Although the composition of Earth’s lower mantle is still poorly constrained, bridgmanite (Mg,Fe)(Si,Al)O3, formed from both the primary pyrolite mineral ringwoodite and the secondary mineral majorite garnet in the transition zone, is widely thought to be the dominant phase of this region. Bridgmanite dominated by the MgSiO3 component is stable over a wide range of depths from 660 km to several hundred kilometers above the core-mantle boundary (~ 2700 km) and as such its physical properties are primarily responsible for the seismic and transport properties of this region.
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1.4.1 Proportions of bridgmanite in different bulk compositions
As described in section 1.3.1 and 1.3.2, the proportions of bridgmanite and other lower mantle minerals depend on the assumed composition of the mantle. For a most exhaustive comparison, average compositions reported in the literature for different chondritic meteorites (Wasson and Kallemeyn, 1988) have been used to derive mineral proportions at conditions of the lower mantle. To this end, only the major elements of the chondritic compositions have been considered and the following assumptions have been used to calculate the oxides wt.% reported in Table 1.3 following a minimization procedure: (1) highly volatile elements like C, H, N as well as the moderately volatile S were neglected; (2) The core was assumed to consist exclusively of Fe and Ni, with the total amount of Ni partitioning completely into the core; (3) The Fe content of the mantle has been fixed at 6.2 atomic wt.% according to the value accepted for the upper mantle of the Earth. This resulted in a value of 7.98 wt.% of FeO for all chondritic compositions; (4) the oxygen content was calculated in order to obtain a final mantle composition expressed in oxide wt.% (Table 1.3) close to 100%. From these compositions it was then possible to calculate the mineral proportions; partitioning all elements between bridgmanite and ferropericlse according to the experimental compositions reported by Irifune et al. (2010) at 36.4 GPa and 1973 K, and assuming both CaSiO3 perovskite and stishovite SiO2 to be pure end-members (Table 1.3). Although these calculations are only a crude approximation, the trend obtained for different chondrite compositions shows that bridgmanite is always the most abundant phase in the lower mantle, that CaSiO3 perovskite amount is pretty constant among all chondritic compositions. Only Carbonaceous chondrites would form ferriopericlase in the lower mantle whereas enstatite chondrites produce a lower mantle containing an excess SiO2 phase. A number of studies have proposed such meteorite based mantle models for the bulk silicate Earth (Fitoussi et al., 2016; Javoy et al., 2010). If suitable mineral models and elasticity data were available, the extent to which these different mantle compositions fit lower mantle seismic velocities could be evaluated.
28 Table 1.3 Calculated mantle compositions, lower mantle mineral proportions for different chondrite compositions. The lower mantle mineral proportion calculated from pyrolite composition from McDonough and Sun (1995) is also shown for comparison.
Pyrolite
MgSiO3 bridgmanite has a perovskite-type structure with an orthorhombic distortion and space group Pbnm (Horiuchi et al., 1987). It consists of a three-dimensional framework of tilted corner-linked SiO6 octahedra (B site) forming cavities in the shape of bicapped trigonal prisms (A site) occupied by Mg. This orthorhombic structure derives from an ideal cubic structure (Space group Pm3̅m) through in-phase and out-of-phase tilting of the SiO6
octahedra in addition to cation displacements at the A site due to the relatively small Mg2+
ion (e.g. Glazer, 1972; Howard and Stokes, 1998, 2005) (see Fig. 1.7). Compared with the aristotype structure, four of the Mg-O bonds are lengthened and the remaining eight are
29 shortened (Fiquet et al., 2000; Ross and Hazen, 1990). This distortion increases with pressure and eventually leads to a phase transition into “post-perovskite” above 120 GPa (Murakami et al., 2004).
Figure 1.7 Structural model of MgSiO3 bridgmanite consisting of two cation sites. The A site is occupied by Mg (orange) and the octahedral B site is occupied by Si (blue). The orthorhombic distortion of the perovskite-like structure is due to the in-phase and out-of-phase tilting of the octahedral framework.
1.4.3 Composition of bridgmanite
High-pressure and high-temperature experiments indicate that bridgmanite can accommodate a substantial amount of Fe and Al (Frost and Langenhorst, 2002; Lauterbach et al., 2000). For a pyrolitic mantle composition, bridgmanite incorporates most of the Al and much of the Fe (Irifune, 1994; Irifune et al., 2010), with a nominal FeO content of ~ 6-7 wt.% and Al2O3 content of ~ 4-5 wt.% at the top lower mantle. The Al2O3 content increases gradually from 1 wt.% to 4-5 wt.% over the first 50 km of the lower mantle due to the transformation of majoritic garnet, the main host for Al in the upper mantle and transition zone. The Fe content will also vary as a result of the effect of pressure on the partitioning of Fe between bridgmanite and coexisting ferropericlase, which may be further influenced by an electronic spin transition of FeO in ferropericlase at pressures between 70-125 GPa (Lin
30 et al., 2013; Mao et al., 2011). Harzburgite has a lower Al content than pyrolite, due to melt extraction beneath ridges, thus bridgmanite in harzburgitic compositions has an Al2O3
content of only 1-2 wt.%; on the contrary, MORB is enriched in Al while depleted in Mg, thus bridgmanite in a MORB composition has an extremely high Al2O3 and Fe2O3 content of ~ 16 wt.% and ~ 23 wt.%, respectively (Hirose et al., 1999; Hirose et al., 2005; Irifune and Ringwood, 1987b). Therefore, although bridgmanite in Earth’s lower mantle has a nominal Al and Fe content of ~ 0.1 atoms per formula unit (pfu), such content may vary between 0 to 0.40 atoms pfu due to the presence of chemical heterogeneities that may arise either from the subduction of oceanic lithosphere or the presence of primordial material (Kellogg et al., 1999). Cation substitutions in bridgmanite may strongly influence the behavior of MgSiO3
bridgmanite such as density and elastic properties in Earth’s lower mantle.
