Apatite as an indicator of fluid salinity: An experimental study of chlorine and fluorine partitioning in subducted sediments

In order to constrain the salinity of subduction zone fluids, piston-cylinder experiments have been conducted to investigate the partitioning behaviour of Cl and F in subducted sediments. These experiments were performed at H₂O-undersaturated conditions with a synthetic pelite starting composition containing 800ppm Cl, over a pressure and temperature range of 2.5-4.5GPa and 630-900°C. Repetitive experiments were conducted with 1900ppm Cl+1000ppm F, and 2100ppm Cl. Apatite represents the most Cl-abundant mineral phase, with Cl concentration varying in the range 0.1-2.82wt%. Affinity for Cl decreases over the following sequence: aqueous fluid>apatite≥melt>other hydrous minerals (phengite, biotite and amphibole). It was found that addition of F to the Cl-bearing starting composition significantly lowers the Cl partition coefficients between apatite and melt (D_Cl^Ap-melt) and apatite and aqueous fluid (D_Cl^Ap-aq). Cl-OH exchange coefficients between apatite and melt (K_dCl-OH^Ap-melt) and apatite and aqueous fluid (K_dCl-OH^Ap-aq) were subsequently calculated. K_dCl-OH^Ap-melt was found to vary from 1 to 58, showing an increase with temperature and a decrease with pressure and displaying a regular decrease with increasing H₂O content in melt. Mole fractions of Cl and OH in melt were calculated based on an ideal mixing model for H₂O, OH, O, Cl and F. The Cl contents of other hydrous minerals (phengite, biotite and amphibole) fall between 200 and 800ppm, with resultant Cl partition coefficients from 0.02 to 0.49, appearing independent of the bulk Cl and F content. Preliminary data from this study show that the partitioning behaviour of F is strongly in favour of apatite relative to melt and phengite, with D_F^Ap-melt=15-51.Apatites from representative eclogite facies metasediments were examined and found to have low Cl contents close to ~100ppm. Calculations using our experimentally determined K_dCl-OH^Ap-aq of 0.004 at 2.5GPa, 630°C indicate a low salinity character (0.5-2wt% NaCl_eq) for the fluid formed during dehydration of subducted oceanic sediment at ~80km depth.