Magmatic volatiles: A melt inclusion study of Taupo Volcanic Zone rhyolites,New Zealand

Abstract

The central segment of the Taupo Volcanic Zone (TVZ) is one of the world’s most productive areas of silicic volcanism and geothermal activity. Rhyolites largely predominate the eruptive output in the central TVZ, with only minor basalts, andesites and dacites. The rhyolites show diversity in composition, and form a compositional continuum between two end-member types (R1 and R2), as suggested in previous studies. In this thesis I present results from a quartz- (and rare plagioclase-) hosted melt inclusions study, focussing on the volatile concentration (i.e. H2O, Cl, F, CO2) and their relative distribution between R1 and R2 rhyolites. The main objective is to add further constraints on the magmatic systems with regard to their contribution to the hydrothermal systems in the central TVZ. A comparative study between R1 and R2 melt inclusions show distinct volatile, fluid-mobile, and highly incompatible element compositions. Differences in the bulk volatile concentration of the parental magmas (i.e. basalts intruding the lower crust) are suggested to be at the origin of these volatile disparities. Further analysis on the volatile exsolution of R1 and R2 melts lead to the observation that the two rhyolite types exsolve a volatile phase at different stages in their magmatic history. From Cl and H2O concentrations, it is suggested that R1 magmas exsolve a vapour phase first, whereas R2 rhyolites more likely exsolve a hydrosaline fluid phase. These results have considerable implications for the magmatic contribution into the hydrothermal systems in the central TVZ, as differences in the composition of the resulting volatile phase may be expected. The hydrothermal systems in the central TVZ are subdivided into two groups based on their gas and fluid chemistry; and the current model suggests that there are two distinct contributions: a typical ‘arc’ system, with geochemical affinity with andesitic fluids, located along the eastern margin of the TVZ, and a typical ‘rift’ system, with geochemical affinity with rhyolitic/basaltic fluids, located along the central and/or western region of the TVZ. The addition of the new data on the rhyolitic melt inclusions, leads to a re-evaluation of the magmatic contribution into the hydrothermal systems, with a particular focus on B and Cl. The results indicate a more diverse variety of contributions to the meteoric water in the hydrothermal systems, and also show that the east-west distribution of ‘arc’ and ‘rift’ fluids is not a viable model for the central TVZ. This work emphasises that melt inclusion data and their volatile degassing history cannot be underestimated when characterising and quantifying the magmatic component in hydrothermal fluids. The melt inclusion data also provide further insight into the pre-eruptive magmatic plumbing systems and are particularly important from a hazard perspective. Included in the thesis is a detailed petrological analysis of rhyolite melt inclusions across the central TVZ and an interpretation that large silicic magma systems (in the TVZ) are typically comprised of multiple batches of magma emplaced at some of the shallowest depths on Earth. Tectonic activity is suggested to play an important role in triggering large caldera-forming eruptions as the evacuation of one magma batch could cause a regional-scale readjustment that is sufficient enough to trigger and allow simultaneous eruption of an adjacent melt batch.