The System KCl-CaCO3-MgCO3 at 3 GPa

Inclusions in mantle minerals and xenoliths from kimberlites worldwide derived from depths exceeding 100 km vary in composition from alkali-rich saline to carbonatitic. Despite the wide distribution of these melts and their geochemical importance as metasomatic agents that altered the mineralogy and geochemistry of mantle rocks, the P-T range of stability of these melts remains largely undefined. Here we report new experimental data on phase relations in the system KCl-CaCO3-MgCO3 at 3 GPa obtained using a multianvil press. We found that the KCl-CaCO3 and KCl-MgCO3 binaries have the eutectic type of T-X diagrams. The KCl-calcite eutectic is situated at K2# 56 and 1000 degrees C, while the KCl-magnesite eutectic is located at K2# 79 and 1100 degrees C, where K2# = 2KCl/(2KCl + CaCO3 + MgCO3) x 100 mol%. Just below solidus, the KCl-CaCO3-MgCO3 system is divided into two partial ternaries: KCl + magnesite + dolomite and KCl + calcite-dolomite solid solutions. Both ternaries start to melt near 1000 degrees C. The minimum on the liquidus/solidus surface corresponds to the KCl + Ca0.73Mg0.27CO3 dolomite eutectic situated at K2#/Ca# 39/73, where Ca# = 100 center dot Ca/(Ca + Mg) x 100 mol%. At bulk Ca# <= 68, the melting is controlled by a ternary peritectic: KCl + dolomite = magnesite + liquid with K2#/Ca# 40/68. Based on our present and previous data, the KCl + dolomite melting reaction, expected to control solidus of KCl-bearing carbonated eclogite, passes through 1000 degrees C at 3 GPa and 1200 degrees C at 6 GPa and crossovers a 43-mW/m(2) geotherm at a depth of 120 km and 37-mW/m(2) geotherm at a depth of 190 km.

Automated summary

Inclusions in mantle minerals and xenoliths from kimberlites derived from deep depths showed great variations in composition. The stability range of these inclusions remains unclear. New experimental data on the KCl-CaCO3-MgCO3 system obtained at 3 GPa show the eutectic type of phase relations. The melting reactions of KCl-bearing carbonated eclogite at different pressures and depths are also identified.