Collaborative Research: Partial Melting of the Shallow Mantle from Thermodynamic Modeling of Minerals and Melt

这笔拨款是为了完善和改进熔化体项目，以应用于与地幔部分熔化有关的问题。melt是一个多功能的热力学算法包，允许岩浆过程的正演建模。将熔融体应用于地幔熔融问题，对部分熔融和熔体输送过程，特别是与亚脊过程有关的过程产生了相当大的新见解，但熔融体的缺陷阻碍了这些问题的应用。在提议的资助期内，我们将通过提高硅酸盐液体和辉石的热力学模型的复杂性，并通过将所得模型更强地与高压矿物-熔体相平衡结合，来解决这些缺陷。我们还将扩展melt的能力，使其能够应用于与地幔熔融过程相关的重要突出问题仍然存在的深度和条件。其中包括(1)在脊下较深的区域（4-6 GPa），少量挥发物被认为产生了小程度的熔融，这对次脊过程的地球动力学和喷发的MORB的地球化学具有重要意义；(2)在俯冲带上方的地幔楔，H2O和/或H2O+CO2对熔融的位置和方式以及熔体的组成和体积起着重要作用。对熔融体提出的改进将包括：(1)改变液体混合熵的模型，从一个类似矿物成分的分子混合模型转变为一个由温度、压力和组分相关的可变化学计量聚合物单元混合的模型。(2)完成我们已经开始的扩展辉石中加入铬的工作，并重新校准辉石热动力学模型，重点是匹配高压辉石相平衡。(3)添加复杂的C-O-H流体，既作为一个独特的相，又溶解在硅酸盐液体中，以及相关的碳质矿物（碳酸盐，石墨，金刚石）。(4)继续扩大用于校准的矿物-熔体相平衡实验数据库。(5)进行“地面真相”相平衡实验，测试和改进原型模型。

This grant is to refine and improve the MELTS program for application to problems related to partial melting of the mantle. MELTS is a versatile package of thermodynamic algorithms that allows forward modeling of magmatic processes. Application of MELTS to mantle melting problems has yielded considerable new insight into processes of partial melting and melt transport, particularly related to sub-ridge processes, but imperfections in MELTS have hampered applications to these problems. During the proposed grant period, we will address these defects by increasing the sophistication of the thermodynamic models of silicate liquids and pyroxenes and by binding the resulting model more strongly to high pressure mineral-melt phase equilibria. We will also extend MELTS' capabilities to allow application to depths and conditions where important outstanding problems related to mantle melting processes remain. These include (1) the deeper regions beneath ridges (4-6 GPa), where small quantities of volatiles are believed to generate small-degree melts that have great significance to the geodynamics of sub-ridge processes and to the geochemistry of erupted MORB, and (2) the mantle wedge above subduction zones, where H2O and/or H2O+CO2 play a significant role in determining the locus and style of melting, and the composition and volume of melts. Proposed improvements in MELTS will include (1) changing the model for the entropy of mixing of the liquid from one of molecular mixing of mineral-like components to one of mixing of temperature, pressure and compositionally dependent polymeric units of variable stoichiometry. (2) Completing our already-initiated work of expanding pyroxenes to incorporate Cr and recalibrating the thermodynamic model for pyroxenes with an emphasis on matching high pressure pyroxene-liquid phase equilibria (3) Adding complex C-O-H fluids, both as a distinct phase and dissolved in silicate liquids, as well as associated carbonaceous minerals (carbonates, graphite, diamond). (4) Continuing expansion of the database of mineral-melt phase equilibria experiments used in calibration. (5) Conducting "ground-truthing" phase equilibria experiments to test and improve prototype models.