Na-bearing bridgmanite: Synthesis, phase relations and application to the origin of alkaline melts in the uppermost lower mantle

Abstract

Experimental studies of melting relations in the MgSiO3–Na2CO3 (±Al2O3, Fe2O3) multicomponent alkaline carbonate–silicate system were carried out at 23–24 GPa and 1100–1700 °C to shed light on the conditions and mechanisms of formation of Na-bearing bridgmanite. At 1100–1300 °C, an assemblage of low-Na bridgmanite (<0.6 wt% Na2O), ringwoodite, periclase (ferropericlase), alkaline carbonate with composition close to Na2Mg(CO3)2, and sodic carbonate–silicate melts is formed. With increase in temperature up to 1700 °C, carbonate disappears and the content of Na2O increases in all mantle phases (up to 1.6, 4.4, and 2.7 wt% in bridgmanite, ringwoodite, and ferropericlase, respectively), which indicates that Na-bearing bridgmanite may crystallize at moderate degrees of partial melting. The clear positive correlation between the contents of Na and Al, and negative correlation between Na and Fe in bridgmanite, as well as the preferable structural incorporation of Al and Na with temperature, provide evidence for the predominant crystallization of Na-rich bridgmanite via the vacancy mechanism Si4+B + Mg2+A + O2−O = Al3+B + Na+A + VO from Al-bearing alkaline carbonate–silicate melts, with composition similar to those detected as inclusions in lower-mantle diamonds.