Helium‐3 plume over the East Pacific Rise at 25°S

We have measured helium isotopic ratios of sixty‐eight water samples collected in the southeastern Pacific at 25°S. The maximum excess 3 He of 47.5% is observed just above the East Pacific Rise, which is comparable to that of 50.5% at 15°S in literature. A negative correlation between helium isotopes and neon to helium ratios of seawater samples suggests that the source of excess helium‐3 in the mid‐depth water is considered to be a MORB‐type mantle. Contour of excess 3 He shows symmetrical pattern, that is, plume of 30% excess can be traced up to 1000 km on both east and west of the East Pacific Rise. This pattern is consistent with a northward flow along the rise at 25°S estimated by a physical model. Our observation also supports a flow pattern of deep ocean currents, which has been suggested by contour of excess 3 He obtained from previous studies.