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

Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218, U.S.A.

Correspondence: ^* E-mail sarahpd{at}geol.umd.edu. Present address: Department of Geology, University of Maryland, College Park, MD 20742.

The X_CO2 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 X_CO2 that results from infiltration of chemically reactive H₂O-rich fluid. The other records relatively high X_CO2, 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-X_CO2 group is close to an F-OH solid solution. Apatite in the high-X_CO2 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 a_HF/a_H2O, a_HF/a_HCl, a_HF, and m_FT (the total F molality of fluid) are systematically higher in fluid coexisting with the low-X_CO2 group. In contrast, a_HCl/a_H2O in the high-X_CO2 group may be higher than or overlap with a_HCl/a_H2O in the low-X_CO2 group. Calculated a_HCl and m_ClT in the high-X_CO2 group are lower than or overlap with a_HCl and m_ClT in the low-X_CO2 group. The Cl-rich apatites in the high-X_CO2 group are explained by crystallization at relatively low a_H2O, a_HF, and m_FT rather than at high a_HCl or m_ClT. In comparison, the F-OH apatites in the low-X_CO2 group formed by infiltration of rock by and equilibration with relatively H₂O-rich, high m_FT/m_ClT 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-X_CO2 group had a plutonic source.