A Major Helium-3 Source at 15°S on the East Pacific Rise

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Abstract
An extensive plume of water enriched with helium-3 has been discovered in the deep Pacific Ocean at latitude 15°S on the East Pacific Rise. In the core of the plume, at a depth of 2500 meters over the ridge crest, the helium-3/helium-4 ratio is 50 percent higher than the ratio in atmospheric helium, indicating a strong injection of mantle or primordial helium at the spreading center axis through local hydrothermal systems. The helium-3 plume is completely absent east of the rise, but it can be traced over 2000 kilometers to the west above a newly observed physical feature: a density discontinuity here called the "ridge-crest front." The injected plume provides a unique deep-sea tracer with an asymmetric distribution which shows that the deep circulation across the rise is from east to west. The striking intensity and lateral extent of this helium-3 anomaly, compared to observations at known oceanic hydrothermal sites, suggest that the largest hydrothermal fields in the ocean are yet to be discovered and that they will be found near 15°S on the East Pacific Rise.

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