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

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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. ********