Effect of chlorine on the melting of the subcratonic lithospheric mantle

The presence of chlorine in the subcratonic lithospheric mantle (SCLM) has been evaluated by compiling the compositional data of fluid inclusions in fibrous diamonds. Chlorine associates with potassium, dissolving in water and forming a KCl-bearing brine with the Cl/(Cl+H2O) molar ratio of 0.05-0.68.;To examine the effect of such a KCl-bearing brine on the melting behavior of the SCLM, we conducted experiments in the Mg2SiO4-MgSiO 3-H2O and Mg2SiO4-MgSiO 3-KCl-H2O systems at 5 GPa and 1100-1700°C. In the Mg 2SiO4-MgSiO3-H2O system, the solidus temperature of forsterite+enstatite is ∼1230°C. In the Mg2SiO 4-MgSiO3-KCl-H2O systems with molar Cl/(Cl+H 2O) ratios of 0.2, 0.4 and 0.6, the solidus temperatures are ∼1430°C, ∼1530°C and ∼1580°C, respectively. The increase in the temperature of the solidus demonstrates that KCl elevates the solidus of the Mg2SiO 4-MgSiO3-H2O system. Therefore, KCl in the SCLM can prevent melting at the H2O-saturated solidus, and a KCl-bearing fluid can be a robust agent for mantle metasomatism.