High-enthalpy supercritical geothermal fluids were obtained from the IDDP-1 well at Krafla, Iceland, which had a discharge temperature of similar to 440 degrees C and specific enthalpy of similar to 3200 kJ kg(-1). Utilization of such fluids may multiply power production from geothermal well fields. However, the origin of supercritical fluids in the roots of volcanic geothermal systems is poorly understood. Here, we propose that an important mechanism of such supercritical fluid formation is conductive heating of surrounding subcritical geothermal fluids near a shallow intrusion. Predictions from hydrologic and chemical models of supercritical fluid formation and ascent are compared with measured fluid compositions from the Krafla geothermal system. Supercritical fluids formed by close to isobaric heating of liquid geothermal groundwater display similar volatile concentrations (C, S, B, Cl, F) as the initial fluid. In contrast, the low concentrations of non-volatile elements (Si, Na, K, Ca, Mg, Al, Fe) in the supercritical fluid result from intensive, quartz-dominated mineral deposition near the magmatic intrusion during boiling of liquid to dryness. Liquid condensed out of ascending supercritical fluid has a low pH (similar to 2) due to the dissociation of volatile components like HCl. Such 'acid' geothermal fluids have been encountered in wells in the Krafla system. However, the chemical signatures of supercritical fluid ascent are likely to be overprinted by mixing of the acid fluids with cooler subcritical fluids, fluid-rock interaction, depressurization boiling and phase segregation.