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。在它们相撞的地方，一个板块被消耗，创造了物质流向地球更深的地方，以及新的岩浆添加到另一个板块的表面，表现为一系列火山(弧线)。在俯冲过程中，元素流入地球深处，并通过火山作用返回地表，这使得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)是这一循环中的关键成分，影响大气的组成和演变，并影响地壳中金属的赋存，这些金属是活的有机体作为营养物质所需的，或被人类为技术用途而开发的。该研究项目研究了S及其伴生的痕量金属在俯冲岩石中的封存，以及在板块会聚过程中通过流体、熔体和气体释放到上覆地幔和地壳中。通过室内实验研究了含S矿物(硫化物)在流体作用下的同一性和稳定性，研究了俯冲的地球富钙、富铝、富铁岩石成分所经历的压力-温度关系。在实验中测量到的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转移如何将元素隔离到地球更深的地方，用矿石金属丰富地壳，或者控制排放到大气中的火山气体的组成。这些实验的结果阐明了S和亲磷元素从俯冲地壳转移到深部地幔的稳定性和赋存状态是许多变量的函数。对S及其同系物在流体和固体中行为的详细研究，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释放。俯冲到上板块的岩浆回流也释放出气体。由火山活动排放到大气中的这些气体中有毒微量金属的产量将在实验室用一种模拟天然火山喷口的新设备进行调查。该仪器将被用来测量从冷却的火山气体中以固体形式在喷口和火山灰颗粒上沉淀的痕量金属。首要目标将是隔离金属在气相中如何运输、在低压金属运输过程中以什么物种存在以及可能发生什么分馏的变量。这些测量的影响将有助于更好地了解火山脱气过程和矿藏的形成，并有助于量化现代喷发的毒性。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痕量金属，以影响生物或导致大规模物种灭绝。

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.