Influence of temperature, pressure, and fluid salinity on the distribution of chlorine into serpentine minerals

Abstract

Serpentinization produces serpentine minerals that have abundant water and fluid-mobile elements (e.g., Ba, Cs, and Cl). The dehydration of serpentine minerals produces chlorine-rich fluids that may be linked with the genesis of arc magmas. However, the factors that control the distribution of chlorine into serpentine minerals remain poorly constrained. We performed serpentinization experiments at 80–500°C and pressures from vapor saturated pressures to 20kbar on peridotite, orthopyroxene, and olivine with <5% pyroxene. The results show that the concentrations of chlorine in serpentine minerals were up to 1.2wt% at 200°C, whereas they decreased slightly at 311–400°C and 3.0kbar and became significantly lower at 485°C and 3.0kbar, ∼0.1wt%. Fluid salinity greatly decreased chlorine concentrations of olivine-derived serpentine produced at 400°C and 3.0kbar, which was associated with a decrease in silica mobility during serpentinization. By contrast, influence of fluid salinity at 311°C and 3.0kbar is minor. Moreover, chlorine distribution into serpentine can be influenced by primary minerals of serpentine. Serpentine formed in olivine-only experiments at 311°C and 3.0kbar had 0.08±0.03wt% Cl, which is significantly lower than chlorine concentrations of serpentine minerals (0.49±0.36wt%) produced in orthopyroxene-only experiments. By contrast, for experiments at 311°C and 3.0kbar, olivine- and orthopyroxene-derived serpentine had comparable amounts of chlorine. In particular, olivine-derived serpentine had 0.16±0.09wt% Cl that was slightly higher than chlorine concentrations of serpentine formed in olivine-only experiments, whereas orthopyroxene-derived serpentine had significantly lower chlorine concentrations than that formed in orthopyroxene-only experiments. The contrast may be associated with releases of aluminum and silica from pyroxene minerals, which possibly results in a decrease in chlorine concentrations of serpentine. The concentrations of chlorine in serpentine formed in experiments at 311°C and 3.0kbar were slightly lower than those in serpentine produced at 300°C and 8.0kbar, which may be associated with influence of pressure on the mobility of iron and silica. The experimental results of this study indicate that serpentine minerals are important carriers of chlorine in subduction zones. It also suggests that chlorine is significant for the redistribution of cations during serpentinization.