Anomalous helium and heat signatures associated with the 1998 Axial Volcano Event, Juan de Fuca Ridge

Water-column plumes sampled before and after the 1998 eruptive event on Axi al Volcano, Juan de Fuca Ridge, showed a complex pattern in He-3 and heat v ery different from that observed during previous magma injection events. Pl umes detected in 1997 before the event had relatively high He-3/heat consis tent with the volatile-rich character of the known hydrothermal vents withi n the summit caldera. A response cruise in February 1998 immediately after the event failed to detect any event plumes, but instead found an intense s teady-state type plume with high-temperature signals, moderate 3He enrichme nts, and low He-3/heat ratios. This plume was present over the caldera and extended a considerable distance off axis. In contrast, all of the plumes d etected during follow-up cruises in July-September 1998 had moderate temper ature signals, high He-3 concentrations, and elevated He-3/heat ratios. Com parison with previous events suggests that the low He-3/heat plume sampled in February was either the remnant of an event plume which had already migr ated off axis, or an event plume-like discharge which was smeared out by st rong currents. It is likely that the high He-3/heat plumes observed in July -September were derived from the injected dike or from the new lava flow.