10 Reviews in Mineralogy & Geochemistry Vol. 65, pp. 313-362, 2007 Copyright © Mineralogical Society of America 1529-6466/07/0065-0010$05.00 DOI: 10.2138/rmg.2007.65.10 Fluid Immiscibility in Volcanic Environments James D. Webster and Charles W. Mandeville Department of Earth and Planetary Sciences American Museum of Natural History Central Park West at 79th Street New York, New York 10024-5192 U.S.A. jdw@amnh.org cmandy@amnh.org INTRODUCTION Volatile components enter the atmosphere, oceans, and other surface waters through pre-, syn-, and post-eruptive volcanic processes that involve a variety of fuids. Aluminosilicate-poor fuids include aqueous or carbonic to sulfde-, sulfate-, chloride-, fuoride-, carbonate-, and phosphate-rich compositions in volcanic environments, but other more complex combinations of these constituents may be involved (Giggenbach 1977; Roedder 1984, 1992; Lowenstern 1995). Other, rare gases and dissolved constituents (e.g., H 2 , N 2 , He, Ar, H 3 BO 3 , Hg, CH 4 and other hydrocarbon compounds, metals, and metalloids) are also present, but in general they are not suffciently concentrated to form their own phases or control bulk-fuid composition and volcanic processes. As a result of their diverse compositions, volcanic fuids ranging from vapor to liquid exhibit widely different densities and show strong distinctions in heat capacity, dielectric behavior, viscosity, and other chemical and physical characteristics (Geiger et al. 2006a). The density of a saline liquid in the system NaCl-H 2 O at 50 MPa and 800 °C, for example, is more than 7 times that of the coexisting aqueous vapor (e.g., 0.2 gm/cm 3 ) (Henley and McNabb 1978). Multiple fuids move, mix, and/or unmix in magma chambers and volcanic conduits, in the root zone of fumaroles, and in the convective hydrothermal systems that are ubiquitous to volcanic environments. Hydrothermal systems are typically centered on magmatic intrusions and may include crater lakes at the top of eruptive conduits. Hydrothermal processes involving two or more fuid phases operate through a variety of geologically relevant, shallow- crustal pressure-temperature-composition conditions. Volcanic fuids occur at supersolidus to subsolidus conditions, meaning that the dense aluminosilicate melt that is common to volcanism may or may not be present. This chapter addresses the evidence for and occurrence of two or more low-density, aluminosilicate-poor fuids in volcanic and related environments. It also describes the role of multiple fuids in the chemical and physical processes that occur in these environments—even though many aspects of volcanic processes are still poorly understood. BACKGROUND Volcanoes are surface loci where heat and matter are transferred from the inner Earth and where the temperatures and pressures of magma, rocks, and fuids change rapidly over short distances. Fluid phase equilibria are strong functions of temperature and pressure; so, two-phase volcanic fuids are common given the broad temperature and pressure ranges that characterize volcanoes and their plumbing systems. The involvement of multiple fuids in volcanic processes is important because heat and mass transport in two-fuid systems can be