North-Central Section - 42nd Annual Meeting (24–25 April 2008)

Paper No. 1
Presentation Time: 8:00 AM

VOLATILES IN MAGMAS: FROM SOURCE TO SURFACE


FISCHER, Tobias P., Earth and Planetary Sciences, Univ of New Mexico, 200 Yale Blvd NE, Northrop Hall, Albuquerque, NM 87131, fischer@unm.edu

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.