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。我计划研究两种岩石，以便更好地了解俯冲带的元素转移，目的如下：

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.