Exsolved volatiles in magma reservoirs

Abstract

We review our understanding of the exsolved volatile phase co-existing with magmas during pre-eruptive storage at the pressures and temperatures corresponding to crustal magma reservoirs. We explore the consequences and implications of such a volatile phase for magma and ore body petrogenesis and the fluid dynamics of magma reservoirs. We outline the geochemical constraints on the size and composition of the exsolved volatile phase that may co-exist with magmas in the crust. We distinguish between decompression-driven and crystallization-driven exsolution, and describe the implications of the volatiles for the dynamics of the magma reservoir, using key natural examples and case studies. We discuss eruptions triggered by second boiling, and the various regimes of magma mixing and magma overturn that may be induced by second boiling in a layered reservoir. We also explore the control of the volatile content of the magma on the mass erupted during an eruption episode, and compare our models to eruption datasets. We then turn to the mechanisms for magma-volatile separation, noting that in crystal-poor melts convective separation of exsolved volatiles may dominate while in crystal-rich melts, volatiles may generate channels or permeable-flow pathways through the crystal mush, thereby separating from the parent magma. We discuss the implications of the accumulation of the exsolved volatile phase at the roof zones of crystal-rich reservoirs for the large gas emissions observed during explosive eruptions, and for the development of metal-rich porphyry deposits.