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Changes in submarine hydrothermal 3He/heat ratios as an indicator of
magmatic/tectonic activity
Edward T. Baker
Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric
Administration, 7600 Sand Point Way NE, Seattle, Washington 98115, USA
John E. Lupton
Marine Science Institute and Department of Geological Sciences, University of
California, Santa Barbara, California 93106, USA
Nature, 346, 556-558 (1990)
Copyright ©1990 Macmillan Publishers Ltd. Further electronic distribution not allowed.
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' [Baker et al., 1987;
Baker et al., 1989; Cann and Strens, 1989], 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
[Rosenberg et al., 1988; Jenkins et al., 1978; Welhan and Craig,
1983; Merlivat et al., 1987; Lupton et al., 1980]. 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.
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