Helium‐3 in the Guaymas Basin: Evidence for injection of mantle volatiles in the Gulf of California

Helium isotope measurements in six major basins in the Gulf of California show that the deep Guaymas Basin has 3 He/ 4 He 65–70% higher than atmospheric helium, clear evidence of mantle helium injection. Smaller 3 He excesses observed in the Carmen and Farallon basins may be derived from this Guaymas Basin anomaly. The 3 He concentrations in the Mazatlan Basin in the mouth of the Gulf of California are similar to average eastern Pacific values, indicating that the Gulf does not provide a significant flux of 3 He into the general Pacific circulation. On the basis of temperature and salinity measurements an upper limit of 0.28°C can be placed on the amount of geothermal heating observed in any of the basins. The isotopic ratio of the injected Guaymas Basin helium is found to be 3 He/ 4 He = (1.10±0.06) × 10 −5 , almost identical to the helium signature observed at the Galapagos Rift but somewhat lower than the average ratio in oceanic basalt glasses.