Natural occurrence and significance of fluids indicating high pressure and temperature

Abstract

Most natural minerals have formed from a fluid phase such as a silicate melt or a saline aqueous solution. Fluid inclusions are tiny volumes of such fluids that were trapped within the growing crystals. These inclusions can provide valuable but sometimes ambiguous data on the temperature, pressure, and composition of these fluids, many of which are not available from any other source. They also provide “visual autoclaves” in which it is possible to watch, through the microscope, the actual phase changes take place as the inclusions are heated. This paper reviews the methods of study and the results obtained, mainly on inclusions formed from highly concentrated solutions, at temperatures ≥500°C. Many such fluids have formed as a result of immiscibility with silicate melt in igneous or high-temperature metamorphic rocks. These include fluids consisting of CO 2 , H 2 O, or hydrosaline melts that were <50% H 2 O. From the fluid inclusion evidence it is clear that a boiling, very hot, very saline fluid was present during the formation of most of the porphyry copper deposits in the world. Similarly, from the inclusion evidence it is clear that early (common) pegmatites formed from essentially silicate melts and that the late, rare-element-bearing and chamber-type pegmatites formed from a hydrosaline melt or a more dilute water solution. The evidence on whether this change in composition from early to late solutions was generally continuous or involved immiscibility is not as clear.