SILICOTHERMAL FLUID - A NOVEL MEDIUM FOR MASS-TRANSPORT IN THE LITHOSPHERE

New experimental data from synthetic fluid-inclusion studies in the sy stem K2O-CO2-SiO2-H2O (KCSH) show that a potassic, silica-rich (approx imate to 90 wt% SiO2) fluid can coexist immiscibly with a supercritica l, alkaline, aqueo-carbonic fluid and quartz from temperatures as low as 300 degrees C to more than 750 degrees C at relatively low geologic pressures (<200 MPa). This type of fluid phase may form in a range of geologic environments, including carbonatite complexes, alkaline subv olcanic-plutonic systems, and subduction zones. With a probable polyme rized (silica-rich, melt-like) structure, such SiO2-rich fluids, if th ey form in the lithosphere, are likely to be important in the mobiliza tion and transport of silica and large ion lithophile elements (e.g., K, Cs, Ba) and metals of economic significance (e.g., An, Ag, U).