A Pyrolitic Lower Mantle with (Mg,Fe3+)(Si,Al3+)O3 Bridgmanite

To better understand the Earth’s lower mantle (LM), thermodynamic properties (TDPs) of LM minerals should be illustrated clearly. We have so far reported the TDPs of Fe (and Al)-bearing MgO, MgSiO₃ bridgmanite (Br) and post bridgmanite [1-5] by using the internally consistent LSDA+U method and the lattice dynamics method. In this work, two spin states, the high (HS) and low spin (LS) state, and several possible distribution configurations are considered in the LM pressure range. For Fe incorporated in Br, only Fe³⁺ at the Si site undergoes a HS to LS transition. However, this is suppressed by Al incorporation, because Al³⁺ prefers the Si site and attracts HS Fe³⁺ at the adjacent Mg site forming Fe³⁺-Al³⁺ pair. Br with geophysically relevant 6.25 mol% Fe²⁺ or Fe³⁺-Al³⁺ pair is found vibrationally stable. Incorporation of these elements increases the Br volume a little but gives marginal effects on the TDPs.

Simulated densities, adiabatic bulk moduli, and bulk sound velocities of possible LM mineral aggregations show that a composition close to pyrolite with (Mg,Fe³⁺)(Si,Al³⁺)O₃ Br is accountable for the reference Earth model, while Fe²⁺-bearing Br instead gives unignorable disagreements in deeper part. Neither Si-richer nor Si-poorer composition improves the disagreements. This indicates that Fe in LM bridgmanite should predominantly be ferric acquiring the HS state, and pyrolitic composition with (Mg,Fe³⁺)(Si,Al³⁺)O₃ Br is a reasonable LM model.

References:

[1] A. Metsue, and T. Tsuchiya, J. Geophys. Res. 116, B08207 (2011).

[2] A. Metsue, and T. Tsuchiya, Geophys. J. Int. 190, 310 (2012).

[3] H. Fukui, T. Tsuchiya, and A. Q. R. Baron, J. Geophys. Res. 117, B12202 (2012).

[4] T. Tsuchiya, and X. Wang, J. Geophys. 118, 83 (2013).

[5] X. Wang, and T. Tsuchiya, under reviewing.