Rapid decrease of MgAlO2.5 component in bridgmanite with pressure

doi: 10.7185/geochemlet.1739 The solubility of the MgAlO2.5 component in bridgmanite was measured at pressures of 27, 35 and 40 GPa and a temperature of 2000 oK using an ultra-high pressure multi-anvil press. Compositional analysis of recovered samples demonstrated that the MgAlO2.5 component decreases with increasing pressure, and approaches virtually zero at 40 GPa. Above this pressure, the MgAlO2.5 component, i.e. the oxygen-vacancy substitution, becomes negligible, and Al is incorporated in bridgmanite by the charge-coupled substitution only. These results are supported by the volume change associated with the change from the oxygen-vacancy substitution to charge-coupled substitution. The present result may explain the seismically observed slab stagnation in the mid-lower mantle. Although bridgmanite has been put forward as a potential host for water and argon in the lower mantle by trapping them in oxygen vacancies, such capabilities will rapidly decrease with depth and be lost in regions deeper than 1000 km. Received 10 May 2017 | Accepted 8 September 2017 | Published 12 October 2017 1. Bayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany * Corresponding author (email: zhaodong.liu@uni–bayreuth.de) Introduction Aluminium (Al) is the fifth most abundant element in the Earth’s mantle (McDonough and Sun, 1995). Under the Earth’s lower mantle conditions, Al3+ is mainly accommodated in bridgmanite in a pyrolite mantle (Irifune, 1994). Al substitution in the Mg, Si or both sites modifies the crystal chemistry of bridgmanite, and thereby changes the physical properties of bridgmanite (Zhang and Weidner, 1999; Brodholt, 2000). Therefore, understanding the Al substitution mechanism is essential to investigating the mineralogy and dynamics of the lower mantle. Two competing Al substitution mechanisms occur in bridgmanite (Hirsch and Shankland, 1991). One is the charge-coupled substitution (Mg2+ + Si4+ = Al3+ + Al3+; components along the MgSiO3–Al2O3 binary system), where two Al cations substitute for one Mg with the roughly 8-fold coordination (A site) and one Si with the 6-fold coordination (B site). The other mechanism is the oxygen-vacancy substitution (2Si4+ + O2– = 2Al3+ + VO, where VO is an oxygen vacancy; components along the MgSiO3–MgAlO2.5 binary system), where two Al cations replace two Si in the B sites and one oxygen vacancy forms to compensate for the excess negative charges. The oxygen vacancies in bridgmanite have special importance for understanding lower mantle processes because oxygen vacancies may induce water (Vo + O2–+ H2O = 2OH–1; Navrotsky, 1999) and noble gases into bridgmanite (Shcheka and Keppler, 2012). Earlier theoretical calculations investigated whether oxygen vacancies exist in bridgmanite (Brodholt, 2000; Yamamoto et al.; 2003; Akber–Knutson and Bukowinski, 2004). Brodholt (2000) suggested that oxygen-vacancy substitution should dominate at pressures only up to 30 GPa. Yamamoto et al. (2003) argued oxygen-vacancy substitution should be unfavourable in the entire lower mantle. Furthermore, energetic calculations by Akber–Knutson and Bukowinski (2004) suggested that oxygen-vacancy substitution could only account for 3–4 % and less than 1 % of Al substitutions in the shallower and deeper parts of the lower mantle, respectively. High pressure and high temperature synthesis (Navrotsky et al., 2003) and nuclear magnetic resonance (NMR) spectroscopy (Stebbins et al., 2003) suggested that oxygen-vacancy substitution dominates in bridgmanite with 5–10 mol. % MgAlO2.5 at 27 GPa and 1873 oK. On the other hand, X-ray diffraction measurements in a laser-heated diamond anvil demonstrated that bridgmanite coexisted with periclase from a glass starting material with a composition of 90 mol. % MgSiO3 and 10 mol. % MgAlO2.5 at 40 ± 10 GPa and 2000–2500 oK (Walter et al., 2006), indicating that the MgAlO2.5 component was not present in bridgmanite and thereby oxygen-vacancy substitution was unfavoured under these conditions. Note that pressure and temperature uncertainties in this study were relatively large, and the composition of bridgmanite was not determined owing to its very small sample size. Thus, it is still uncertain whether MgAlO2.5 can be a dominant component in bridgmanite in the lower mantle.