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Numerous observations provide strong evidence that fluid-fluid coexistence is a widespread phenomenon in many geological processes: i) Oceanic hydrothermal fluids display a wide range in salinity, which clearly indicates fluid phase separation of the circulating seawater (see Foustoukos and Seyfried 2007a); ii) Selective enrichment of certain elements and coeval entrapped inclusions of different, co-existing fluids highlight the role immiscible fluids play in ore formation (see Heinrich 2007a); iii) Trace element and stable isotope characteristic as well as equilibrium mineral assemblages in contact aureoles prove immiscible fluids to be a ubiquitous feature in most contact metamorphism (see Heinrich 2007b); iv) Small scale fluid heterogeneity in eclogitic rocks indicate fluid immiscibility to occur in metamorphic rocks up to high or even ultra-high pressure (see Heinrich 2007b). Unfortunately, fluids are principally non-quenchable phases and direct quantitative information on fluid properties at P and T is in most cases not available. Experimental studies are thus central in studying geologic fluids in general and co-existing fluids in particular. They are still the primary source of quantitative information on physicochemical properties of co-existing crustal fluids: they allow, e.g., determine i) fluid phase relations as function of P, T, and x; ii) physical fluid properties like density and viscosity, and iii) trace element and isotope fractionation between co-existing fluids but also between co-existing fluids and solids. The experimental data can then be used as input in forward modeling approaches, the results and/or predictions of which may then be tested in natural systems (see Driesner and Geiger 2007).
Experimental data on co-existing fluids in systems of geological relevance already date back to the flrst half of last century (e.g., Keevil 1942). Since the 1960ies, numerous systematic experimental studies on fluid-fluid phase relations at crustal P and T conditions were performed (e.g., Sourirajan and Kennedy …