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Recently discovered hydrothermal vents at 21°N on the East Pacific Rise are discharging turbid waters at up to 400°C; mixtures of the plumes with ambient seawater contain significant amounts of dissolved H2 and CH4as well as He. The first grab samples of these waters were diluted 50‐100 foldbut they contained as much as 20 × 10−5 cc(STP) of H2 and 2 × 10−5 cc(STP) of CH4 per gram of water. H2/CH4 ratios in the vents increase with temperaturea result that is tentatively attributed to chemical equilibrium and/or the redox state of the individual waters. The phase diagram for the NaCl‐H2O "surrogate‐seawater" system shows that liquid‐vapor separation may take place prior to dischargeand mixing of a vapor phase with entrained cooler sea‐water would profoundly alter original concentrations of volatiles as well as dissolved salts. H2 and CH4 ratios to basalt‐derived helium are respectively about 550 and 70 in these waters. The total fluxes from the world‐ocean ridge systemestimated from the He‐3 fluxare of the order of 1.3 × 109 m³/y for H2 and 1.6 × 108 m³/y for CH4. The CH4 flux so calculated is sufficient to replace the deep‐sea methane in ∼ 30 yearsimplying a very rapid bacterial consumption rate below the thermocline.

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Helium isotope ratios (3He/4He) in Lassen Park and Yellowstone Park volcanic gases show large 3He enrichments relative to atmospheric and crustal helium indicating the presence of a dominant mantle-helium component. The ratios in Lassen helium are 8 times atmospheric in acid hot spring gases, and about 3 times atmospheric in the gas phase in near-neutral high-temperature waters; the acid-spring helium is thus isotopically similar to helium in island-arc and other continental-margin orogenic areas. At Yellowstone, however, •he isotopic ratio in helium from the Mud Volcano area, in the eastern part of Yellowstone caldera, is 15.6 times atmospheric, similar to the ratio in helium from Kilauea and Iceland. These high ratios, 15 times atmospheric or greater, are probably distinctively associated with deep-mantle plumes under hot-spots, while ratios about 10 times atmospheric characterize mantle helium in basalt glasses at the crests of mid-ocean rises. The presence of "Kilauea-type" helium at Yellowstone indicates that at least in certain areas the continental crust is essentially transparent to mantle volatiles.