1.4.4 Substitution mechanisms in bridgmanite
Although iron exists predominantly as Fe2+ in the upper mantle and transition zone, significant amounts of Fe3+ have been identified in bridgmanite (Fe3+/ΣFe= ~ 16% without Al and ~ 50-75% with Al), even when in equilibrium with Fe metal (Frost et al., 2004;
Lauterbach et al., 2000; McCammon, 1997). The abundance of Fe3+ appears to depend on the Al content of bridgmanite and arises from the unusual crystal chemistry of silicate perovskite (McCammon, 2005). The higher Fe3+ content seemingly required in the lower mantle relative to the upper mantle could be explained in an isochemical mantle if the disproportionation of Fe2+ (2Fe2+→Fe3++Fe0) occurs. This expansion in the stabilization of Fe3+ is induced by the coupled substitution in bridgmanite of Fe3+ + Al3+ replacing Mg + Si.
This results in a rather complex crystal chemistry of lower mantle bridgmanite as a result of the different possible substitution mechanisms.
Ferrous iron, Fe2+, incorporates into the bridgmanite structure through the substitution of Mg2+ on the A site creating compositions along the MgSiO3-FeSiO3 join (Andrault et al., 1998; Lauterbach et al., 2000; McCammon et al., 1992). For the incorporation of trivalent cations M3+ (M3+=Fe3+ or Al3+) into bridgmanite at least two mechanisms need to be taken into account. One is a charge-coupled substitution (CCS) forming compositions along the
31 MgSiO3-M2O3 join:
MgMgX + SiSiX → MMg∙ + MSi′ (1.1) where two trivalent cations substitute for Mg at the A site and Si at the B site, maintaining electrical neutrality without the formation of vacancies (here the superscripts ˙ and
‘ represent a positive or negative net charge on the site, respectively and X indicates no charge). This reaction is analogous to the substitution of aluminum for tetrahedral silicon and octahedral magnesium along the diopside-Ca-Tschermak’s pyroxene join, CaMgSi2O6 -CaAlAlSiO6, therefore it also is referred to as stoichiometric or Tschermakitic substitution (Navrotsky et al., 2003). Although Fe3+ and Al may occupy both cation sites in bridgmanite, there is some evidence that in the presence of Al, Fe3+ preferentially occupy the A site (McCammon et al., 2013).
The other mechanism is oxygen vacancy substitution (OVS) along the MgSiO3-MgMO2.5
join
2SiSiX → 2MSi′ + VO∙∙ (1.2) where trivalent cations only replace Si4+ at the B site and oxygen vacancies are required for charge balance (VO∙∙ is an oxygen vacancy). This mechanism is analogous to the formation of defect perovskites along the CaTiO3 pervoskite-CaFeO2.5 brownmillerite join (Becerro et al., 1999), thus it also is referred to as nonstoichiometric, defect, or brownmilleritic substitution.
Most low-pressure ceramic perovskites incorporate trivalent cations according to this second mechanism (Navrotsky, 1999) and this also may be an important mechanism in the Earth’s lower mantle, which contains ferropericlase coexisting with bridgmanite and thus buffers low SiO2 activity favouring Mg/Si>1. The oxygen vacancies created by this substitution mechanism are of particular interest since they may provide a way of incorporating water into the dense structure of bridgmanite (Murakami et al., 2002;
Navrotsky et al., 2003) andmay affect the diffusivity, conductivity, compressibility and creep rate of the mantle.
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1.4.5 Spin transition of bridgmanite
Fe2+ and Fe3+ in bridgmanite exist in the high-spin state under ambient conditions (e.g.
McCammon, 2006). For Fe2+, it is still under debate whether the abnormal increase in quadrupole splitting of Fe2+ at the A site of bridgmanite at high pressures is related to a high-spin (HS) to intermediate-spin (IS) transition or enhanced lattice distortion of the A site (Hsu et al., 2010; Lin et al., 2012; Lin et al., 2013; McCammon et al., 2008). On the other hand, there is general consensus that Fe3+ at the A site remains in the high-spin state to at least 100 GPa (Catalli et al., 2011; 2010; Fujino et al., 2012; Glazyrin et al., 2014; Kupenko et al., 2014; Lin et al., 2012; Potapkin et al., 2013) and only Fe3+ at the B site undergoes a spin crossover from high-spin to low-spin at about 30 to 60 GPa (Catalli et al., 2011; 2010; Fujino et al., 2012; Kupenko et al., 2015; Lin et al., 2012; Liu et al., 2018a; Mao et al., 2015). The spin transition pressure of Fe3+ is positively related to the Fe3+ content. Knowledge of the Fe3+ distribution among the two structural sites of the bridgmanite structure is therefore important in order to assess whether its spin state may influence the elastic properties of bridgmanite or whether it may be invisible to seismic observations.