VOLATILES IN MAGMAS: FROM SOURCE TO SURFACE

Magma ascent towards the surface results in the release of volatiles to the atmosphere. The composition of these volatiles as they discharge from active volcanoes is dominated by H₂O, CO₂ and S species. Trace components include the noble gases, HCl, HF, N₂, H₂, CH₄ and CO. Systematic variations of gases and isotopic compositions (δ¹³C, δ¹⁵N, δD, δ¹⁸O) with tectonic setting show that gas emissions are strongly influenced by varying contributions of volatiles from the crust, the mantle and the subducting slab. In particular, time-series data of δD and δ¹⁸O values of discharging steam shows that H₂O in arc volcanoes is a mixture of magmatic water (from the mantle and subducted slab) and surface derived meteoric water. Recent melt inclusion work on δD of arc lavas confirmed that slab derived water is the dominant source of water in these magmas.

When considering the mass balance of volatiles emitted from arc volcanoes and those subducted into the zones of arc magmas generation several observations emerge: 1) globally, the water available to arc magmas is approximately balanced with the amount sudbucted 2) nitrogen mass balance is likely dependant on the thermal regime of the subduction zone: input equals output in Central America but input exceeds output in Izu Bonin Marianas 3) CO₂ input exceeds output in Central America and globally. As both input and output parameters become better constrained and include fore-arc fluxes, and more systematic across-arc studies, volatile mass balances of subduction zones will be further constrained.

Future directions regarding sources of volatiles may benefit from 1) development of instrumentation and methods for obtaining continuous records of gas emissions (composition and fluxes), 2) direct comparisons with petrologic studies, in particular volatiles measured in melt inclusions and studies of short-lived radiogenic isotopes and 3) integration of gas emissions into global element cycling models.