Experimental investigation of volatile elements in mantle minerals and melts

氢和碳等挥发性元素对地球内部的性质具有根本性的影响，影响导致化学分异的熔化过程、地幔的对流活力以及地幔岩石的电导率，这是地球内部可测量的性质。此外，从行星内部提取挥发性元素会导致大气的形成，因此对创造宜居世界具有影响力。水通过俯冲到地球深处进行循环利用，并以矿物晶格中的点缺陷的形式储存在高压相中；正是这些含水缺陷的存在，解释了水对地幔物理性质的强烈影响。然而，人们对含水缺陷的确切性质知之甚少，这导致预测地幔流变特性和解释物理测量结果存在不确定性。当挥发物溶解到硅酸盐液体中时，会从地球内部释放出来。这些液体的粘度、密度和电导率取决于其挥发物含量，但这些关系尚未在高压下进行系统研究。溶解的挥发物的形态随化学环境而变化，使得H2在低氧逸度条件下成为主要的含水物种，例如在早期地球中可能普遍存在。为了了解岩浆海洋中可以溶解多少氢气，以及该过程如何影响行星的氧化还原演化，需要研究熔体结构对减少的挥发性物质溶解度的影响。 本提案中描述的研究计划旨在 1) 表征地幔矿物中含水缺陷的性质，2) 准确测量在高压和高温下合成的矿物的氢含量，以限制地幔中可能发生熔融的条件，3) 系统地研究挥发性元素浓度对碳酸盐硅酸盐（金伯利岩）熔体的粘度和电导率的影响，4) 测量在还原条件下硅酸盐熔体中挥发物的溶解度，例如可能适用于早期地球或其他行星体。 在我的研究中，我使用多种工具：高压设备（将在西部大学创建一个新的活塞缸实验室）、红外光谱（我的目标是创建一个针对实验产品研究进行优化的设施并培训我的学生偏振显微光谱技术）、核技术（通过现有合作）和离子束技术（将作为学生项目开发，以创建加拿大独一无二的分析能力）。

Volatile elements, such as hydrogen and carbon, have a fundamental influence on the properties of the Earth’s interior, affecting the melting processes that lead to chemical differentiation, the convective vigor of the mantle, and the electrical conductivity of mantle rocks, which is a measurable property of the Earth’s interior. Moreover, the extraction of volatile elements from planetary interiors leads to the formation of atmospheres and hence is influential in creating habitable worlds. Water is recycled through subduction into the deep Earth, where it is stored within high pressure phases as point defects in the mineral lattice; it is the presence of these hydrous defects that accounts for the strong influence of water on the physical properties of the mantle. The exact nature of the hydrous defects, however, is poorly understood, which leads to uncertainties in predicting the rheological properties of the mantle and in interpreting physical measurements. Volatiles are released from the Earth’s interior when they dissolve into silicate liquids. The viscosity, density and electrical conductivity of those liquids depend on their volatile content, but these relationships have yet to be investigated systematically at high pressure. The speciation of dissolved volatiles varies with chemical environment, such that H2 becomes the dominant hydrous species under conditions of low oxygen fugacity, such as may prevail in the early Earth. In order to know how much H2 could dissolve in a magma ocean, and how that process may affect the redox evolution of a planet, the effect of melt structure on the solubility of reduced volatile species needs to be investigated. The research program described in this proposal aims 1) to characterise the nature of hydrous defects in mantle minerals, 2) to measure accurately the hydrogen contents of minerals synthesised at high pressure and temperature, in order to constrain the conditions under which melting may occur in the mantle, 3) to investigate systematically the effects of volatile element concentrations on the viscosity and conductivity of carbonated silicate (kimberlitic) melts and 4) to measure the solubility of volatiles in silicate melts under reduced conditions, such as may apply to early Earth or to other planetary bodies. In my research I use multiple tools: high pressure apparatus (a new piston cylinder lab will be created at Western), infrared spectroscopy (my aim is to create a facility that is optimised for studying experimental products and train my students in polarised micro-spectroscopic techniques), nuclear techniques (through existing collaboration) and ion beam techniques (which will be developed as a student project to create an analytical capability that will be unique within Canada).