The role of Al-defects on the equation of state of Al–(Mg,Fe)SiO 3 perovskite

Abstract

We performed compression curves of aluminous silicate perovskite (Al–Pv) synthesized under various conditions of pressure, temperature and MgO or SiO 2 activities, using laser-heated diamond anvil cell at the ESRF (Grenoble). We refined bulk moduli ( K 0) from 235 to 270 GPa, in agreement with the wide range of values reported in the literature. We observe that Al–Pv phase synthesized at high temperature, in the SiO 2-rich system, is more compressible than Al–Pv phase synthesized at high pressure, in the MgO-rich system. As suggested by various authors, the resolution of this controversy rests on a better understanding of the crystal chemistry of Al in perovskite, which involves at least two competitive mechanisms, substitution of Si in the octahedral site only, or a coupled substitution on both Mg and Si sites. The vacancy mechanism is expected to reduce the K 0 significantly, due to the presence of oxygen vacancies. All compression curves performed in this study can be explained by considering that the vacancy mechanism is favored at high temperatures and that the coupled mechanism is favored at high pressures. These trends agree well with previous reports. For (Mg,Fe)(Si,Al)O 3 perovskite compositions relevant to the lower mantle, the two previous reports and our new data set for a MORB-type perovskite phase agree well with each other with higher K 0 values between 260 and 270 GPa, compared with K 0 = 253 GPa for the pure MgSiO 3 phase suggested from previous studies. In these compounds, coupled substitution of Al 3+ and Fe 3+ cations leads to a well constrained crystal chemistry. Therefore, the low K 0 value observed in some of the previous studies for Fe-free Al–Pv is likely to be irrelevant for mantle perovskite. Some questions may remain only for the mantle region just below the 670 km discontinuity, where pressures remain moderate, thus potentially allowing for at most 2% of oxygen vacancies. However, it is clear that using low K 0 values to extrapolate to greater depths is unjustified.