 的预测相比，HPx-mb16 中的单斜辉石和斜长石稳定性扩展到更高的温度。与 HPx-mb16 相比，斜方辉石和橄榄石在流纹岩-MELTS 中的比例更大且温度范围更广，因此是稳定的。重要的是，HPx-mb16 预测此处介绍的所有运行中的闪石，而闪石仅在流纹岩熔体中的高 P-高 H2O（1 GPa 和 4 wt% H2O）下预测，并且数量较少。石榴石稳定性在较高温度下系统性地扩大，并且在流纹岩-熔体中的比例更大。除了相组合差异外，相组成也可能不同。例如，HPx-mb16 中的斜长石钙长石含量系统性较高（对于相同的条件），而石榴石 Mg# 在流纹岩-MELTS 中较高。计算的闪石组成在两种模型之间也有很大不同。液体组合物也显示出重要的差异。High-T 液体的 SiO2 含量通常相似，但在较低温度下会发散；在这些情况下，与流纹岩-MELTS 生产的液体相比，HPx-mb16 液体减少了 SiO2。HPx-mb16 中的液体也系统性地和显着地更加镁铁质，氧化铝和碱的浓度相对不同，并且在恒压下显示出不同的趋势作为温度的函数。总体而言，液体成分在固相线附近表现出最大的差异。相和液体成分的模态丰度差异会影响液体微量元素特征，这些差异会影响地质解释。最后，玄武岩块状成分的熔融实验与两种热力学模型的比较表明，流纹岩-MELTS 更好地再现了较高温度的实验，而 HPx-mb16 更好地再现了低温实验。我们讨论这些和其他相似点和不同点，以突出每个模型的优势和局限性，并认识到建模结果对解释地质过程具有重要意义。我们认识到，我们的结果是由有限范围条件下的一小部分计算得出的——但鼓励在更广泛的条件和成分上进行进一步比较。