The System KCl–CaCO3–MgCO3 at 3 GPa

Abstract

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 °C, while the KCl-magnesite eutectic is located at K2# 79 and 1100 °C, where K2# = 2KCl/(2KCl + CaCO3 + MgCO3) × 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 °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∙Ca/(Ca + Mg) × 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 °C at 3 GPa and 1200 °C at 6 GPa and crossovers a 43-mW/m2 geotherm at a depth of 120 km and 37-mW/m2 geotherm at a depth of 190 km.