Behaviour of Igneous Systems and Metal endowment across Arcs (BISMArc)

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Rifting of arc crust may host large hydrothermal systems with the potential of forming large mineral deposits of economic relevance. The highly variable structural and magmatic conditions across arcs into the backarc environment provide a unique opportunity to investigate a large diversity of magmatic and hydrothermal systems (e.g., Hannington et al., 2005). Here, we aim at quantifying both the changes in the melting regime (physical conditions, melting and mantle sources) in the transition from arc front into backarc and the impact on metal potential. A direct contribution of magmatic volatiles to (ore-forming) hydrothermal fluids is known from island arc volcanoes but the general links between oxidation state, sulphur saturation and magmatic degassing on metal behaviour in silicate melts are still a matter of active debate. Jenner et al. (2010, 2015) pointed at the importance of the onset of magnetite crystallization for sulphur saturation and thus chalcophile element behaviour. However, their magnetite crisis may be restricted to specific physicochemical circumstances (such as closed system behaviour, oxidation state and melt composition etc.) and needs to be investigated in different magmatic systems (e.g., arc to backarc transition) and spatial resolution. Here, we are aiming at disentangling the influence of mantle sources and melting on the metal enrichment in melts and volatiles and the geological framework of pathways for melts and volatiles at a scale that is relevant to resource exploration. This project consists of two sub-projects, one with an emphasis on mantle sources, melting conditions and magmatic differentiation providing the basic framework and the other focused on seafloor geology (ascent paths and geodynamics) and the distinct behaviour of metals (especially Cu, Au and the so-called critical metals) and volatiles (H2O, CO2, Cl, S) in the melts. However, these two projects are closely linked (especially through the aspect of magma evolution), requiring close collaboration and frequent exchange. The focus of this study will be on the Tonga-Kermadec subduction system, where extensive sample material is readily available and two additional research cruises have been recently approved (ARCHIMEDES I and TongaRIFT).