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

岩浆弧中的火山喷发出大量挥发性元素；伴生的热液活动通常形成金属矿物的聚集，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岩浆弧中的矿化和岩浆之间存在着遗传联系。这些元素（挥发物、金属和许多其他元素集中在沉积物中）在这些岩浆产生的地幔中丰度很低，而在地表环境（即海洋中的水和卤素）中丰度很高。火山和岩浆弧中的热液系统不断释放这些元素需要向地幔供应；也就是说，从地表回收元素。在俯冲带中，板块从地表向下拉向地幔，这种物理运动伴随着元素从地表向地幔的化学转移，尽管这种转移的机制尚不清楚。我们早期的工作表明，拥有大型斑岩铜矿床的火成岩本质上是氧化的，富含S，氧化的岩浆有利于金属和S从地幔运输到地壳。然而，控制岩浆和源幔氧化条件的因素尚不清楚。我计划考察两种岩石，以更好地了解俯冲带的元素转移，目的如下：是什么控制了岩浆的氧化条件？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弧下地幔到基性火山岩的氧化条件的继承。这种关系是否也适用于作为金属矿床主体的花岗质岩石？岩浆上升到浅层地壳时，乡村岩石的同化作用是否起作用？* * * * * * 2。挥发性元素是如何从地表移动到热地幔的？挥发性元素，如卤素，对金属的运输是必不可少的。我们的研究表明，其中一些与变质矿物结合并俯冲到地幔深处，例如，蛇纹石中的砷，次生橄榄石和磷灰石中的氟，云母岩和褐铁矿中的硼。******提出的研究解决了地球化学中的基本问题：元素如何在俯冲带中循环，以及它们通过俯冲和地幔从地表到弧岩浆的途径是什么。这项研究也与行星科学有关，因为类地行星的内部由金属核减少，但一些火星火山岩在岩浆阶段本质上被氧化。氧化的原因已经讨论过了。除了基本的科学知识外，拟议的研究还将为有效勘探大型金属矿床提供有用的信息，使肥沃的寄主区域成为目标。* * * * * * * *

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. ********