New data on the system Na2CO3–CaCO3–MgCO3 at 6 GPa with implications to the composition and stability of carbonatite melts at the base of continental lithosphere

Abstract

Subsolidus and melting phase relationships in the system Na2CO3–CaCO3–MgCO3 have been studied at 6 GPa and 900–1250 °C using a Kawai-type multianvil press. At 900 and 1000 °C, the system has four intermediate compounds: Na2Ca4(CO3)5 burbankite, Na2Ca3(CO3)4, Na4Ca(CO3)3, and Na2Mg(CO3)2 eitelite. The Na-Ca compounds dissolve noticeable amounts of Mg component, whereas eitelite dissolves a few percents of Ca component: Na2(Ca≥0.91Mg≤0.09)4(CO3)5, Na2(Ca≥0.94Mg≤0.06)3(CO3)4, Na4(Ca≥0.67Mg≤0.33)(CO3)3, and Na2(Mg≥.93Ca≤0.07)(CO3)2. At 1050 °C, the system is complicated by an appearance of dolomite. Na-Ca burbankite decomposes at 1075 ± 25 °C to aragonite plus Na2Ca3(CO3)4. Na4Ca(CO3)3 and eitelite disappear via congruent melting between 1200 and 1250 °C. Na2Ca3(CO3)4 remains stable through the whole studied temperature range. The liquidus projection of the studied ternary system has eight primary solidification phase regions for magnesite, dolomite, calcite-dolomite solid solutions, aragonite, Na2Ca3(CO3)4, Na4Ca(CO3)3, and Na2CO3 solid solutions. The system has five ternary peritectic reaction points and one minimum on the liquidus at 1050 °C and 48Na2CO3·52(Ca0.75Mg0.25)CO3. The minimum point resembles a eutectic controlled by a four-phase reaction, by which a liquid transforms into three solid phases upon cooling: Na2(Ca0.94Mg0.06)3(CO3)4, Na4(Ca0.67Mg0.33)(CO3)3, and Na2(Mg0.93Ca0.07)(CO3)2 eitelite. Since at 6 GPa, the system has a single eutectic, there is no thermal barrier preventing continuous liquid fractionation from alkali-poor toward Na-rich dolomitic compositions. Cooling of the Na-Ca-Mg carbonatite melt from 1400 to 1100 °C within the lherzolite substrate will be accompanied by magnesite crystallization and wehrlitization keeping calcium number of the melt at 40 and shifting the Na2CO3 content to ≥40 mol%. In the case of the eclogitic wall rock, the cooling will be accompanied by dolomite crystallization keeping calcium number of the melt at 60–65 and shifting the Na2CO3 content to ≥30 mol%.