Abstract
A model has been developed that relates the 3He content of the reservoir fluid to the fluid enthalpy. In contrast to oceanic hydrothermal systems that display a uniform 3He/enthalpy ratio (0.4–14 × 10 6 atoms of 3HE/joule), Icelandic systems vary by ∼ 1000 (0.4–400 × 10 6 atoms/joule). A process that preferentially extracts helium relative to heat from a cooling magma produces high 3He/enthalpy ratios for young systems (e.g., Krafla) and low 3He/enthalpy ratios for aging magmatic systems (e.g., Hveragaerdi). Using the highest 3He / heat ratios measured in the fluid, estimates of initial gas concentrations in the basalt are made for 3He (5 × 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°C) and cold groundwater along a curve of constant 3He/enthalpy.
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