Evaluating Earth degassing in subduction zones by measuring helium fluxes from the ocean floor

Volatiles are lost from the Earth's mantle to the atmosphere, hydrosphere and the crust through subaerial and submarine volcanism. Quantifying the volatile sources bears fundamental information on a number of issues in Earth sciences, from the evolution of the atmosphere and oceans to the nature of chemical heterogeneity of the Earth's mantle. The primordial noble gas isotope 3He provides an unambiguous measure of the volatile flux from the mantle, yet so far in the ocean region; it has been only measured at a mid-ocean ridge. Here, we present original measurements of the 3He flux at the Mid-Okinawa Trough back-arc basin. The 3He flux was estimated from 3He/20Ne vertical profiles measured in deep-sea sediment pore water. Diffusive 3He fluxes vary from 1.6 3He atoms cm−2 s−1 at the hydrothermally active Izena Cauldron to 0.57 3He atoms cm−2 s−1 at the background site, 13 km away. These values are about 20% of the 3He flux measured at the East Pacific Rise, supporting the never−proven hypothesis that 3He mantle flux from subduction zones is a quarter of that at MOR. Measured ocean−floor 4He flux ranges from 3.3 × 105 to 4.8 × 1054He atoms cm−2 s−1, higher than that measured worldwide, suggesting that 4He flux at subduction zones might have been previously underestimated.