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.

岩浆弧中的火山排放大量挥发性元件；相关的水热活动通常形成金属矿物质的积累，例如是世界铜的主要来源的斑岩沉积物。这种金属沉积物与火山的密切关联表明岩浆弧中的矿化与岩浆之间存在遗传联系。这些元素（挥发物，金属和许多集中在沉积物中的元素）在产生这些岩浆的地幔中的丰度较低，而它们在表面环境（即海洋中的水和卤素）中很丰富。从岩浆弧中的火山和热液系统中连续释放这些元素需要向地幔供应。即，从表面回收元素。在俯冲带中，将平板从表面拖到地幔，这种物理运动伴随着元素从表面到地幔的化学转移，尽管这种转移的机制尚不清楚。我们较早的工作表明，富含大型斑岩铜沉积物的火成岩本质上被氧化和富含S的岩石，氧化岩浆有利于金属和S从地幔到外壳的运输。但是，控制岩浆和源地幔的氧化条件的因素知之甚少。我计划检查两种类型的岩石，以更好地了解俯冲区中的元素转移，并具有以下目标： 1。是什么控制岩浆的氧化条件？岩浆的氧化条件是否反映了地幔的氧化条件？我们以前的工作证明了从超级斗篷到镁铁质火山岩的氧化条件的遗传。这种关系还适用于金属矿物质矿床的主要寄主的花岗岩岩石吗？在岩浆上升期间，乡村岩石的同化是否起着任何作用？ 2。挥发性元素如何从表面转移到热地幔？挥发性元素（例如卤素）对于金属的运输至关重要。我们的工作表明，有些人被掺入变质矿物质中，并深入地幔中，例如，蛇纹石中的砷，二级橄榄石和磷灰石中的氟，以及Phengite和Lawsonite中的硼。 拟议的研究解决了地球化学的基本问题：俯冲带中的元素如何回收，以及它们通过俯冲和地幔从表面到弧岩浆的途径是什么。这项研究也与行星科学有关，因为金属岩心可以减少地球状行星的内部，但是在岩浆阶段，某些火星火山岩在内在氧化。氧化的原因一直在讨论中。除了基本的科学知识外，拟议的研究还将通过允许将肥沃的寄主区域作为目标提供有效探索大型金属沉积物的信息。