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Carbonate recycling in subduction zones has a strong influence on arc magmatism and carbon reservoirs in the mantle. However, geochemical evidence of carbonate recycling during subduction, especially at sub-arc depths, is rare and the related process is poorly understood. In this study, we document high-Mg olivines (81–91 mol.% forsterite) with extremely low Ni concentrations (mostly <600 ppm), which occur in host dacites recovered from the southwestern Okinawa Trough (SWOT). The temperature–pressure of crystallization had a negligible effect on the Ni contents of the olivines, and reactions between primary basaltic magmas and the lithospheric mantle were also insignificant. Small sulphide droplets are present in the olivines, which suggest that sulphide–silicate melt immiscibility occurred during the early stages of magmatic evolution. Although early segregation of sulphide melts could reduce the Ni contents of the primitive magmas, our forward modelling suggests that sulphide segregation cannot solely explain the low Ni contents. Therefore, the most likely explanation for low Ni concentrations is the interaction between Ca-rich carbonate melts and the mantle wedge, forming an olivine-rich mantle source by consumption of orthopyroxene, as indicated by the extremely low Ni/Mg and high Ca/Fe ratios of magmatic olivines. Given that the SWOT is located just ~100 km above the Wadati–Benioff zone, our results provide rare evidence for carbonate recycling at sub-arc depths in an active subduction zone. [ABSTRACT FROM AUTHOR]

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 20 kbar on peridotite, orthopyroxene, and olivine with <5% pyroxene. The results show that the concentrations of chlorine in serpentine minerals were up to 1.2 wt% at 200 °C, whereas they decreased slightly at 311–400 °C and 3.0 kbar and became significantly lower at 485 °C and 3.0 kbar, ∼0.1 wt%. Fluid salinity greatly decreased chlorine concentrations of olivine-derived serpentine produced at 400 °C and 3.0 kbar, which was associated with a decrease in silica mobility during serpentinization. By contrast, influence of fluid salinity at 311 °C and 3.0 kbar 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 3.0 kbar had 0.08 ± 0.03 wt% Cl, which is significantly lower than chlorine concentrations of serpentine minerals (0.49 ± 0.36 wt%) produced in orthopyroxene-only experiments. By contrast, for experiments at 311 °C and 3.0 kbar, olivine- and orthopyroxene-derived serpentine had comparable amounts of chlorine. In particular, olivine-derived serpentine had 0.16 ± 0.09 wt% 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 3.0 kbar were slightly lower than those in serpentine produced at 300 °C and 8.0 kbar, 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. [ABSTRACT FROM AUTHOR]