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.

地球表面表现的许多构造过程都是植根于地球更深处的过程的表现。 上地幔（地壳下面的层）的温度和成分的变化与驱动运动的力相结合，导致了地表的变形模式和/或熔体的生成（可能导致火山）。 地幔模型进一步加深了我们对这些过程演化的理解，但这些模型需要准确估计上地幔的温度和成分——而这两者都无法直接测量。 为了估计地幔的状态，通常使用对温度和成分敏感的物理特性的地球物理模型作为代理（例如地震速度和电导率）。 使用电导率作为替代指标需要在地幔预期的一系列条件下对矿物进行实验室测量。 大多数对上地幔电导率的解释仅基于矿物橄榄石，橄榄石占地幔的60-70%，并且常常忽略其余矿物成分可能的影响。 虽然橄榄石通常在电导率中占主导地位，但在某些情况下，特别是当结合水含量较高时，斜方辉石和单斜辉石（第二丰富的上地幔矿物）的电导率贡献可能与其体积不成比例。该项目将涉及收集地幔电导率在实验室控制的环境下，在各种物理条件下对辉石进行了研究，从而改进了干辉石电导率模型。 该模型将有助于改善当前并在未来帮助解释地幔电导率模型以及可能驱动熔体生成和表面动力学的过程。该项目还将包括建造可用于未来研究和培训的电导率装置对斜方辉石的电导率研究很少，对单斜辉石的电导率研究更少。 虽然他们认为斜方辉石的无水电导率与橄榄石相似，但单斜辉石大约低一个数量级，但它们是沿着预定的氧逸度路径传导的，因此限制了它们在特定条件下的应用。 该项目将重点收集与地幔相关的各种温度和氧逸度范围内地幔衍生辉石的电导率和热电测量值。 通过收集几种氧逸度状态和温度下的热电势测量值和电导率，将有可能估计各种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比较的标准，并在最近的实验中得到反复验证。在过去的几年里，水对橄榄石电导率的影响引起了人们的广泛关注。 然而，水分配实验表明，辉石所容纳的结合水量是橄榄石的十倍。 除了对整体成分有贡献之外，辉石岩（含有 50% 辉石的岩石）在区域性的矿脉中被发现，这对地球化学预算很重要，并且可能是洋中脊和洋岛玄武岩中“石榴石特征”的原因。 如果这些静脉形成相互连接的网络，它们将对地幔电导率产生不成比例的影响。 作为本研究的一部分开发的可靠的无水辉石电导率模型将有助于解释未来含水辉石的电导率实验，并改进对电磁测深推断的地幔电导率的解释。