ESTABLISHING ISLAND-WIDE WATER CHARACTERIZATION IN A VOLCANO-HYDROTHERMAL SYSTEM IN DOMINICA, LESSER ANTILLES

The island of Dominica is located in the center of the Lesser Antilles Island Arc in the Caribbean. It is dominated by intermediate effusive and explosive volcanic deposits, with significant activity < 200 ka. The most recent eruptions were small phreatic explosions in 1880 and 1997 in the Valley of Desolation, one of more than 30 hydrothermal areas on Dominica. In June 2014, we began an island-wide survey of meteoric and hydrothermal waters on the island. Our goal was to provide a baseline for future monitoring. By establishing a baseline, we can potentially assess whether there has been a localized increase in magmatic degassing, which could serve as a pre-cursor to an eruption. As magmas degas, they affect water chemistry: raise temperature, lower pH, increase Li, B, SO₄ concentrations, and make carbon isotopes of DIC samples heavier. In 2014, field measurements (temperature, pH, and conductivity) and filtered water samples for chemical (alkalinity, elements, anions, cations) and isotopic characterization (carbon, oxygen, deuterium) were collected from 72 sites. In June 2015, the sites from 2014 were re-sampled and an additional 8 samples from the Valley of Desolation were added. These samples included meteoric streams, hydrothermal springs, and Boiling Lake. Several of the samples were cloudy and colored (gray, black, and orange), potentially due to mineral precipitation or biologic activity. Meteoric samples were ~21°C and varied in pH from 6.5-7.6, whereas hot springs varied in temperature from 41°-93˚C and ranged in pH from 2.3-5.9. Boiling Lake was >82°C, with a pH of 3.5, its lowest value since 2003. Several hot springs in the valley and Boiling Lake have been periodically monitored since 2000 and both the field measurements and compositions appear to vary independently for each location. The present levels are within the normal range observed. The streams on the periphery of the valley were classified as bicarbonate, the hot springs were classified as acid sulphate, steam heated waters and Boiling Lake is considered a chloride water. The waters experienced a variable amount of H₂S dissolution, with the hot springs particularly sulphate-rich. Trace element ternary plots suggest the waters may have been affected by illite formation, although none of the samples have attained equilibrium with the host rocks.