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

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Abstract
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.

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