What are volatiles?

Volatiles are chemical species that under normal conditions of pressure and temperature exist in the gaseous phase. The main volatiles associated with volcanoes are water (H₂O), in the form of steam, and carbon dioxide (CO₂) gas.

There are 3 possible reservoirs that can contribute volatiles to subduction zone volcanoes: 1) the subducting slab, 2) the mantle wedge, and 3) the crust. The subducting slab has a veneer of marine sediments on its surface, which are taken into the mantle as part of the subduction process. The sediments, in particular, tend to be very water and carbonate rich. Thermal breakdown of the carbonate as the subducting plate enters the mantle produces CO₂. Another source of volatiles is the mantle trapped between the downgoing plate and the over-riding plate or crust upon which the volcano is built. This region of the mantle is called the mantle wedge. Finally, the arc crust itself can release volatiles by thermal contact with ascending magmas from the mantle. Distinguishing between these possible sources of volatiles in subduction zone volcanoes is a primary objective of our study in Costa Rica.