Carbonatite formation in continental settings via high pressure– high temperature liquid immiscibility

Abstract

The goal of this study is to compare compositions of high temperature silicate-carbonatite immiscible melts, known from melt inclusions and experiments, to compositions of silica-undersaturated volcanic rocks from continental settings, in order to improve understanding of the formation of calcite carbonatite rocks worldwide. Melt inclusions in this study are abundant in perovskites crystals from magnetite-perovskite cumulates sampled at the Kerimasi volcano in the East African Rift System. The temperature of complete dissolution of daughter minerals in the melt inclusions and the high CO2−content of the silicate melt (5.4–9.8 wt%) support early formation of the rock and entrapment of melts at high temperatures (∼1100 °C) and pressures (≥1 GPa). Heated-quenched melt inclusions indicate the presence of immiscible mafic-melilitite and Ca-Na-K-carbonatite melts together with a fluid phase at entrapment. Melilitite melts are low in SiO2 (29.4–33.5 wt%), moderate in MgO (4.0–5.9 wt%) but high in CaO (16.3–24.4 wt%), FeOT (9.0–13.5 wt%), Na2O (6.8–12.7 wt%) and K2O (1.8–3.2 wt%). Coexisting carbonatite melts also show high CaO-content (28.4–39.0 wt%), along with moderate-to-high Na2O (8.2–20.2 wt%) + K2O (4.1–6.6 wt%) compositions (total alkalis ranges between 13.1 and 24.3 wt%). We compared the studied silicate melts in the inclusions with a global dataset of 146 continental melilitite and 640 nephelinite compositions (GEOROC database). We argue that the studied calcite-saturated melilitite melts formed in a continental rift setting and were able to exsolve carbonatite melts that crystallized voluminous calcite carbonatite rocks during their evolution. In contrast, magnesian melilitite and nephelinite volcanic rocks from intracontinental settings are compositionally far away from any immiscibility field at feasible pressures and were only able to unmix carbonatite melts during late-stage evolution, leaving little opportunity for calcite crystallization. CaO- and alkali-rich, but extremely SiO2-undersaturated melilitite melts play a key role in early silicate-carbonatite immiscibility, can only be preserved in melt inclusions and cannot be represented by volcanic rocks. The parental melts of the studied melilitite-carbonatite melts probably formed via low-degree partial melting of modally metasomatized continental lithosphere.