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The XCO 2  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 XCO 2  that results from infiltration of chemically reactive H 2 O-rich fluid. The other records relatively high XCO 2 , 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-XCO 2  group is close to an F-OH solid solution. Apatite in the high-XCO 2  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 αHF/aH 2 O, αHF/αHCl, αHF, and mF T  (the total F molality of fluid) are systematically higher in fluid coexisting with the low-XCO 2  group. In contrast, αHCl/αH 2 O in the high-XCO 2  group may be higher than or overlap with αHCl/αH 2 O in the low-XCO 2  group. Calculated αHCl and mCl T  in the high-XCO 2  group are lower than or overlap with aHCl and mCl T  in the low-XCO 2  group. The Cl-rich apatites in the high-XCO 2  group are explained by crystallization at relatively low αH 2 O, αHF, and mF T  rather than at high αHCl or mCl T . In comparison, the F-OH apatites in the low-XCO 2  group formed by infiltration of rock by and equilibration with relatively H 2 O-rich, high mF T /mCl T  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-CO 2  fraction of fluid in equilibrium with both groups had modest, seawater-like salinity, and that the reactive H 2 O -  and F-rich fluid that infiltrated the low-XCO 2  group had a plutonic source.

Coupled dichotomies of apatite and fluid composition during contact metamorphism of siliceous carbonate rocks