Chalcophile Evolution in Arc Magmas: The Story of Boninites

Abstract

Boninites are magmas derived by partial melting of highly refractory (prior melt-depleted) mantle sources. Their high-MgO at intermediate silica contents are generally restricted to supra-subduction zone, mantle wedge settings. A comprehensive study of silicate glass (formerly melt) inclusions (MI), volcanic groundmass glasses, and sulfide inclusions in boninites from the Bonin (Ogasawara) Ridge has been undertaken to constrain the behaviour of trace elements, with a particular focus on chalcophile element (Cu, Ag, PGE) abundances in these types of melt. An emphasis on high-resolution, in situ analyses has shed light on processes that control chalcophile element behaviour including, but not exclusively sulfide saturation. A broad compositional array of boninite series melts were characterised and compared to existing datasets of both in-situ glass (groundmass glass and MI), and whole rock analyses. Direct comparison between bulk and in situ datasets in the literature has revealed erroneous conclusions regarding the petrogenesis of different parental boninite melts along the Ogasawara Ridge. An alternative classification method is presented here, utilising trace elements rather than major elements, and an unsupervised machine learning algorithm to classify boninite series glasses in high dimension geochemical space. The results show that the most fractionated boninite series melts preserve geochemical fingerprints of their parentage. In conjunction with the comprehensive trace element dataset, Fe K-edge XANES of MI are presented. Fe-XANES of MI show the first direct evidence that parental boninites are equally or more reduced than MORB and become oxidised during fractionation. This suggests that the oxidised nature of arc magmas may not be a parental trait, which is of profound significance to the speciation of S and therefore the behaviour of chalcophile elements. Sulfide saturation is considered the dominant process controlling chalcophile elements in magmas, however the formation of metallic nanoparticles is evident in boninites and influences the behaviour of Ag and PGE. The formation of nanoparticles is perhaps enhanced in boninites relative to other melts due to their reduced nature and lack of S. Understanding the processes that control chalcophile elements in evolving magmas is important for constraining models describing the genesis of magmatic ore deposits, for example porphyry Cu-Au (Mo, Ag, W) deposits. To complement the study of magmatic chalcophile evolution in boninites, an experimental study of magmatic volatile phase (MVP) saturation was undertaken. Previous studies have shown that major ore-forming components, Cl and S, decouple during staggered MVP saturation. However, the behaviour of chalcophile elements during these processes has not previously been characterized. Results from this study show that Cu and Cl decouple from S during MVP saturation, which has potentially far-reaching implications for our understanding of porphyry deposit formation. Show more