FY 1990

Changes in submarine hydrothermal ³He/heat ratios as an indicator of magnetic/tectonic activity

Baker, E.T., and J.E. Lupton

Nature, 346, 556–558 (1990)

An important question in submarine hydrothermal research concerns the connection between hydrothermal discharge from a spreading centre and variations in local magmatic and tectonic activity. Because it is likely that tectonic stretching and concomitant shallow magmatic activity triggered the cataclysmic venting that created the Juan de Fuca Ridge "megaplumes," we have for three years monitored the He concentration and temperature anomaly of the underlying steady-state plume at the site of the original megaplume. We report here that the apparent He/heat ratio in the steady-state plume has progressively decreased from 4.4 to 2.4 to 1.3 × 10 cm STP cal, changing from a uniquely high ratio to one characteristic of established vent fields on other ridge segments. We propose that the initially high He/heat ratio, sampled within days of the megaplume eruption, resulted from magma degassing into a hydrothermal circulation system of high permeability and short fluid residence time. Thus, high He/heat ratios may indicate venting created or profoundly perturbed by a magmatic-tectonic event, and lower ratios may typify systems at equilibrium.