Experimental Investigation of Magma Generation in Subduction Zones: Hydrous Liquidus Phase Relations of Primitive Magmas from the Trans-Mexican Volcanic Belt

知识价值。实验岩石学和含橄榄石的熔融包裹体有力地证明了H2O在超俯冲带基性岩浆的形成中起重要作用。然而，尽管部分基于这些数据的地球化学和地球动力学模型越来越复杂，但我们对俯冲板块上方地幔楔中岩浆生成的P-T-XH2O条件的了解仍然存在重大差距。该项目将通过解决以下问题来测试关于弧岩浆生成的一些基本观点：在含水条件下的地幔压力下，来自TMVB的原始岩浆是否与橄榄岩、辉石岩或辉石岩组合饱和？富氢岩浆（可能由通量熔融形成）和贫氢岩浆（可能由减压熔融形成）最后与地幔组合平衡时的压力是否存在差异？这种差异可能与楔体内部俯冲衍生成分的运输和储存有关（例如，富氢熔体是否来自更靠近板块的地方）。3 .贫氢原始岩浆的地幔平衡温度和压力是否与弧后热地幔的平流和上涌一致？平衡温度和压力如何与基于二维和三维地球动力学模型的地幔楔热结构预测相比较？该项目将通过提供地幔楔内不同深度的温度和水含量数据，为评估流体通量和减压融化在弧中的相对作用以及测试俯冲系统的地球动力学模型提供重要的约束条件。本文将确定跨墨西哥火山带（TMVB）五种原始熔体组成的液相相关系，作为地幔楔块压力下H2O含量的函数。原始弧岩浆的起始成分跨越了K2O和H2O的大部分全球范围。将评估深层地壳分馏结晶（如母玄武岩）的成分效应，以测试更进化的成分是否可以“反向校正”到原始熔体成分。更广泛的影响。该项目将通过俄勒冈大学的一名博士生的参与，将研究和教育结合起来。学生将参与实验研究的各个方面，并将学习一系列现代分析和成像技术（FTIR，电子探针，扫描电镜）。一名本科生还将参与一篇毕业论文项目。对结果的解释将涉及与墨西哥的一位年轻科学家Vlad Manea博士（墨西哥国立大学、尤里奎拉、墨西哥和加州理工学院）的合作，他正在研究俯冲带的地球动力学建模。约翰·多诺万是一位备受推崇的电子束仪器操作员，他将参与开发分析含水产品的可靠方法。

Intellectual Merit. There is strong evidence from experimental petrology and olivine-hosted melt inclusions that H2O is important in the formation of supra-subduction zone mafic magmas. However, despite the increasing sophistication of geochemical and geodynamic models based in part upon these data, there is still a major gap in our knowledge of the P-T-XH2O conditions of magma generation in mantle wedges above subducting slabs. This project will test some fundamental ideas about arc magma generation by addressing the following questions:1. Do primitive magmas from the TMVB saturate with a lherzolite, harzburgite or pyroxenite assemblage at mantle pressures under hydrous conditions?2. Are there differences in the pressures at which H2O-rich magmas (probably formed by flux melting) and H2O-poor magmas (probably formed by decompression melting) last equilibrated with a mantle assemblage? Such differences could relate to transport and storage of subduction derived components within the wedge (e.g. do H2O-rich melts derive from closer to the slab).3. Are mantle equilibration temperatures and pressures for the H2O-poor primitive magmas consistent with advection and upwelling of hotter mantle from behind the arc?4. How do equilibration temperatures and pressures compare with predictions for the thermal structure of the mantle wedge based on 2D and 3D geodynamic models?This project will provide important constraints for evaluating the relative roles of fluid-flux and decompression melting in arcs and for testing geodynamic models of subduction systems by providing data on temperatures and H2O contents at various depths within the mantle wedge.Liquidus phase relations will be determined for five primitive melt compositions from the Trans-Mexican Volcanic Belt (TMVB) as a function of H2O content at mantle wedge pressures. Starting compositions span much of the global range of K2O and H2O for primitive arc magmas. Compositional effects of deep crustal fractional crystallization such parental basalts will be evaluated to test whether more evolved compositions can be "back-corrected" to primary melt compositions.Broader Impacts. This project will integrate research and education through the involvement of a Ph.D. student at the University of Oregon. The student will participate in all aspects of the experimental research and will learn a spectrum of modern analytical and imaging techniques (FTIR, electron probe, SEM). An undergraduate student will also be involved in a senior thesis project. The interpretation of results will involve collaboration with a young scientist in Mexico, Dr. Vlad Manea (UNAM, Juriquilla, Mexico and Caltech), who is working on geodynamic modeling of subduction zones. John Donovan, a highly regarded electron beam instrument operator, will be involved in developing robust methods for analyzing hydrous run products.