Behaviour of trace metals in the solid earth and atmosphere during subduction and volcanism

地球的上表面由构造板块组成。在它们碰撞的地方，一个板块被消耗，创造了一个物质流到地球深处，以及新的岩浆添加到另一个板块的表面，表现为一连串的火山（弧）。在俯冲过程中，元素流入地球深部，并通过火山作用返回地表，形成了一个重要的地球化学循环，对地球的演化至关重要。硫（S）和与之配对的其他元素（亲硫金属）是这一循环的关键组成部分，影响大气的组成和演变，以及地壳中作为生物体所需的营养物质或人类为技术用途开发的金属的禀赋。本研究计划调查俯冲岩石中S和相关微量金属的封存，以及它们在板块会聚期间通过流体，熔体和气体释放到上覆地幔和地壳中。在流体存在下，含硫矿物（硫化物）的身份和稳定性进行了研究，通过实验室实验的压力-温度制度所经历的俯冲的Ca-，Al-和Fe丰富的岩石成分在地球上。在实验过程中测量的流体中的S和亲铜金属（As，Zn，Pb，Cu，Cd，Mo）的分区可以应用于天然岩石，以了解弧下深处的质量传递如何将元素隔离到更深的地球，用矿石金属富集地壳，或控制排放到大气中的火山气体的成分。这些实验的结果阐明了稳定性和主机的S和亲铜元素转移预期从俯冲地壳到深地幔作为功能的许多变量。详细检查的行为S和盟军在流体和固体中的chalcophiles将有助于回答一个长期存在的问题，俯冲带下的地幔如何氧化，或产生岩浆与氧化气体，控制演化的氧气在大气中随着时间的推移。岩浆通过俯冲作用返回到上板块也释放出气体。将在实验室里用一种模拟天然火山口的新装置研究火山活动排放到大气中的这些气体中有毒微量金属的产出。该仪器将用于测量微量金属从冷却的火山气体中沉淀到喷口和火山灰颗粒上的固体形式。总体目标将是隔离的变量，在如何金属在气相中传输，在什么样的物种和什么可能的分馏在低压金属运输。这些测量的影响将有助于更好地了解火山脱气过程和矿床的形成，并有助于量化现代火山爆发的毒性。最终，我们可以测试过去的火山喷发是否给地表环境带来了足够的微量金属来影响生物体或导致大规模灭绝。

The upper surface of the Earth consists of tectonic plates. Where they collide, one plate is consumed, creating a flux of material to the deeper earth, as well as the addition of new magma to the surface of the other plate, expressed as a chain of volcanoes (arcs). The flux of elements into the deeper earth during subduction, and their return to the surface by volcanism, makes an important geochemical cycle critical to the evolution of the earth. Sulfur (S) and other elements that pair with it (chalcophile metals) are key components in this cycle that affect the composition and evolution of the atmosphere, and the endowment of the crust in metals required as nutrients by living organisms, or exploited by humans for technological use. This research program investigates the sequestration of S and allied trace metals in subducted rocks, and their release to overlying mantle and crust by fluids, melts and gases during plate convergence. The identity and stability of S-bearing minerals (sulfides) in the presence of fluids is investigated by laboratory experiments over the pressure-temperature regime experienced by subducted Ca-, Al- and Fe-rich rock compositions in the earth. The partitioning of S and chalcophile metals (As, Zn, Pb, Cu, Cd, Mo) into fluids measured during the experiments can be applied to natural rocks to understand how mass transfer deep beneath arcs can either sequester elements into the deeper earth, enrich the crust with ore metals, or control the composition of volcanic gases emitted to the atmosphere. The results of such experiments elucidate the stability and host for S and chalcophile element transfer expected from the subducted crust into the deep mantle as functions of many variables. The detailed examination of behaviour S and allied chalcophiles in fluids and solids will help answer a long standing question as to how the mantle beneath subduction zones can oxidize, or produce magmas with oxidized gases, controlling the evolution of oxygen in the atmosphere over time. The return flux of magma by subduction to the upper plate also emits gases. The output of toxic trace metals in these gases emitted to the atmosphere by volcanism will be investigated in the laboratory with a new apparatus that simulates natural volcanic vents. The apparatus will be used to measure the precipitation of trace metals from cooling volcanic gases into solid forms in vents and on volcanic ash particles. The overarching goal will be to isolate the variables in how metals are transported in a gas phase, in what species and what possible fractionations ensue during low pressure metal transport. The impact of these measurements will inform a better understanding of volcanic degassing processes and the formation of ore deposits and help quantify the toxicity of modern eruptions. Ultimately we can test if volcanic eruptions in the past loaded the surface environment with enough trace metals to influence living organisms or cause mass extinctions.