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

Abstract

The X C O 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 X C O 2 that results from infiltration of chemically reactive H 2 O-rich fluid. The other records relatively high X C O 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-X C O 2 group is close to an F-OH solid solution. Apatite in the high-X C O 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 a H F /a H 2 O , a H F /a H C l , a H F , and m F T (the total F molality of fluid) are systematically higher in fluid coexisting with the low-X C O 2 group. In contrast, a H C l /a H 2 O in the high-X C O 2 group may be higher than or overlap with a H C l /a H 2 O in the low-X C O 2 group. Calculated a H C l and m C l T in the high-X C O 2 group are lower than or overlap with a H C l and m C l T in the low-X C O 2 group. The Cl-rich apatites in the high-X C O 2 group are explained by crystallization at relatively low a H 2 O , a H F , and m F T rather than at high a H C l or M C l T . In comparison, the F-OH apatites in the low-X C O 2 group formed by infiltration of rock by and equilibration with relatively H 2 O-rich, high m F T /m C l 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-X C O 2 group had a plutonic source.