| 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 and their pyroclastic equivalents largely predominate the eruptive output in the central TVZ, with only minor basalts, andesites and dacites. Identifying the magmatic input into the hydrothermal systems is not straightforward due to the diluted nature of the hydrothermal fluids. The high temperature geothermal systems have previously been subdivided into two groups; 1) low-gas (i.e. CO2), high Cl, low B and Li/Cs ratio systems suggested to have chemical affinities with basaltic (and rhyolitic) magmas related to back-arc fluids, or hereafter ‘rift fluids’, on the western side of the central TVZ, and 2) high-gas, low Cl, high B and Li/Cs ratio systems having chemical affinities with andesitic magmas, related to ‘arc fluids’ along the eastern border. Here, we investigate the relationship between the chemistry of the geothermal fields and the volatiles dissolved in the magmas erupted nearby. We present magmatic volatile and fluid-mobile element compositions (i.e. H2O, Cl, F, Li, B, Cs) from rhyolitic melt inclusions of the central TVZ. Rhyolitic magma compositions are subdivided into two types (R1 and R2) showing significant differences in their volatile concentrations in melt inclusions. The R1 melt inclusions have a Cl/H2O ratio of ~ 0.04 indicating exsolution dominated by a vapour phase, and R2 melts have a ratio of ~ 0.075 indicating exsolution dominated by a hydrosaline phase. We calculate degassing trends for R1 and R2 melt compositions and estimate Cl concentrations of the magmatic volatile phase (vapour and hydrosaline phase). These results indicate that there should be a considerable effect on the magmatic fluid composition contributing to the hydrothermal systems depending on the type of rhyolite residing in the subsurface. Andesitic melt inclusions from the southern TVZ (Mt. Ruapehu) have a significantly lower Cl concentration and are not likely to exsolve a hydrosaline fluid phase (at the time of melt inclusion entrapment). Hydrothermal fluids in the central TVZ have maximum Cl concentrations of ~2500 ppm (in geothermal wells), considering their diluted nature ( more than 90 % meteoric water), these fluids must have interacted with another Cl-rich fluid of magmatic origin. Considering the likely exsolution of a highly concentrated hydrosaline fluid from R2 magmas, there is a possibility that the ‘arc’ and ‘rift’ hydrothermal systems have a significant rhyolitic fluid component (even the systems on the eastern border of the central TVZ). A comparison of B among greywacke samples, melt inclusions, and hydrothermal fluids indicates that there is a B enrichment likely related to fluid-rock interaction with greywacke basement. In particular, the southern TVZ melt inclusions from Mt. Ruapehu have two to four times more B than the central TVZ melt inclusions. Boron and Cl concentrations in the hydrothermal system for the central and southern TVZ plot along distinct trends suggesting mixing between meteoric water and at least four end-member fluid compositions. Among these end-members are: three distinct magmatic fluids compositions, with R1, R2 and andesitic fluids, and chemical constituents leached from the greywacke basement and/or other host-rocks in the geothermal fields. |