Degassing of the H_2O-rich rhyolites of the Okataina Volcanic Center, Taupo Volcanic Zone, New Zealand

摘要

The Taupo Volcanic Zone (TVZ), New Zealand, is the most active region of rhyolitic volcanism on Earth, with >50 rhyolitic eruptions and ~780 km~3 of magma erupted in the past 60 k.y. Here we use analyses of quartz-hosted melt inclusions from eight eruptions in the Okataina Volcanic Center (OVC) of the TVZ to constrain magmatic volatile contents, pressures, and temperatures of quartz crystallization, and degassing of H_2O, Cl, and minor CO_2 from the rhyolitic magmas. The OVC melt inclusions trapped volatile-rich melts with δ6 wt% H_2O and δ0.27 wt% Cl. Our data indicate that vapor-saturated crystallization of quartz occurred at low temperatures (760-805 oC) over a wide range of pressures (50-200 MPa). For some eruptions, variations in volatiles and major and trace elements provide evidence for simultaneous crystallization and partial loss of H_2O, Cl, and CO_2 into a vapor phase. Using the combination of melt inclusion and pumice glass volatile contents, we calculate minimum volatile emissions of ~3 × 10~(11) to 8 × 10~(12) kg H_2O and ~7 × 10~9 to 4 × 10~(10) kg Cl during the OVC eruptions. We estimate that emissions from the smaller volume (<13 km~3 magma) OVC rhyolitic eruptions would have been equivalent to ~15%-40% of the yearly global arc flux of H_2O and ~10%-50% of the global arc flux of Cl, whereas the large-volume (ε100 km~3) Rotoiti eruption ca. 60 ka would have been equivalent to >100% of the global arc H_2O flux and as much as 90% of the global arc Cl flux. These results underscore the importance of individual magmatic provinces in creating large temporal variations in global arc volatile fluxes to Earth's hydrosphere.