Variable 3He/heat ratios in submarine hydrothermal systems: evidence from two plumes over the Juan de Fuca ridge

Abstract

The first vent fluid samples recovered from submarine hydrother-mal systems on the Galapagos Rift1 and at 21° N on the East Pacific Rise (EPR)2 had a nearly identical ratio of 3He/heat of ~0.5 x l0−12cm3STPcar−1, even though the two hydrothermal systems were separated geographically and had widely differing fluid exit temperatures (~20 and ~350 °C, respectively)3–5. Jenkins et al.3 combined this ratio with independent estimates of the flux of mantle 3He through the oceans6, to calculate a global oceanic hydrothermal heat flux of 4.9 x 1019cal yr−1, which is in excellent agreement with geophysical estimates for this flux7,8. Other investigators then combined this 3He flux with measured ratios of various chemicals in vent fluids to 3He (such as Mn/3He and Si/3He) to estimate global hydrothermal fluxes for these species1,9,10. Here we show that 3He/heat ratios vary by over an order of magnitude between submarine hydrothermal systems, suggesting that early measurements of the 3He/heat relation are not representative of all hydrothermal systems, and that flux calculations based on the oceanic 3He flux must be undertaken with caution.