Mantle helium in hydrothermal plumes in the Galapagos Rift

A discussion is presented on the 3He/4He ratio in deep Pacific water, which is found to be 30 to 40% higher than in atmospheric He, due to the injection of primordial He from the mantle. The largest 3He enrichments have been found in water on the crest of the east Pacific rise. It has also been found that the hot brines in the axial rift of the Red Sea are very highly enriched in mantle He, the 3He/4He ratios being about 8.6 times that in atmospheric He. Nine samples of bottom water were collected in May 1976 along the central axis of the Galapagos Spreading Center, using a newly designed sampling sled attached to the Scripps Institution of Oceanography deep-tow instrument. Three of these samples were collected during 'hydrothermal spike' temperature excursions, whose origin could not be explained by turbulent mixing within the ambient water mass. The samples were taken about 10 m above the axial fissure opening. Samples were subjected for isotopic analysis on a 3He/4He ratio mass spectrometer. Rn activities were also measured on some of the samples. Results are presented in tabular form and discussed. The results established clear connection between the mantle He in oceanic basalts and the excess 3He observed in the deep oceans, by seawater penetration and circulation in the basalts. Unequivocal evidence was obtained for the existence of a hydrothermal basalt-seawater system in the axial region of the Galapagos Rift. (U.K.)