Helium isotopes in volcanic rocks from the Okinawa Trough—impact of volatile recycling and crustal contamination

Abstract

The Okinawa Trough, located between Japan and Taiwan, is a back‐arc basin at an early stage of evolution from arc to back‐arc. Relative to other western Pacific back‐arc basins, it has a shallow Wadati–Benioff zone and thick continental crust. In this paper, helium and CO 2 concentrations and helium isotope data are presented for three basalts and three pumices from the middle and southern parts of the Okinawa Trough, with the aim of assessing the impact of volatile recycling and crustal contamination on magma composition. In the southern trough, a basalt sample sourced from magma with a large subduction component has a helium isotope ratio similar to that of typical MORB and a high CO 2 / 3 He ratio. This result indicates that subduction is an important barrier for helium but not for carbon. The contribution of recycled helium to the underlying mantle wedge is negligible in the middle and southern parts of the trough. Crustal‐helium contamination of the basaltic magma from the southern trough is weak but is significant for volcanic rocks from the middle trough that were sourced from magma with significant crustal residence times. Degassing has affected all samples, producing hydrothermal fluids rich in volatiles. Deep, violent degassing may have occurred in the mantle wedge in the southern trough, and pre‐eruptive magma degassing during storage is particularly extensive in the middle trough. Copyright © 2016 John Wiley & Sons, Ltd.