The oxygen vacancy component (Mg, Fe2+)(Fe3+, Al)O2.5 in bridgmanite does not contain Si in its formula. This fact leads to the idea that the oxygen vacancy component should dominate in systems with high Mg/Si ratios, namely ultramafic systems rather than basaltic systems. Based on this idea, relations of the MgAlO2.5 (OV) and AlAlO3 (CC) contents to the bulk Mg/Si ratio were investigated in the system MgO-Al2O3-SiO2 at a pressure of 27 GPa and a temperature of 2000 K (Fig. 1). The starting materials have two series: one series (A, C, E, and G) has bulk compositions close to MgSiO3, from which no excess alumina phase was expected, whereas the other series (B, D, F, and H) has intermediate bulk composition between MgSiO3 and aluminous phases.
Bulk compositions of starting materials (A – G) used in this study.
Fig. 2 shows experimental results, namely, the OV and CC contents as a function of the bulk Mg/Si ratio. In both Al-undersaturated and -saturated systems, the OV content in bridgmanite decreases with increasing the bulk Mg/Si ratio, as expected. More are noted: 1) the OV contents in the Al-undersaturated systems are higher than the Al-saturated systems, which implies that bridgmanite could contain excess OV components than equilibrium in systems without excess phases.
Variation of the MgAlO2.5 (OV) and AlAlO3 (CC) components in bridgmanite with Al pfu = 0.1 (a) and Al pfu > 0.1 (b) as a function of the bulk Mg/Si ratio. The bridgmanite with Al pfu > 0.1 was saturated with the aluminous phases (Ca-ferrite type MgAl2O4 and/or corundum), whereas that with Al pfu = 0.1 did not coexist with aluminous phase. In both cases, the OV component decreases with increasing the bulk Mg/Si ratio.
This study concludes that bridgmanite in the ambient lower mantle contains the OV component, whereas basaltic layers in subducted slabs do not contain any.