Student, J.J. and Bodnar, R.J.
Fluids Research Laboratory, Department of Geological Sciences, Virginia Tech, Blacksburg, VA 24061
Partial melting experiments have been conducted to produce synthetic melt and coeval aqueous fluid inclusions trapped under known PTX conditions. These inclusions have been used to characterize melt inclusion types and mechanisms of melt entrapment in quartz and to evaluate microthermometric results and interpretations obtained from melt and coeval aqueous fluid inclusions. Experiments were conducted in the H2O-saturated haplogranite system (Albite-Orthoclase-Quartz) within the quartz stability field at 800oC and 2000 bars. Starting materials included albite, orthoclase and silica gels, and quartz core. The starting aqueous fluid composition was 10 wt % NaCl. These parameters were chosen to simulate conditions inferred for the formation of melt inclusions which have been found in quartz phenocrysts from calc-alkaline rocks associated with porphyry copper systems.
Quartz core from the experimental run products contains secondary aqueous fluid inclusions, melt inclusions, and planes of silicate glass, all within healed fractures. Primary melt inclusions occur at the interface between the quartz core and quartz overgrowths and within synthetically grown beta-quartz phenocrysts. The quench glass contains clear regions of inclusion free glass, white frosted regions containing minute (< 1.0 micron) bubbles, beta-quartz phenocrysts, isolated aqueous fluid inclusions, feldspar crystallites, and miarolitic cavities which contain quartz, feldspar, and glass. Inclusion, mineral and glass textures produced in the experimental run products are similar to textures observed in natural calc-alkaline rocks.
An NaCl equivalent salinity of 11.9 wt % was determined by ice melting temperatures from aqueous fluid inclusions in the quartz core. The increase in aqueous fluid salinity can be explained by the larger partitioning coefficient of H2O into the melt relative to Cl-. Isochores (iso-Th lines) for the aqueous inclusions pass through the experimental trapping conditions. Homogenization of silicate melt inclusions trapped within quartz, determined using standard heating stage techniques, exceeded the maximum run conditions by as much as 39oC. In spite of this difference, coeval aqueous fluid inclusion isochores and melt inclusion homogenization temperatures constrain the original entrapment P-T conditions with a high degree of accuracy (Figure 1).
Figure 1. Entrapment conditions (819-839oC, 2080-2200 bars) of coeval aqueous and melt inclusions estimated from microthermometric data compared to actual experimental trapping conditions (800oC, 2000 bars). The isochore corresponding to the homogenization temperature of the aqueous inclusions (474oC) and the H2O-saturated haplo-granite minimum melt curve are also shown for reference.