Abstract
Volcanoes in the 'Ring of Fire' surrounding the Pacific Ocean are sited on tectonic arc segments marking the great subduction zones where oceanic crust returns to the mantle. Helium isotope ratios in volcanic gases along these arcs are close to those found in mid-ocean-ridge basalts, revealing the presence of primordial 3He released from the wedge of mantle material above the sinking plate. These results show that although the subduction of oceanic crust drives the arc volcanism, the subducted crust itself does not contribute a major fraction of the upwelling magma.
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