Combined stable isotope systematics and thermodynamic models: Constraints on hydrothermal alteration prior to phreatic eruptions at Te Maari, Tongariro, New Zealand

Hydrothermal systems hosted on active volcanoes exhibit complex interactions of magmatic heat and fluids, which facilitate changes to host rock physical and chemical properties. These resulting changes from hydrothermal alteration often lead to a decrease in flank stability, enhance sealing of the degassing conduit, and modulate eruption dynamics. However, the timescales of these processes are not well constrained. This study utilizes new stable isotope systematics (S, O, and H) and thermodynamic models of fresh and altered samples from the Te Maari vent, Tongariro volcano, New Zealand, to constrain a maximum timescale over which the observed secondary mineralogy formed as well as refine the physiochemical conditions of their formation. Tongariro hosts a complex multivent hydrothermal system with the most recent phreatic eruptions occurring at the Upper Te Maari crater in 2012. The δ 34 S S O 4 values of alunite from Te Maari are near 0 (−0.7 to 2.7‰), suggesting very limited SO 4 2 − -H 2 S exchange. Elevated δ 18 O a l u n ( S O 4 ) (15.85 to 19.35‰) values indicate SO 4 2 − exchange with high- 18 O water, typical of low temperature, low pH fumarole environments. Thermodynamic models of the alteration observed at Te Maari yielded refined physicochemical conditions of pH 1.25 to 2.53. Stable isotope fractionation calculations indicate temperature ranges of 100-150 °C, which is supported by Te Maari field measurements. Experimental equilibrium calculations returned a maximum time frame of 10 years to 200 days to reach 90% equilibrium for temperatures of 100 °C and 150 °C, respectively. These timescales indicate that alteration-induced conduit-sealing mineralization at Te Maari prior to the 2012 eruptions progressed quite rapidly.