Genetic link between saline and carbonatitic mantle fluids:The system NaCl-CaCO_(3)-MgCO_(3)±H_(2)O±Fe^(0) at 6 GPa

Melt inclusions in kimberlitic minerals and diamonds indicate that chlorides are important constituents of mantle carbonatite melts.Besides,alkaline chlorides are important constituents of saline high-density fluids(HDFs)found in diamonds from kimberlites and placers around the world.Continuous compositional variations suggest that saline and carbonatitic HDFs could be genetically linked.However,the essence of this link remains unclear owing to the lack of data on phase relations in the chloridecarbonate systems under pressure.Here we studied subsolidus and melting phase relations in the system NaCl–CaCO_(3)–MgCO_(3)at 6 GPa and 1000–1600℃using a Kawai-type multianvil press.We found that at 1000℃,subsolidus assemblage consists of halite,magnesite,and aragonite.At higher temperatures,the stabilization of dolomite splits the subsolidus area into two partial ternary fields:halite+magnesite+dolomite and halite+dolomite+aragonite.The minimum on the liquidus surface corresponds to the halite-dolomite-aragonite ternary eutectic,situated at 1100℃.The eutectic melt has Ca#89 and contains 30 wt.%Na Cl(26 mol%2NaCl).The system has two ternary peritectics:halite+dolomite=magnesite+liquid located near the ternary eutectic and magnesite+dolomite=Mg-dolomite+liquid situated between 1300 and 1400℃.Although under dry conditions incipient melting yields carbonatedominated melt,the addition of water facilitates the fusion of Na Cl and expands the liquid field to Na Cl-rich compositions with up to 70 wt.%Na Cl.The obtained results favor the idea that hydrous saline melts/fluids(brines)found as inclusions in diamonds could be a lower temperature derivative of mantle carbonatite melts and disagree with the hypothesis on chloride melt generation owing to the chloridecarbonate liquid immiscibility since no such immiscibility was established.We also studied the interaction of the NaCl–CaCO_(3)–MgCO_(3)system with iron metal and found that carbonate reduction produces Cbearing species(Fe^(0),Fe-C melt,Fe_(3)C,Fe_(7)C_(3),C^(0))and wüstite containing Na_(2)O,CaO,and MgO.Besides,a carbonate chloride compound,Ca_(2)Cl_(2)CO_(3),was established among the reaction products.The interaction between Na Cl-bearing carbonate melt shifts its composition toward Mg-poor and Na Cl-rich.Given the above,an alternative hypothesis can be proposed,according to which the interaction of alkaline chloride-bearing carbonate melts formed in the subduction zones with the reduced mantle should be accompanied by diamond crystallization and shift the composition of the melt from carbonatitic to alkali-rich saline.