Recycling of elements in subduction zones and the metal fertility of magmas in volcanic arcs

俯冲带元素的循环和火山弧岩浆的金属肥力

• 批准号: RGPIN-2016-05040
• 负责人: Hattori, Keiko
• 金额: $ 2.4万
• 依托单位: University of Ottawa
• 依托单位国家: 加拿大
• 项目类别: Discovery Grants Program - Individual
• 财政年份: 2016
• 资助国家: 加拿大
• 起止时间: 2016-01-01 至 2017-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=615461
• 关键词: Recycling; elements; subduction; zones; metal

Volcanoes in magmatic arcs discharge large quantities of volatile elements; associated hydrothermal activity commonly forms accumulations of metallic minerals, such as porphyry deposits that are the principal source of world copper. The close association of such metallic deposits to volcanoes indicates a genetic link between mineralization and magmas in magmatic arcs. These elements (volatiles, metals and many other elements concentrated in the deposits) have a low abundance in the mantle where these magmas are generated, whereas they are abundant in surficial environments (i.e., water and halogens in the ocean). Continuous release of these elements from volcanoes and hydrothermal systems in magmatic arcs require a supply to the mantle; i.e., recycling of elements from the surface. In subduction zones, slabs are dragged down from the surface to the mantle, and this physical movement is accompanied by chemical transfer of elements from the surface to the mantle, although the mechanism of this transfer is not well understood. Our earlier work showed that igneous rocks that host large porphyry copper deposits are intrinsically oxidized and S-rich, with the oxidized magmas favouring transport of metals and S from the mantle to the crust. However, the factors controlling the oxidation conditions of magmas and the source mantle are poorly understood. I plan to examine two types of rocks to better understand the element transfer in subduction zones, with the following objectives: 1. What controls the oxidation conditions of magmas? Do oxidation conditions of magmas reflect those of the mantle? Our previous work demonstrated the inheritance of oxidation conditions from the subarc mantle to mafic volcanic rocks. Does this relationship also apply to granitic rocks that are the major host of metallic mineral deposit? Does the assimilation of country rocks during magma ascent to shallow crust play any role? 2. How do volatile elements move from the surface to the hot mantle? Volatile elements, such as halogens, are essential for the transport of metals. Our work shows that some are incorporated in metamorphic minerals and subducted deep into the mantle, e.g., arsenic in serpentine, fluorine in secondary olivine and apatite, and boron in phengite and lawsonite. The proposed study addresses fundamental questions in geochemistry: how are elements recycled in subduction zones, and what are their pathways from surface to arc magmas via subduction and the mantle. This study is also relevant to Planetary Science as the interior of earth-like planets are reduced with metallic cores, but some Martian volcanic rocks are intrinsically oxidized during the magmatic stage. The cause for the oxidation has been in discussion. Apart from basic scientific knowledge, the proposed study will provide information useful for efficient exploration of large metallic deposits by allowing fertile host regions to be targeted.

岩浆弧中的火山释放出大量的挥发性元素；相关的热液活动通常形成金属矿物的堆积，例如斑岩矿床，它是世界铜的主要来源。这种金属矿床与火山的密切联系表明，成矿作用与岩浆弧中的岩浆之间存在成因联系。这些元素(挥发物、金属和许多其他元素集中在矿床中)在产生岩浆的地幔中丰度较低，而在地表环境(即海洋中的水和卤素)中丰度较高。这些元素从火山和岩浆弧中的热液系统持续释放需要向地幔供应，即从地表回收元素。在俯冲带，板块被从地表拖到地幔，这种物理运动伴随着元素从地表到地幔的化学转移，尽管这种转移的机制还不是很清楚。我们早期的工作表明，赋存大型斑岩铜矿床的火成岩本质上是氧化的，富含S，氧化后的岩浆有利于金属和S从地幔向地壳的输送。然而，控制岩浆和源地幔氧化条件的因素却知之甚少。我计划研究两种类型的岩石，以更好地了解俯冲带中的元素转移，目标如下： 1.是什么控制了岩浆的氧化条件？岩浆的氧化条件是否反映地幔的氧化条件？我们以前的工作证明了从弧下地幔到镁铁质火山岩的氧化条件的继承性。这种关系是否也适用于作为金属矿床主要寄主的花岗岩？在岩浆上升到浅地壳的过程中，围岩的同化作用起到了什么作用吗？ 2.挥发性元素是如何从地表运移到热地幔的？挥发性元素，如卤素，对于金属的运输是必不可少的。我们的工作表明，一些变质矿物中的砷，如蛇纹石中的砷，次生橄榄石和磷灰石中的氟，多硅白云母和钠铝石中的硼，都存在于变质矿物中并深入地幔中。 这项拟议的研究解决了地球化学中的基本问题：元素是如何在俯冲带中循环的，以及它们通过俯冲和地幔从地表到弧状岩浆的路径是什么。这项研究也与行星科学有关，因为类地行星的内部被金属核心还原，但一些火星火山岩在岩浆阶段被内在氧化。氧化的原因一直在讨论中。除了基本的科学知识外，拟议的研究还将提供有助于有效勘探大型金属矿床的信息，使富饶的宿主地区成为目标。