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

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