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

俯冲和火山作用期间固体地球和大气中痕量金属的行为

• 批准号: RGPIN-2019-03956
• 负责人: Canil, Dante
• 金额: $ 3.72万
• 依托单位: University of Victoria
• 依托单位国家: 加拿大
• 项目类别: Discovery Grants Program - Individual
• 财政年份: 2020
• 资助国家: 加拿大
• 起止时间: 2020-01-01 至 2021-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=714533
• 关键词: Behaviour; trace; metals; solid; earth

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.

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，产生一股物质流到地球深处，同时向另一个板块的表面添加新的岩浆，表现为一连串火山（弧）。俯冲过程中元素流入地球深处，并通过火山作用返回地表，形成了对地球演化至关重要的重要地球化学循环。硫 (S) 和与其配对的其他元素（亲铜金属）是该循环中的关键组成部分，影响大气的组成和演化，以及地壳中生物体所需金属的禀赋，或被人类用于技术用途的利用。该研究项目研究了俯冲岩石中硫和相关微量金属的封存，以及板块汇聚过程中流体、熔体和气体将它们释放到上覆地幔和地壳中的情况。通过实验室实验，研究了地球中俯冲富含钙、铝和铁的岩石成分所经历的压力-温度范围内含硫矿物（硫化物）在流体存在下的特性和稳定性。实验期间测得的硫和亲铜金属（As、Zn、Pb、Cu、Cd、Mo）在流体中的分配可应用于天然岩石，以了解弧下深处的传质如何将元素隔离到地球深处，使地壳富含矿石金属，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硫和亲铜元素预期从俯冲地壳转移到深部地幔的稳定性和宿主，作为许多变量的函数。对流体和固体中的行为S和相关亲铜元素的详细研究将有助于回答一个长期存在的问题，即俯冲带下方的地幔如何氧化，或产生含有氧化气体的岩浆，从而控制大气中氧气随时间的演变。俯冲到上板块的岩浆回流也会释放出气体。火山活动排放到大气中的这些气体中有毒微量金属的排放量将在实验室中使用模拟天然火山喷口的新设备进行研究。该设备将用于测量冷却火山气体在喷口和火山灰颗粒上形成固体形式的微量金属沉淀。总体目标是隔离金属如何在气相中运输、以什么种类以及在低压金属运输过程中可能发生的分馏等变量。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喷发是否给地表环境带来了足够的微量金属，足以影响生物体或导致大规模灭绝。