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

俯冲带岩浆生成的实验研究：跨墨西哥火山带原始岩浆的含水液相相关系

• 批准号: 0739065
• 负责人: Paul Wallace
• 金额: $ 30万
• 依托单位: University of Oregon Eugene
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2008
• 资助国家: 美国
• 起止时间: 2008-06-01 至 2012-11-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0739065&HistoricalAwards=false
• 关键词: Experimental; Investigation; Magma; Generation; Subduction

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.

智力上的功绩。实验岩石学和橄榄石赋存熔体包裹体的有力证据表明，H2O在超俯冲带镁铁质岩浆的形成中起着重要作用。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日益复杂，但我们对俯冲板块上方地幔楔体产生岩浆的P-T-XH2O条件的了解仍有很大差距。这个项目将通过解决以下问题来检验关于弧状岩浆生成的一些基本观点：1.在含水条件下，来自TMVB的原始岩浆是否与二辉橄榄岩、方辉橄榄岩或辉石岩在地幔压力下饱和？2.富水岩浆(可能是熔融形成的)和贫水岩浆(可能是减压熔融形成的)最后与地幔组合平衡的压力是否存在差异？这种差异可能与俯冲衍生成分在楔形体内的运输和储存有关(例如，富含H2O的熔体是否来自更靠近冰块的熔体)。贫水原始岩浆的地幔平衡温度和压力是否与较热的地幔从弧后平流和上涌相一致？4.平衡温度和压力与基于2D和3D地球动力学模型的地幔楔形热结构预测如何比较？该项目将通过提供地幔楔形压力下不同深度的温度和H2O含量的数据，为评估弧内流体流动和减压熔融的相对作用和检验俯冲系统的地球动力学模型提供重要约束。原始弧岩浆的起始成分跨越了全球K2O和H2O的大部分范围。地壳深部分离结晶的成分效应将对这类母玄武岩进行评估，以测试是否可以将更多的演化成分“修正”到原始熔体成分。这个项目将通过俄勒冈大学的一名博士生的参与来整合研究和教育。学生将参与实验研究的所有方面，并将学习现代分析和成像技术的光谱(FTIR、电子探针、扫描电子显微镜)。一名本科生也将参与一项高级论文项目。对结果的解释将涉及与墨西哥一位年轻科学家Vlad Manea博士(UNAM、Juriquilla、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模型。约翰·多诺万是一位备受尊敬的电子束仪器操作员，他将参与开发分析含水RUN产品的可靠方法。