The Behavior of Metals (Pb, Zn, As, Mo, Cu) During Crystallization and Degassing of Rhyolites from the Okataina Volcanic Center, Taupo Volcanic Zone, New Zealand

Abstract

The Taupo Volcanic Zone (TVZ), New Zealand is a region of voluminous and frequent rhyolitic volcanism and widespread geothermal activity. Additionally, the hydrothermal systems of the TVZ contain relatively high concentrations of base and precious metals. Here we present an extensive dataset of major element, volatile, and trace element (including Pb, Zn, As, Mo, Cu) abundances in melt inclusions, pumice glasses and minerals from eight eruptions within the Okataina Volcanic Center (OVC) of the TVZ to investigate the behavior of metals during melt evolution. The high-SiO2 rhyolites of the OVC contain high concentrations of volatiles (≤6 wt % H2O, ≤0·25 wt % Cl) and underwent significant degassing prior to and during eruption. The OVC melts contain moderate concentrations of metals (11–24 ppm Pb, 20–50 ppm Zn, 2–7 ppm As, <2·5 ppm Mo, <∼5 ppm Cu). Ferromagnesian minerals (amphibole, biotite and orthopyroxene) in the OVC pumice have high concentrations of Zn (≤1500 ppm), and plagioclase and biotite contain moderate amounts of Pb (≤11 ppm). The melt inclusion and pumice glass trace element data reveal complex histories of magma mixing and mingling prior to eruption; however, discrete melt batches are easily identified based on trace element geochemistry. Variations in incompatible trace elements within these melt batches suggest that the OVC rhyolites underwent at least ∼20–25% fractional crystallization during quartz crystallization and melt inclusion entrapment (at pressures of ∼100–200 MPa) and little to no crystallization (≤5%) during ascent and eruption. Comparison of melt inclusion metal and incompatible element (e.g. U) concentrations reveals that melt Pb, Mo and As increase, whereas melt Zn decreases, during fractional crystallization at depth (∼100–200 MPa). These observations can be explained by minor partitioning of the metals Pb, Mo and As into the fractionating minerals and stronger partitioning of Zn into the ferromagnesian phases, supported by calculated metal D values and analyzed metal concentrations in OVC minerals. Interestingly, throughout both deep, vapor-saturated crystallization and during extensive degassing during magma ascent and eruption (as recorded by pumice glasses), the metals analyzed here do not appear to partition appreciably into the vapor. We propose that the lack of volatility of the metals analyzed in this study can be attributed to a combination of several factors. First, vapor–melt partitioning requires the presence of ligands—commonly Cl, S and OH—with which the metals may complex. Given the low Cl/H2O ratios in the OVC melts and the extensive degassing of H2O compared with Cl, it seems likely that the rhyolites would have exsolved H2O-rich vapor with insufficient Cl to transport metals (in particular Pb and Zn) into the vapor phase, either at depth or during magma ascent. Second, the overall small volumes of vapor present during crystallization at pressures of 100–200 MPa would have impeded significant vapor–melt partitioning of the metals. Finally, the estimated very rapid ascent of the OVC melts from depths of 4–8 km suggests that there was insufficient time at low pressure for diffusion of metals out of the melt. These results imply that there may be an indirect connection between the rhyolites and the metals of the hydrothermal systems of the TVZ. As the metals, and other species such as Cl, remain in the rhyolitic magmas upon eruption, they are available in the large volumes of rhyolite emplaced in the upper crust of the TVZ for leaching by heated meteoric waters.