HELIUM-ISOTOPES IN ICELANDIC GEOTHERMAL SYSTEMS .2. HELIUM-HEAT RELATIONSHIPS

A model has been developed that relates the He-3 content of the reservoir fluid to the fluid enthalpy. In contrast to oceanic hydrothermal systems that display a uniform He-3/enthalpy ratio (0.4-14 X 10(6) atoms of He-3/joule), Icelandic systems vary by approximately 1000 (0.4-400 X 10(6) atoms/joule). A process that preferentially extracts helium relative to heat from a cooling magma produces high He-3/ enthalpy ratios for young systems (e.g., Krafla) and low He-3/enthalpy ratios for aging magmatic systems (e.g., Hveragaerdi). Using the highest He-3/heat ratios measured in the fluid, estimates of initial gas concentrations in the basalt are made for 3He (5 x 10(-10) cc/g) and carbon (1200 ppm). Low temperature wells near Geysir in southwest Iceland appear to result from mixing of high temperature fluid (>200-degrees-C) and cold groundwater along a curve of constant He-3/enthalpy.