Geochemical Exploration of a "Difficult" Geothermal System, Paraso, Vella Lavella, Solomon Islands

Abstract

Thermal activity at the Paraso geothermal field, Vella Lavella, Solomon Islands, consists largely of steam-heated features, usually associated with vapor-dominated zones of a geothermal system. Chloride contents of up to 2400 in some of the waters, however, point to the simultaneous discharge of waters from liquid-dominated parts of the system. Varying contamination of the neutral waters with the steam-heated, acid waters greatly impedes application of standard solute thermometers; only ratios provide an indication for the occurrence of high temperatures of 290?C at depth. Gas geotliermometers H2) are more reliable and confirm deep temperatures of close to 300?C. The isotopic composition of the high waters corresponds to an of 6%0 or the presence of about 40% of "andesitic water". The isotopic composition of = and low (75 000) and (450) ratios, however, suggest a largely mantle only little contaminated by volatiles from subducted sediments. The taneous discharge of fluids representative of vapor-and liquiddominated zones is likely to be due to the Paraso field being situated in a narrow valley, at close to sealevel, preventing clear separation of vapor- and liquid-dominated discharge features.