Volatile abundances in the sub-arc mantle: insights from volcanic and hydrothermal gas discharges

Volatile emissions from arc-related volcanic and hydrothermal systems are characterized by elevated CO2/He, N2/He and N2/Ar ratios compared to MORB. The source of CO2 and N2 is thought to be subducted sediments and oceanic crust, whereas helium is mainly derived from the mantle wedge and Ar is contributed from the atmosphere. The CO2 and H2O concentrations of arc-related magmas have been measured in melt inclusions from explosively erupted tephra and suggest that arc-magmas have CO2 contents of up to ∼2000 ppm and H2O contents up to ∼6 wt.%, supporting the idea of elevated volatile contents in the sub-arc mantle compared to MORB. We present here a new approach to estimating the volatile content of primary undegassed sub-arc magmas taking advantage of the fact that 3He originates solely from the asthenospheric mantle wedge. Recent estimates of 3He flux from arcs, 3He concentration in the crystalline mantle, and arc S, CO2, N2, and Ar fluxes allow to estimate concentrations of these volatiles in primitive undegassed sub-arc melts. Our results suggest ∼8 to 16 wt.% H2O, ∼500 to 2200 ppm S, ∼3500 to ∼7600 ppm CO2, ∼11 to 107 ppm N2, 0.1 to 0.3 ppm Ar for undifferentiated melts produced by 20% partial melting of the crystalline mantle wedge. Although the uncertainties are large, the results for H2O and CO2 are generally higher than what has previously been measured in olivine-hosted melt inclusions from arcs. We reconcile this difference being due to deep volatile degassing of sub-arc magmas prior to melt inclusion entrapment at shallower levels in the crust.