A mineral-physics based model of mantle electrical conductivity

在地球表面表达的许多构造过程都是扎根于地球更深层次的过程的表现。上地幔(地壳下面的一层)的温度和组成的变化，再加上驱动运动的力量，导致了地表和/或熔体的变形模式(可能导致火山爆发)。地幔模型进一步加深了我们对这些过程演化的理解，但这些模型需要对上地幔的温度和成分进行准确的估计-这两者都不能直接测量。为了估计地幔的状态，通常使用对温度和成分敏感的物理性质的地球物理模型作为替代(例如地震速度和电导率)。使用电导率作为指标，要求在地幔预期的条件范围内对矿物进行实验室测量。大多数对上地幔电导率的解释只基于矿物橄榄石，它占地幔的60%-70%，往往忽略了剩余矿物成分的可能影响。虽然橄榄石通常在电导率中占主导地位，但在某些情况下，特别是在结合水含量较高的情况下，斜方辉石和单斜辉石的电导率贡献可能与其体积不成比例。该项目将涉及在实验室控制的环境中收集广泛物理条件下的地幔辉石的电导率，从而改进干辉石导电性模型。这个模型将有助于改进当前的地幔传导性模型的解释，并在未来帮助解释潜在驱动熔体生成和表面动力学的过程。这个项目还将包括建立一个传导性仪器，而不是用于未来的研究以及博士后的培训。关于斜方辉石的传导性研究很少，关于单斜辉石的传导性研究也很少。虽然他们认为辉石的无水导电性类似于橄榄石，但单斜辉石的导电性比橄榄石低一个数量级，但它们是沿着预定的氧逸度路径进行的，因此它们的应用仅限于特定条件。该项目将重点收集与地幔有关的广泛温度和氧逸度范围内来自地幔的辉石的电导率和热电势测量结果。通过收集在几个氧逸度状态和温度下的热电势测量和电导率，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的地幔条件范围。类似的橄榄石模型已经成为与实验室实验进行比较的标准，并在最近的实验中反复得到验证。在过去的几年里，水对橄榄石传导性的影响受到了极大的关注。然而，水分配实验表明，辉石的结合水含量可能是橄榄石的十倍。除了对整体成分有贡献外，地区性的辉石岩(含50%辉石的岩石)在矿脉中被发现，这对地球化学收支很重要，并可能是大洋中脊和大洋岛屿玄武岩“加仑特征”的原因。如果这些静脉形成相互连接的网络，它们将对地幔传导性产生不成比例的影响。作为这项研究的一部分，建立了可靠的无水辉石电导率模型，这将有助于解释未来关于含水辉石的电导率实验，并改进从电磁测深推断的地幔电导率的解释。

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.