Te Puia Springs, Gisborne, past and present fluids

Abstract

Currently, the Te Puia hot springs discharge up to 71-77°C CH4-rich Na-Ca-Cl fluids with Cl concentrations of 5100-9800 mg/kg, gas:water ratio of 0.1, and an estimated annual water flowrate of ~2.85 x 105 m3
/a. Fluid inclusions in calcite veins from sinter mounds adjacent to hot springs yield homogenization temperatures (median: 80°C) and salinity (8300 mg/kg Cl), similar to present-day surface discharge
measurements suggesting that spring temperatures and Cl contents have been relatively constant since the hot springs appeared, probably <140k years ago. However, gas ratios in fluid inclusions deviate from present-day measurements. In particular, fluid inclusions contain higher relative concentrations of CO2 and H2S and lower relative concentrations of hydrocarbons. Within the hydrocarbon fraction, fluid inclusion gas contains higher concentrations of C2+ alkanes, as compared to vent gases, and detectable benzene. This suggests some temporal variability in volatile composition, while temperatures and TDS remained
relatively unchanged.