Mantle and crustal controls on the distribution of chalcophile elements in silicate melts
In silicate melts, sulfur-loving chalcophile and siderophile elements (collectively termed CSE) are an important suite of elements both in terms of their economic importance (e.g., porphyry Cu deposits) and use as petrogenetic tracers. During the formation and evolution of basaltic melts, the fate of chalcophile elements is linked to that of sulfur, which has a finite solubility in silicate magmas. When this solubility limit is exceeded, immiscible liquid or crystalline sulfide forms and sequesters CSE from the melt. The solubility of sulfur in basaltic melts is a complex function of melt composition, temperature, pressure and oxygen fugacity. Given that these variables can vary substantially across different tectonic settings, the mechanisms which control the abundance and distribution of CSE in different magmatic environments are poorly constrained. In this thesis, I use the major, trace and volatile element systematics, together with in situ Fe3+/∑Fe and S6+/∑S measurements, of basalts from three key sample suites; (i) oceanic plateau basalts (OPB), (ii) ocean island basalts (OIB) and (iii) Iceland, to constrain the mechanisms controlling the distribution of CSE during both partial melting and the cooling and differentiation of basaltic melts in the Earth’s crust. Combined with chemical data for global mid-ocean ridge basalts and experimental models of sulfur solubility, the data presented in this thesis are used to isolate and assess the effects of crustal thickness, mantle heterogeneity and oxygen fugacity on the sulfur and CSE systematics of mantle-derived basalts. Conclusions drawn from each sample set offer unique insights into how chalcophile elements are processed and distributed in the Earth’s crust, with important implications for our understanding of how magmatic ore deposits form. I further explore methods by which accurate Fe3+/∑Fe and S6+/∑S measurements are obtained from hydrous, magnetite-saturated basalts, which benefits our understanding of how the relative valence states of Fe and S vary across different magmatic environments.
geochemistry, spectroscopy, petrology, chalcophile elements
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