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，产生了物质流到更深的地球，同时在另一个板块的表面增加了新的岩浆，表现为火山链（弧）。元素在俯冲过程中进入更深的地球，并通过火山作用返回地表，这是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（亲铜金属）是这个循环中的关键成分，它们影响大气的组成和进化，以及生物作为营养物质所需的地壳金属资源，或被人类用于技术用途。这个研究项目调查了S和相关的微量金属在俯冲岩石中的封存，以及它们在板块收敛过程中通过流体、熔体和气体释放到上覆的地幔和地壳。在流体存在下，含s矿物（硫化物）的特性和稳定性通过实验室实验研究了地球上俯冲的富钙、富铝和富铁岩石组成所经历的压力-温度状态。实验中测量到的S和亲铜金属（As, Zn, Pb, Cu, Cd, Mo）在流体中的分配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中，使地壳丰富矿石金属，或控制火山气体排放到大气中的成分。这些实验结果阐明了S和亲铜元素从俯冲地壳向深部地幔转移的稳定性和寄主是许多变量的函数。对流体和固体中S和相关的亲铜酸盐行为的详细检查将有助于回答一个长期存在的问题，即俯冲带下的地幔如何氧化，或产生带有氧化气体的岩浆，从而控制大气中氧气的演化。通过俯冲回到上板块的岩浆流也释放出气体。由火山活动排放到大气中的这些气体中的有毒微量金属的输出将在实验室中用一种模拟天然火山喷口的新装置进行研究。该仪器将用于测量从冷却的火山气体中析出的微量金属，这些金属在火山口和火山灰颗粒中形成固体形式。总体目标将是分离出金属在气相中如何运输的变量，在低压金属运输过程中，金属的种类和可能发生的分馏。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喷发的毒性。最终，我们可以测试过去的火山爆发是否给地表环境带来了足够的微量金属，从而影响生物或导致大规模灭绝。

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.