Niobian rutile from the McGuire granitic pegmatite, Park County, Colorado; solid solution, exsolution, and oxidation

Over 90% of the fluid inclusions in quartz, topaz, and beryl from the Morefield pegmatite, Amelia Co., Virginia, contain mixed CO 2 -H 2 O fluids. In order of abundance, there are four inclusion types: (1) three- phase, 1-1-v; (2) two-phase, H 2 O-rich; (3) three- or four-phase, 1-v-s and 1-1-v-s; and (4) two-phase, CO 2 - rich. CO 2 -rich inclusions formed by necking of inclusions containing mixed fluids. Topaz and quartz contain all four inclusion types, and all types occur throughout the pegmatite, although no H 2 O-rich inclusions occur in beryl. Quartz from gneiss that hosts the pegmatite contains dominantly H 2 O-rich inclusions. Most inclusions are secondary and occur along healed fractures. Possible primary inclu­sions occur only among those that contain solids. All inclusions homogenize into the liquid phase; a few homogenize to liquid CO 2 . H 2 O-rich inclusions with <2 wt% NaCl (eq.) and homogenization tem­peratures <200 ℃ occur in the host gneiss and throughout the pegmatite and are interpreted to contain meteoric waters. H 2 O-rich and mixed fluid inclusions with 3-7 wt% NaCl (eq.) and homogenization temperatures >290 ℃ contain fluids interpreted to derive primarily from the pegmatite. Inclusions with salinities between 3-13 wt% NaCl (eq.) and intermediate homogenization temperatures are in part cooled equivalents of the pegmatite fluids, but the origin of those with salinity >7 wt% NaCl (eq.) is uncertain. Homogenization temperatures for CO 2 indicate average CO 2 densities near 0.65 g/cm 3 . For mixed fluids homogenizing near 300 ℃ and this CO 2 density, the trapping pressure is estimated to be 2.5 kb. Neither age relations nor physical proximity tie the Morefield pegmatite to known granitic intrusions. Fluid compositions preclude a genetic relation between the pegmatite and its host gneisses.