Problematic issues regarding estimates of volatile output at arcs

Problematic issues regarding estimates of volatile output at arcs 

There are two major issues of concern with the approach of using C02/ He and N2/ Ar (or N2/ He) ratios in combination with and values to constrain the sources of volatiles at arcs. The first issue is the selection of end-member isotopic and relative elemental abundances— this factor has a profound effect on the deduced provenance of the volatile of interest. The second is the assumption that various elemental (and isotopic) ratios observed in the volcanic products are representative of the magma source. Both have the potential to compromise the accuracy of the output flux estimates. 

In addition to supplying volatiles that are lost via arc-related volcanism, the subducting slab may also contribute volatiles to both the back-arc and fore-arc regions. To complete a realistic mass balance for subduction zones, therefore, it is essential to quantify volatile fluxes at the back-arc and fore-arc. As we discuss below, both fluxes are severely underconstrained at present. 

The same uncertainty in back-arc flux rates applies also to N2. Limited data on 

Although volatile flux estimates from back-arc regions vary greatly, the situation is even more poorly constrained for fore-arc regions. We are unaware of any flux estimates for either major volatiles or noble gases. This is in spite of abundant circumstantial evidence for fluid venting in fore-arc regions e.g., serpentinite diapirism in the Marianas (Fryer et al. 1985) and mud volcanism at various accretionary prisms worldwide (Brown 

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