Liquid immiscibility and its role at the magmatic–hydrothermal transition: a summary of experimental studies

Abstract

Several types of fluid immiscibility may affect the evolution of volatile-rich magmatic systems at the magmatic–hydrothermal transition. The topology of silicate–salt–H 2O systems implies that three-fluid immiscibility (silicate melt+hydrosaline melt+vapour) should be stable in a broad range of compositions and P– T conditions. The most important factor controlling the immiscibility appears to be the Coulombic properties (electric charges Z and ionic radii r) of the main network-modifying cations and the capacity for immiscibility appears to decrease in the following sequence: Mg>Ca>Sr>Ba>Li>Na>K. Liquid immiscibility is enhanced in peralkaline compositions and in the presence of nonsilicate anions such as F −, Cl −, CO 3 2− and BO 3 3−. In volatile-rich magmatic systems, the H 2O is likely to react with the chloride, fluoride, borate and carbonate species and the chemical effects of high-temperature hydrolysis may be greatly enhanced by phase separation in systems with multiple immiscible fluid phases. Natural granitic magmas can thus exsolve a range of chemically and physically diverse hydrosaline liquids and the role of these fluid phases is likely to be especially significant in pegmatites and Li–F rare-metal granites.