Strong correlation of oxygen vacancies in bridgmanite with Mg/Si ratio

Abstract

The variation of an oxygen vacancy (OV) in the form of an MgAlO2.5 component in bridgmanite with Mg/Si ratios of bulk compositions was clarified using phase relations in the ternary system MgO–AlO1.5–SiO2 at a pressure of 27 GPa and a temperature of 2000 K using advanced multi-anvil techniques. Both normal and reversed experiments suggest that significant amounts of the OV component exist in bridgmanite synthesized from bulk compositions with Mg/Si ratios above unity. The OV component significantly decreases with decreasing Mg/Si ratio, and it finally becomes negligible for Mg/Si ratios below unity. In contrast, the charge-coupled (CC) component of AlAlO3 becomes more dominant. The ternary phase relations further indicate that bridgmanite in a pyrolitic or peridotitic lower mantle will contain certain amounts of OV component, while that in mid-ocean ridge basalts (MORB) will not contain any amount of this component. Our study suggests that significant amounts of volatiles such as argon trapped by the OV of bridgmanite may be induced into the ambient lower mantle, while they cannot be brought into basaltic slabs by bridgmanite but other phases such as hydrous phases. The decrease of the OV in bridgmanite with decreasing Mg/Si ratio may offer a simple explanation for the occurrence of some stagnant slabs in the lower mantle.