The system KCl−CaCO3−MgCO3 at 6 GPa: A link between saline and carbonatitic diamond-forming fluids

Abstract

Alkaline chlorides and carbonates are abundant in melt inclusions in diamonds, mantle xenoliths, and igneous minerals from kimberlites. Despite this, the phase relationships in chloride‑carbonate systems remain poorly constrained. Here we studied phase equilibria in the system KCl–CaCO3–MgCO3 in multianvil experiments at 6 GPa and 1000–1600 °C. It was found that at 1000 °C, subsolidus assemblage consists of KCl, magnesite, and aragonite. At higher temperatures, the stabilization of dolomite splits the system into two partial ternaries: KCl + dolomite + aragonite and KCl + magnesite + dolomite. Both ternaries start to melt near 1200 °C. The melting of the first ternary is controlled by the KCl-dolomite-aragonite eutectic situated at 30 mol% (24 wt%) KCl and Ca# 92, where Ca# = 100∙Ca/(Ca + Mg). The melting of the second ternary is controlled by the peritectic, KCl + dolomite = magnesite + liquid, producing chloride‑carbonate melt with 33 mol% (27 wt%) KCl and Ca# 69.The established melting temperature of the KCl + Ca-Mg carbonate assemblages is 200–500 °C higher than the subduction geotherms. This indicates that chlorides and carbonates can survive subduction to a depth of 200 km. However, the stagnation and warming of slabs to the ambient mantle temperature must result in partial melting of KCl + Ca-Mg carbonate assemblages producing a KCl-rich low-Mg carbonate melt. Percolation of this melt into the subcontinental lithospheric mantle should yield crystallization of KCl and CaMg carbonates. In the presence of water, KCl will form hydrous saline melt or fluid, while Ca and Mg carbonates, which are poorly soluble in water, will remain solid. The obtained results imply that the hydrous saline fluids (brines) found as inclusions in diamonds are a lower temperature derivative of mantle carbonatite melts and do not support the hypothesis of chloride melt generation owing to the chloride‑carbonate liquid immiscibility since no such immiscibility was established.