Water-column plumes sampled before and after the 1998 eruptive event on Axial Volcano, Juan de Fuca Ridge, showed a complex pattern in 3He and heat very different from that observed during previous magma injection events. Plumes detected in 1997 before the event had relatively high 3He/heat consistent with the volatile-rich character of the known hydrothermal vents within the summit caldera. A response cruise in February 1998 immediately after the event failed to detect any event plumes, but instead found an intense steady-state type plume with high-temperature signals, moderate 3He enrichments, and low 3He/heat ratios. This plume was present over the caldera and extended a considerable distance off axis. In contrast, all of the plumes detected during follow-up cruises in July-September 1998 had moderate temperature signals, high 3He concentrations, and elevated 3He/heat ratios. Comparison with previous events suggests that the low 3He/heat plume sampled in February was either the remnant of an event plume which had already migrated off axis, or an event plume-like discharge which was smeared out by strong currents. It is likely that the high 3He/heat plumes observed in July-September were derived from the injected dike or from the new lava flow. ¿ 1999 American Geophysical Union