Variations in water-column ³He/heat ratios associated with the 1993 CoAxial event, Juan de Fuca Ridge

The 1993 volcanic/hydrothermal event on the CoAxial segment, northern Juan de Fuca Ridge, generated a complex set of event plumes and steady-state type plumes with a pattern of 3 He/heat ratios very similar to that observed during the 1986 megaplume event on the southern JdFR. The event plumes residing -600 m above the seafloor had normal He/heat ratios averaging 0.34 x 10 -12 cm 3 STP cal -1 , while the deeper, steady-state plumes had elevated 3 He/heat values ranging from 0.6 to 4.5 x 10 -12 cm 3 STP cal -1 . These results generally confirm that elevated 3 He/heat ratios in some of the water-column plumes are a reliable indicator of a recent magmatic/tectonic event. However, in contrast to the 1986 JdFR event, there was no evidence for hydrothermal activity on the CoAxial segment prior to 1993, thereby requiring revision of our models for event plume generation.