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 (3)He 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 (3)He flux at the Mid-Okinawa Trough back-arc basin The (3)He flux was estimated from (3)He/(20)Ne vertical profiles measured in deep-sea sediment pore water. Diffusive (3)He fluxes vary from 1.6 (3)He atoms cm(-2) s(-1) at the hydrothermally active Izena Cauldron to 0.57 (3)He atoms cm(-2) s(-1) at the background site, 13 km away. These values are about 20% of the (3)He flux measured at the East Pacific Rise, supporting the never proven hypothesis that (3)He mantle flux from subduction zones is a quarter of that at MOR. Measured ocean floor (4)He flux ranges from 33 x 10(5) to 4.8 x 10(5) (4)He atoms cm(-2) s(-1), higher than that measured worldwide, suggesting that (4)He flux at subduction zones might have been previously underestimated (c) 2010 Elsevier B V. All rights reserved.