Synthesis, Elasticity, and Spin State of an Intermediate MgSiO₃‐FeAlO₃ Bridgmanite: Implications for Iron in Earth's Lower Mantle

Abstract

Fe‐Al‐bearing bridgmanite may be the dominant host for ferric iron in Earth's lower mantle. Here we report the synthesis of (Mg0.5Fe3+0.5)(Al0.5Si0.5)O3 bridgmanite (FA50) with the highest Fe3+‐Al3+ coupled substitution known to date. X‐ray diffraction measurements showed that at ambient conditions, the FA50 adopted the LiNbO3 structure. Upon compression at room temperature to 18 GPa, it transformed back into the bridgmanite structure, which remained stable up to 102 GPa and 2,600 K. Fitting Birch‐Murnaghan equation of state of FA50 bridgmanite yields V0 = 172.1(4) Å3, K0 = 229(4) GPa with K0′ = 4(fixed). The calculated bulk sound velocity of the FA50 bridgmanite is ~7.7% lower than MgSiO3 bridgmanite, mainly because the presence of ferric iron increases the unit‐cell mass by 15.5%. This difference likely represents the upper limit of sound velocity anomaly introduced by Fe3+‐Al3+ substitution. X‐ray emission and synchrotron Mössbauer spectroscopy measurements showed that after laser annealing, ~6% of Fe3+ cations exchanged with Al3+ and underwent the high‐ to low‐spin transition at 59 GPa. The low‐spin proportion of Fe3+ increased gradually with pressure and reached 17–31% at 80 GPa. Since the cation exchange and spin transition in this Fe3+‐Al3+‐enriched bridgmanite do not cause resolvable unit‐cell volume reduction, and the increase of low‐spin Fe3+ fraction with pressure occurs gradually, the spin transition would not produce a distinct seismic signature in the lower mantle. However, it may influence iron partitioning and isotopic fractionation, thus introducing chemical heterogeneity in the lower mantle.

Formation of large low shear velocity provinces through the decomposition of oxidized mantle

“…The Δ V HS-LS discrepancy between experimental and theoretical studies is probably caused by the difference in the pressure range for the mixed-spin (MS) state. The predicted volumes of (Mg 0.5 Fe 0.5 )(Si 0.5 Al 0.5 )O 3 Bdg also show excellent agreement with experimental results at 300 K 42 . These comparisons demonstrate the high reliability of our DFT + U calculations in predicting elastic properties, as suggested by previous studies 43 – 45 .…”

“…The blue, green, orange, and red curves are calculated compression curves of (Mg 0.5 Fe 0.5 )(Si 0.5 Fe 0.5 )O 3 and (Mg 0.5 Fe 0.5 )(Si 0.5 Al 0.5 )O 3 bridgmanite at 300, 1000, 2000, and 3000 K, respectively. The blue squares are experimental measurements from Liu et al 36 , which shows that the spin transition of Fe 3+ in the B-site of (Mg 0.5 Fe 0.5 )(Si 0.5 Fe 0.5 )O 3 bridgmanite occurs between 43 and 53 GPa at 300 K. The blue circles are experimental results of Zhu et al 42 . …”

“…The elastic properties of (Mg 0.5 Fe 0.5 )(Si 0.5 Fe 0.5 )O 3 Bdg up to 130 GPa and 3000 K were theoretically obtained from ab initio calculations. Because Bdg may also accommodate Al in the octahedral site 31 , 38 , 42 , we also conducted calculations on an end-member composition (Mg 0.5 Fe 0.5 )(Si 0.5 Al 0.5 )O 3 , to quantify the effect of Al on the elasticity of Bdg. In these calculations, the octahedral site (B-site) Fe 3+ in (Mg 0.5 Fe 0.5 )(Si 0.5 Fe 0.5 )O 3 Bdg undergoes a high-spin (HS) state to a low-spin (LS) state transition with increasing pressure (Fig.…”

Low-spin ferric iron in primordial bridgmanite crystallized from a deep magma ocean

“…We view our results for MAOS3265 bridgmanite in the following manner: when a single-crystal sample is used, i.e., under chemical composition-fixed conditions, the cation exchange reaction Eq. ( 1 ) is required for Fe 3+ to be incorporated into the B site, but the reaction’s large kinetic barrier 20 , 21 , 25 , 42 hampers achieving the equilibrated site configuration and thus the accommodation of LS Fe 3+ even with further pressurization and annealing; on the other hand, the chemical composition and cation configuration of the DACMSRd01 sample were equilibrated since a direct synthesis of bridgmanite in a DAC at high pressure where Fe 3+ is stable in the LS state does not require the cation exchange in Eq. ( 1 ) to accommodate LS Fe 3+ , which is consistent with the large LS fraction of Fe in bridgmanite formed in only a DAC at high pressure 25 .…”

“…One way to interpret this contradiction is by attributing the invisible spin transition 13 – 18 to the difficulty of achieving equilibrium in the site distribution of Fe 3+ due to the large kinetic barrier of cation exchange 20 – 22 , 25 described below, which is suggested to be a plausible process for bridgmanite containing low spin (LS) Fe 3+ to be generated from initial high spin (HS) 20 . …”