Exchange experiments for chlorine and bromine partitioning in scapolite at variable fluid salinities, pressures, and temperatures: implications for tracing crustal fluid sources via Cl/Br ratios in scapolite

Abstract

Halogen ratios (Cl/Br) preserved in halogen-bearing minerals can be very useful to identify the sources of fluids interacting with crystalline rocks, as different fluid types have distinct halogen ratios. In this study we conduct exchange experiments for chlorine and bromine partitioning between scapolite and brine treated at variable fluid salinities ranging from 0.20 to 0.66 mol fraction of salt (NaCl + NaBr). Experiments involved two different natural scapolites, which were treated in the presence of brine and minor calcite in sealed platinum capsules at 0.32 to 1.52 GPa and 600 °C to 1000 °C for durations of 12–120 h. Neomorphic scapolite appeared as overgrowths on the initial scapolite, or, in some cases, fully recrystallized with no relict scapolite visible. The experiments show that the Ca/Na ratio of scapolite depends on the treatment temperature, the fluid salinity, and pressure. In contrast, the Cl/Br distribution coefficients between neomorphic scapolite and fluids do not depend on the temperature, composition of the mineral or the total salinity of the fluid. The Cl/Br distribution coefficient is, however, markedly pressure-dependent. The experimentally-determined partitioning coefficients of this study and previous work, ranging from 1 atm to 1.5 GPa, enable the use of Cl/Br ratios in scapolite to characterize the halogen ratio of fluids throughout the entire crust. The molar Cl/Br ratio of a fluid can be determined from the measured molar Cl/Br of scapolite via: Cl/Br^fluid = Cl/Br^scapolite x ( – 1.473 × P + 1.119 × P² – 0.299 × P³ + 1.103)⁻¹, where P is pressure in GPa, over the range of 0.0001–1.5 GPa.