Experimental constraints on truly conjugate alkaline silicate – carbonatite melt pairs

Abstract

Physical separation of immiscible carbonatites from CO2-bearing alkaline silicate melts is frequently invoked to explain the common presence of carbonatites in alkaline rock suites. We present new isobaric 1.0 GPa crystallization and liquid immiscibility experiments between 1250 and 925°C on chemically complex CO2-bearing subvolcanic basanitic, nephelinitic and nepheline-syenitic compositions from the intrusive complex on Brava Island, Cape Verdes. In the experiments, carbonatites exsolve from silicate melt along the basanite–nephelinite melt fractionation pathway throughout an unmixing temperature window from 1100 to 950°C. Liquid immiscibility is suppressed by 950°C by the onset of alkali-feldspathoid crystallization, which deflects the residual silicate melt away from the two-liquid field. While evolving along the two-liquid field boundary, silicate melt increases in SiO2 from 46 to 51 wt% and decreases in CaO / (Na2O + K2O) (by weight) from 0.83 to 0.12, reproducing the observed compositional array of alkaline silicate subvolcanic rocks from Brava. The conjugate carbonatite melts are moderately alkaline Ca-carbonatites with 5 to 9 wt% SiO2 and CaO / (Na2O + K2O) that decreases from 5.64 at 1100°C to 1.60 at 950°C. These coexisting silicate and carbonatite melt compositions show that previous high-pressure experimental studies have not yielded truly conjugate melt pairs, mainly due to the use of modified starting materials resulting in melt compositions too enriched in alkalis to match the natural alkaline silicate rock record.