A mineral-physics based model of mantle electrical conductivity

基于矿物物理学的地幔电导率模型

• 批准号: 1112861
• 负责人: Steven Constable
• 金额: $ 35.72万
• 依托单位: University of California-San Diego Scripps Inst of Oceanography
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2011
• 资助国家: 美国
• 起止时间: 2011-06-01 至 2016-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1112861&HistoricalAwards=false
• 关键词: mineral; physics; based; model; mantle

Many of the tectonic processes expressed at Earth's surface are themanifestation of processes rooted deeper within the planet. Variations intemperatures and composition of the upper mantle (the layer below thecrust) combined with the forces that drive motion contribute to thedeformation patterns at the surface and/or melt generation (possiblyresulting in volcanoes). Models of the mantle further our understandingof the evolution of these processes, but these models require accurateestimates of temperature and composition of the upper mantle---neitherof which can be measured directly. To estimate the state of the mantle,geophysical models of a physical property sensitive to temperature andcomposition are typically used as a proxy (e.g. seismic velocity and electricalconductivity). Using electrical conductivity as a proxy requireslaboratory measurements on minerals at a range of conditions expectedfor the mantle. Most interpretations of upper mantleelectrical conductivity are based only on the mineral olivine,which comprises 60-70% of the mantle, and often ignore the possibleinfluence of the remaining mineral components. While olivine typicallydominates the electrical conductivity, there are cases, particularlywhen bound water content is high, where electrical conductivitycontributions from orthopyroxene and clinopyroxene, the next mostabundant upper mantle minerals, may be disproportionate to their volume.This project will involve the collection of electrical conductivity onmantle pyroxenes in a laboratory controlled setting over a wide range ofphysical conditions, leading to an improved model of dry pyroxeneconductivity. This model will help improve current,and aid in future, interpretations of mantle conductivity models and theprocesses that potentially drive melt generation and surface dynamics.This project will also include the building of a conductivity apparatusthan can be used in future studies as well as the training of a postdoc.There have been few electrical conductivity studies on orthopyroxene andfewer on clinopyroxene. While they suggest anhydrous conductivity oforthopyroxene is similar to that of olivine, with clinopyroxene about oneorder of magnitude lower, they were conducted along predetermined oxygenfugacity paths, thus limiting their application to specific conditions. This projectwill focus on the collection of electrical conductivity and thermopowermeasurements on mantle derived pyroxenes over a wide range oftemperatures and oxygen fugacities relevant to the mantle. Bycollecting thermopower measurements in tandem with electricalconductivity at several oxygen fugacity states and temperatures itwill be possible to estimate the concentration and mobility of thevarious charge carriers and build point defect models that extendconductivity estimates to a much larger range of mantle conditions. Asimilar model for olivine has become a standard for comparison withlaboratory experiments and repeatedly verified in recent experiments.In the past few years there has been a great deal of attention paid tothe effect of water on olivine conductivity. However, water partitioningexperiments show that pyroxenes may hold ten times as much bound H2O asolivine. Apart from contributing to bulk composition, pyroxenites (rockswith 50% pyroxene) are found regionally in veins which are important tothe geochemical budget, and may be responsible for the "garnetsignature" in mid-ocean ridges and ocean island basalts. If such veinsform interconnected networks they would have a disproportionate effecton mantle conductivity. A reliable anhydrous pyroxene conductivitymodel developed as part of this study will aid interpretation of futureelectrical conductivity experiments on hydrous pyroxenes, as well as improve the interpretation of mantle conductivities inferred fromelectromagnetic sounding.

许多在地球表面表现出来的构造过程是根植于地球内部更深处的过程的体现。 温度和上地幔（地壳下面的一层）成分的变化与驱动运动的力量相结合，导致了地表的变形模式和/或熔体的产生（可能导致火山）。 地幔模型使我们进一步了解这些过程的演化，但这些模型需要对上地幔的温度和成分进行精确的估计-这两者都不能直接测量。 为了估计地幔的状态，对温度和成分敏感的物理性质的地球物理模型通常被用作替代（例如地震速度和电导率）。 使用电导率作为替代指标需要在预期的地幔条件范围内对矿物进行实验室测量。 大多数上地幔电导率的解释仅基于矿物橄榄石，其占地幔的60-70%，并且通常忽略了其余矿物成分的可能影响。 虽然橄榄石的电导率通常占主导地位，但在某些情况下，特别是当束缚水含量高时，上地幔中含量仅次于橄榄石的斜方辉石和单斜辉石的电导率可能与其体积不成比例。本项目将在实验室控制的环境中收集各种物理条件下地幔辉石的电导率，从而得到改进的干辉石导电率模型。 该模型将有助于改善当前和未来对地幔电导率模型的解释，以及潜在驱动熔体生成和表面动力学的过程。该项目还将包括建立一个电导率仪器，用于未来的研究以及博士后的培训。 虽然他们认为斜方辉石的无水电导率与橄榄石相似，单斜方辉石的电导率低一个数量级，但他们是沿着预定的氧逸度路径进行的，从而限制了它们在特定条件下的应用。 该项目的重点是收集地幔来源的辉石在与地幔有关的广泛温度范围内的电导率和热功率测量结果以及氧逸度。 通过在几个氧逸度状态和温度下收集热电测量值和电导率，将有可能估计各种电荷载流子的浓度和迁移率，并建立点缺陷模型，将电导率估计扩展到更大范围的地幔条件. 橄榄石的类似模型已成为与实验室实验进行比较的标准，并在最近的实验中得到反复验证，水对橄榄石电导率的影响在过去几年中引起了人们的极大关注。 然而，水分配实验表明，辉石可能持有十倍的结合H2O橄榄石。 辉石岩（含50%辉石的岩石）除了对整体成分有贡献外，还在区域性矿脉中发现，这对地球化学收支很重要，可能是造成大洋中脊和洋岛玄武岩中“石榴石特征”的原因。 如果这样的矿脉形成相互连接的网络，它们将对地幔传导性产生不成比例的影响。 一个可靠的无水辉石conductivitymodel开发作为这项研究的一部分，将有助于解释未来的含水辉石的电导率实验，以及提高解释地幔电导率推断从电磁测深。