- © The Mineralogical Society Of America
IMMISCIBILITY IN NATURAL SYSTEMS INVOLVING HYDROUS SILICATE MELTS
The importance of magma degassing and separation of magmatic fluids (aqueous as well as other components) has been recognized in many geologic observations at least since Niggli (1912; see also Morey and Niggli 1913; Bowen 1928; Morey 1957). Improved analytical techniques in recent years have permitted micron scale examination of melt and fluid inclusions inside magmatic minerals. In these inclusions, concentrated aqueous solutions and extreme enrichment of melts and fluids in volatile components (S, CO2, B2O3, Cl, F, and metals) have been observed (Roedder 1992; Webster 1997; Thomas et al. 2000; Kamenetsky et al. 2004). Pegmatite formation reflects the role of exsolution of aqueous fluid from silicate melt (Jahns and Burnham 1969) at the magmatic–hydrothermal transition (e.g., Veksler 2004). Fluid–gas immiscibility is clearly involved in formation of pegmatites, and for some other fluids originating in magmatic or metamorphic environments.
Certain natural fluid systems rich in volatile components are immiscible and others show completely mixed compositions at geological temperatures and pressures (e.g., Roedder 1984; hydrocarbons, Pedersen and Christensen 2007; geothermal and submarine hydrothermal systems, Arnórsson et al. 2007, Foustoukos and Seyfried 2007; ore-forming fluids, Hedenquist and Lowenstern 1994; Heinrich 2007a; mixed metamorphic fluids, Trommsdorff and Skippen 1986; Heinrich 2007b; and magmatic fluids, Webster and Mandeville 2007). This has encouraged the idea that certain common natural melt–fluid–gas systems pass from immiscible (with multiple coexisting fluids) to miscible behavior (a single homogeneous fluid) with changing pressure and/or temperature (increasing or decreasing) or changing composition (see recent summary by Veksler 2004). The term supercritical is applied to any solution (liquid or solid) occurring at a temperature or pressure above its critical point. In the case of supercritical fluid this corresponds to conditions where liquid …