University of Tasmania, Australia

Evolution of metals and volatiles in arc and back-arc magmas: A study of submarine glasses and melt inclusions in phenocrysts

Many deposits of metals like copper, molybdenum, tin, tungsten, zinc and lead are associated with either magmatism or magmatic rocks or both. For example, there is a clear temporal and spatial connection between intermediate to felsic intrusions and major porphyry Cu-Au-Mo deposits in tectonic environments related to subduction, whereas massive sulfide Cu-Zn-Pb ores are hosted within basalt-andesite-dacite-rhyolite rocks in mid-ocean spreading ridges and back-arc settings. Despite the empirical data supporting a clear association between magma type, tectonic location and development of ore deposits, our understanding of the behavior of metallic and volatile elements (S, Cl, F, and H₂O) during magma evolution and the separation of precursor hydrothermal fluid is limited. The aims of this PhD project are to:

• Determine the role of crystal fractionation in driving magma compositions towards enrichment in volatiles and metals for a given magma type;

• Determine the role of magma degassing and melt-fluid immiscibility in depleting magmas in certain elements.

This project involves a combination of volcanological, petrological and geochemical analyses with melt inclusion studies. The main steps are:

1. Detailed characterisation of fractionation series (focusing on two examples) using volcanological approach and whole rock and glass analysis;
2. Heating stage experiments with melt inclusions in different phenocrysts;
3. Electron microprobe analysis for major elements, chlorine and sulfur; laser-ablation and solution ICPMS analysis for lithophile and metallic elements, laser Raman, FTIR and PIXE analysis for volatiles in glasses and fluid inclusions.

These steps will yield data on temperature, and silicate melt and coexisting fluid compositions (including metals and volatiles). This data will be for resolving the question of whether migrating and cooling magma-derived fluids carry enough metals to be responsible for links between mineralization and western Pacific magmatism in general.

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