VARIATIONS IN WATER-COLUMN HE-3 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 He-3/heat ratios very similar to that observed during the 1986 megaplume event on the southern JdFR. Th e event plumes residing similar to 600 m above the seafloor had ''norm al'' SHemeat ratios averaging 0.34 x 10(-12) cm(3)STP cal(-1), while t he deeper, steady-state plumes had elevated He-3/heat values ranging f rom 0.6 to 4.5 x 10(-12) cm(3)STP cal(-1). These results generally con firm that elevated He-3/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 hydrot hermal activity on the CoAxial segment prior to 1993, thereby requirin g revision of our models for event plume generation.