Volatile Transfer from Magma Sources in the Taupo Volcanic Zone

Abstract

The central Taupo Volcanic Zone (TVZ) is a rifted arc where dominantly silicic magmatism and volcanism is closely related to tectonics. Two distinct rhyolite magma types (dry-reducing and wet-oxidizing) have erupted from the central TVZ over the past ~550kyrs. We measured volatile concentrations and Boron (B) isotopes in quartz-hosted melt inclusions from rhyolitic eruptions representing these distinct types to identify spatial and temporal variation in the volatile contributions to the overlying hydrothermal systems. Dry magma in the upper crust may not be volatile saturated and, therefore, would contribute very little to the overlying hydrothermal system. Melt inclusions from the dry Ohakuri and Mamaku eruptions (~240 ka) have high chlorine values (0.25-0.36wt%), and show a positive correlation between chlorine and incompatible elements, suggesting that no vapour phase was exsolved prior to eruption. In comparison, volatile data from the wet Kaharoa eruption (~1314AD) show Cl loss to a coexisting volatile phase, i.e. exsolution of a volatile phase occurred during crystallization. Measured B and ä11B in these two different types of systems also reveal distinct signatures. The boron content of the Kaharoa magma ranges from 20-30ppm, and the isotopic composition is homogeneous, with ä11B of + 4‰. As the Kaharoa was volatile saturated prior to eruption, a much higher boron content may have existed (B preferentially partitions into the volatile phase). The Mamaku and Ohakuri melts, on the other hand, have homogeneous boron contents around 15 ppm, but isotopic ratios ranging from – 3 to + 3‰. We attribute these two different magmatic signatures to a variation in slab-derived fluid flux across the TVZ. The spatial distribution of the two different types of rhyolite (wet vs. dry) can be mapped in the TVZ and directly linked to the input of slab-derived fluids. As such, boron could be an effective tracer for volatile transfer to overlying geothermal systems where magmas are suspected to be saturated.