Update on the fluid geochemistry monitoring time series for geothermal systems in Dominica, Lesser Antilles island arc: 2009–2017

Abstract

With updated geochemical and isotopic compositions obtained over the period 2009 to 2017, this study presents revised fluid characterizations for the active volcanic-hydrothermal systems on the island of Dominica in the Lesser Antilles, which were first reported in 2011. Hydrothermal waters of Dominica cover a wide spectrum of pH, temperatures and chemical composition. The pH of the thermal waters ranges from acidic to neutral (pH values of 1–7.8) and the waters are predominantly Na-SO4 in character (Na = 14–2127 mg/L; SO4 = 1–1725 mg/L), likely formed as a result of dilution of acidic H2S-rich gases in near surface oxygenated groundwater, and have experienced limited water-rock interaction. The geochemical composition of the waters for most of the hydrothermal systems studied indicate no significant changes, with the exception of the Boiling Lake, which experienced a short (~6 week) episode of instability in November 2016 which appeared to be associated with a small mud-rich explosion. Unlike the last such event in December 2004, which was reported to be earthquake-triggered, this event is possibly the consequence of a moderate-sized landslide into the lake. The lake draining episodes have been accompanied by changes in composition between Na-SO4 and Na-Cl, which is attributed to hydrothermal fluid contributions from two different aquifers: a shallower acid-sulphate hydrothermal aquifer and a deeply-sourced brine aquifer.Reservoir temperatures determined by quartz geothermometers have not changed significantly over the monitoring period, suggesting steady-state degassing of the magma chambers. In two areas, temperatures have increased: Watten Waven (from 83–90 °C to 89–139 °C) and Sulphur Springs (from 145–152 °C to 93–243 °C). The elevated reservoir temperatures have affected the isotopic composition of the waters (δ18O = −5.7 to 9.1‰ and δD = −8 to 20.5‰), that reflect a dominantly meteoric source, with boiling/degassing and evaporation also playing an important role. The time series data suggests that some hydrothermal areas are experiencing increased steam evaporation over time whereas other waters are becoming more meteoric. The δ13CDIC is decoupled from the deuterium and oxygen-18 isotopes and shows very little variation over time, but a broad range in values from −11 to +5‰. The dominant process contributing to δ13CDIC is degassing of primarily magmatic CO2, as exhibited by the bubbling pools. Hydrothermal streams have experienced mixing with biogenic CO2 sources, including plant respiration and methanogenesis. The slight variations observed from site to site are likely a consequence of fractional degassing of the magma chamber during exsolution of CO2. Over the sampling period 2014–2017, the temperatures and δ13C values do not change, which suggests a current steady-state of degassing.