Effect of saline fluids on chlorine incorporation in serpentine

Abstract

The incorporation of chlorine in serpentine minerals is crucial for the recycling of chlorine in subduction zones. Fluid inclusions of natural serpentinites have up to 50 wt% Cl, but effect of fluid salinity on chlorine distribution in serpentine minerals is poorly constrained. In this study, natural serpentinites (Lichi Melange, Taiwan) with starting grain sizes of 100–177 μm were equilibrated in saline solutions (2.93 wt%, 8.78 wt%, and 19.30 wt% NaCl) at ambient temperature and pressure for experimental duration from 18 to 43 days. The concentrations of chlorine in serpentine minerals were analyzed using electron microprobe with a detection limit of 33 ppm. Serpentine before experiments has very low chlorine, 0.017 ± 0.009 wt%. In contrast, serpentine equilibrated in saline solutions contains chlorine around three times higher, e.g., serpentine equilibrated in 2.93 wt% NaCl has 0.077 ± 0.033 wt% Cl after 18 days. The serpentine was re-equilibrated in pure water for around 24 h in order to testify chlorine is hosted in a weak-bound or structurally-bound position. For serpentine minerals equilibrated in low-salinity solutions (2.93 wt% and 8.78 wt% NaCl), they lost ∼40% of Cl after re-equilibrated in pure water. Despite such release, chlorine in serpentine minerals is still higher than that of serpentine before experiment, which indicates that saline solutions increase structurally-bound chlorine of serpentine minerals. This is more efficient for high-salinity solution (19.30 wt% NaCl), and chlorine in serpentine re-equilibrated in pure water is essentially the same as that of serpentine equilibrated in the saline solution. Our experimental results suggest that chlorine in serpentine can be greatly modified by saline fluids. The structurally-bound chlorine may not necessarily reflect the T-P information of natural serpentinites.