In the contact metamorphic aureole of the Beinn an Dubhaich granite, Isle of Skye, Scotland, nodular chert (70 percent quartz, 20 percent dolomite, 10 percent calcite) reacted with dolostone (99 percent dolomite, 1 percent calcite) to form reaction rims (mantles) of calc-silicate minerals. In general talc, tremolite, diopside, and forsterite appear successively in the reaction rims as the granite is approached. Mineral assemblages in the aureole can be related by equilibria in the system CaO–MgO–SiO2–CO2–H2O at PT = 500 bars, PT being the estimated lithostatic pressure at the time of granitic intrusion. Fluorine can be neglected as the molecular proportion of fluoro-talc and fluoro-tremolite is less than 0.017. The sequence of talc and tremolite reactions and their areal extent suggests that talc is stable at low pressures.
Two prograde reaction paths, indicating different X(CO2), can be traced at different zone boundaries within the nodules. One path corresponds to reactions within the mantle or at the matrix-mantle boundary. The other path corresponds to reactions at the core-mantle boundary and persisted as long as dolomite and quartz remained in the core of the nodules. Once quartz was consumed in the nodules the two paths converged. Small but significant fluid composition differences [∼0.1 X(CO2)] existed across the reaction rims, which are 1 mm to several cm wide. These X(CO2) differences reflect the buffering capacities of mineral assemblages at zone boundaries, and perhaps the opposing outward flow of CO2-rich fluid derived from the reactions and inward flow of H2O-rich matrix pore fluid. The two prograde paths equilibrated at X(CO2) > 0.85 once talc was consumed.