The XCO2 recorded by mineral-fluid equilibria in contact metamorphosed siliceous carbonates commonly defines two groups of rocks in the same aureole. One group records relatively low XCO2 that results from infiltration of chemically reactive H2O-rich fluid. The other records relatively high XCO2, up to 0.99, that results from decarbonation reactions with little or no infiltration. A complementary dichotomy in apatite compositions exists in five contact aureoles in Italy, Scotland, and the U.S.A. Apatite in the low-XCO2 group is close to an F-OH solid solution. Apatite in the high-XCO2 group is a relatively Cl-rich Cl-F-OH solution. The halogen content of fluid coexisting with analyzed apatite was characterized in two aureoles to determine the origin and significance of the dichotomy in apatite composition. Calculated aHF/aH2O, aHF/aHCl, aHF, and mFT (the total F molality of fluid) are systematically higher in fluid coexisting with the low-XCO2 group. In contrast, aHCl/aH2O in the high-XCO2 group may be higher than or overlap with aHCl/aH2O in the low-XCO2 group. Calculated aHCl and mClT in the high-XCO2 group are lower than or overlap with aHCl and mClT in the low-XCO2 group. The Cl-rich apatites in the high-XCO2 group are explained by crystallization at relatively low aH2O, aHF, and mFT rather than at high aHCl or mClT. In comparison, the F-OH apatites in the low-XCO2 group formed by infiltration of rock by and equilibration with relatively H2O-rich, high mFT/mClT fluid, reflecting the same metasomatic process responsible for F-rich humite-group minerals and skarns in many contact aureoles. Calculated halogen contents indicate that the non-CO2 fraction of fluid in equilibrium with both groups had modest, seawater-like salinity, and that the reactive H2O- and F-rich fluid that infiltrated the low-XCO2 group had a plutonic source.

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