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Water column hydrothermal plumes on the Juan de Fuca Ridge

1990; American Geophysical Union; Volume: 95; Issue: B8 Linguagem: Inglês

10.1029/jb095ib08p12829

2156-2202

J. E. Lupton,

Groundwater and Isotope Geochemistry

Hydrographic surveys on the Juan de Fuca Ridge (JdFR) carried out from 1980 to 1987 show a complex pattern of 3 He and Mn‐rich water column plumes produced by venting from several submarine hot spring areas. In the vicinity of Axial Volcano at latitude 46°N, distinct plumes were detected in 1980, 1982, and 1983 with 3 He signatures up to δ( 3 He) = 64% at ∼1500‐m depth at distances of ∼10 km from the seamount summit. However, the same plumes had no detectable thermal signature, a paradox which is attributed to the high 3 He/heat ratios and low salinities of the fluids venting within the caldera of Axial Volcano. Profiles directly over the seamount show hydrothermal 3 He in the water column up to 300 m above the caldera floor, with the 3 He signal increasing with depth to very high and uniform ratios of δ( 3 He) = 108–150% below the ∼1500‐m caldera sill depth. Another apparent locus of hydrothermal input is Helium Basin, a depression on the northeast flank of Axial Volcano which had δ( 3 He) = 51% when first sampled in 1980. However, subsequent hydrocasts into Helium Basin in 1982 and 1983 yielded lower helium enrichments, suggesting either a decrease in hydrothermal input or flushing of the basin via a mixing event. To the south of Axial Volcano, high δ( 3 He) values of ∼40% observed over the ridge axis at 45°18′N and 45°39′N indicate venting on this previously unexplored section of the ridge. The water column plumes over the U. S. Geological Survey vent site at ∼44°40′N on the southern JdFR have very high Mn/ 3 He ratios of 4600 mol/cm 3 , an apparently unique characteristic which can be used to distinguish these plumes from those originating at other JdFR vent fields.