Experimental study in the Na_2O-CaO-MgO-Al_2O_3-SiO_2-CO_2 system at 3 GPa: the effect of sodium on mantle melting to-carbonate-rich liquids and implications for the petrogenesis of silicocarbonatites

摘要

Melt compositions in equilibrium with peridotite assemblages were determined in the analogue system Na_2O-CaO-MgO-Al_2O_3-SiO_2-CO_2 at 3 GPa with Ca/Ca + Mg = 0.56-0.43 and up to 6 wt.% Na_2O. There is a greater compositional range generated isobarically over a larger temperature interval than in a sodium-absent system: increasing sodium content drives liquids to compositions with lower CaO and higher SiO_2 concentrations. A positive correlation between silica and Na_2O content of liquids produced at constant temperature is due to the depolymerization of silicate tetrahedra in the presence of monovalent cations, as in the volatile-free system. Liquids with Na_2O >6 wt.% occur in association with wehrlites as the composition of diopsidic pyroxene expands towards enstatite with addition of Na_2O, decreasing the orthopyroxene content of peridotite. The orthopyroxene-out curve intersects an enriched mantle solidus at 3 GPa where near-solidus liquids have Na_2O = 7±1.5 wt.%. Sodium partitioning between a metaluminous liquid and clinopyroxene follows the jadeite partitioning models calculated for the dry silicate system but sodium partitions into peralkaline carbonated liquids as both the pyroxene and the carbonate molecules. The peralkaline liquids generated are essentially carbonated silicate melts that are analogous to silica-bearing carbonatites and silicocarbonatites from a range of possible metasomatized mantle sources.