Composition dependence of spin transition in (Mg,Fe)SiO₃bridgmanite

Abstract

Spin transitions in (Mg,Fe)SiO 3 bridgmanite have important implications for the chemistry and dynamics of Earth's lower mantle, but have been complex to characterize in experiments. We examine the spin state of Fe in highly Fe-enriched bridgmanite synthesized from enstatites with measured compositions (Mg 0.61 Fe 0.38 Ca 0.01 )SiO 3 and (Mg 0.25 Fe 0.74 Ca 0.01 )SiO 3 . Bridgmanite was synthesized at 78-88 GPa and 1800-2400 K and X-ray emission spectra were measured on decompression to 1 bar (both compositions) and compression to 126 GPa [(Mg 0.61 Fe 0.38 Ca 0.01 )SiO 3 only] without additional laser heating. Observed spectra confirm that Fe in these bridgmanites is dominantly high spin in the lower mantle. However, the total spin moment begins to decrease at ~50 GPa in the 74% FeSiO 3 composition. These results support density functional theory predictions of a lower spin transition pressure in highly Fe-enriched bridgmanite and potentially explain the high solubility of FeSiO 3 in bridgmanite at pressures corresponding to Earth's deep lower mantle.

Optical signatures of low spin Fe3+ in NAL at high pressure

“…Pressure-induced spin transitions have now been identified in multiple minerals that may be present in the lower mantle [Badro, 2014;Lin et al, 2013]. However, the mechanism, pressure, and compositional dependence of the spin state of bridgmanite (Bdgm), the most abundant lower mantle phase, remains controversial [Badro et al, 2004;Catalli et al, 2010Catalli et al, , 2011Dorfman et al, 2015;Kupenko et al, 2015;Li et al, 2004;Lin et al, 2012;McCammon et al, 2010;Mohn and Trønnes, 2016;Zhang and Oganov, 2006]. The main reason for this controversy arises from its crystal chemical complexity, in which the iron can be distributed between two nonequivalent crystallographic positions: large pseudododecahedral (A) sites and relatively small octahedral (B) sites [McCammon, 1997].…”

“…Mőssbauer spectroscopy (traditional or synchrotron) can be used to infer the iron oxidation/spin states as well as the site occupancies, but the relation of hyperfine parameters to various iron states is not straightforward, and as a result, the interpretation of Mőssbauer spectra is generally inconclusive [Dyar et al, 2006]. Combined XES and Mössbauer studies helped to partially resolve the problem [Catalli et al, 2010[Catalli et al, , 2011Dorfman et al, 2015;Lin et al, 2016] but are still inefficient in probing a broad range of chemical compositions because they require extraordinary access to synchrotron X-ray sources.…”

Abnormal Elasticity of Fe‐Bearing Bridgmanite in the Earth's Lower Mantle

“…Solid black lines are modeled velocity profiles of Run 2 using thermoelastic equations. In contrast, both Fe 3+ and Fe 2+ in the A site have been reported to remain in the HS state throughout the entire lower-mantle pressure range (24-130 GPa) (Dorfman et al, 2015;Hsu et al, 2010Hsu et al, , 2011Lin et al, 2016). In contrast, both Fe 3+ and Fe 2+ in the A site have been reported to remain in the HS state throughout the entire lower-mantle pressure range (24-130 GPa) (Dorfman et al, 2015;Hsu et al, 2010Hsu et al, , 2011Lin et al, 2016).…”

“…Earlier studies have indicated that drastic softening of K S and V P could occur across the spin transition of Fe in lower-mantle Fe-bearing minerals, such as ferropericlase, magnesiosiderite, and the new hexagonal aluminous phase Lin et al, 2013;Wu et al, 2013Wu et al, , 2016Yang et al, 2015). Similarly, but distinct from the aforementioned Fe-bearing minerals, the spin and valence transitions of Fe in bridgmanite have been much debated due to its complicated crystal chemistry as well as the coupled substitution possibility of Al and Fe in crystallographic sites of bridgmanite (Catalli et al, 2010(Catalli et al, , 2011Dorfman et al, 2015;Hsu et al, 2011;Lin et al, 2016;Mao et al, 2015). The current consensus is that both ferrous (Fe 2+ ) and ferric (Fe 3+ ) iron can be incorporated into bridgmanite, where Fe 3+ can occupy both the large dodecahedral site (A site) and octahedral site (B site), while Fe 2+ only occupies the A site Mao et al, 2015;Shukla et al, 2016).…”

“…Geophysical Research Letters in bridgmanite with composition (Mg 0.9 Fe 0.1 )SiO 3 containing 20% Fe 3+ (Mao et al, 2015), and recent experimental study observed 1.9% volume decrease within the spin transition of B-site Fe 3+ in ferric-iron-only bridgmanite (Mg 0.46 Fe 3+ 0.53 )(Fe 3+ 0.51 Si 0.49 )O 3 (Liu et al, 2018). In contrast, both Fe 3+ and Fe 2+ in the A site have been reported to remain in the HS state throughout the entire lower-mantle pressure range (24-130 GPa) (Dorfman et al, 2015;Hsu et al, 2010Hsu et al, , 2011Lin et al, 2016). Additionally, A-site Fe 2+ will undergo an enhanced lattice distortion at~18-30 GPa, which results in extremely high quadrupole splitting (Catalli et al, 2010(Catalli et al, , 2011Lin et al, 2012;McCammon et al, 2010;Narygina et al, 2010).…”