The Role of Redox on Bridgmanite Crystal Chemistry and Calcium Speciation in the Lower Mantle

Abstract

The amount of ferric iron Fe 3+ in the lower mantle is largely unknown and may be influenced by the disproportionation reaction of ferrous iron Fe 2+ into metallic Fe and Fe 3+ triggered by the formation of bridgmanite. Recent work has shown that Fe 3+ has a strong effect on the density and seismic wave speeds of bridgmanite and the incorporation of impurities such as aluminum. In order to further investigate the effects of ferric iron on mineral behavior at lower mantle conditions, we conducted laser-heated diamond-anvil cell (LHDAC) experiments on two sets of samples nearly identical in composition (an aluminum-rich pyroxenite glass) except for the Fe 3+ content; with one sample with more Fe 3+ ("oxidized": Fe 3+ /ΣFe~55%) and the other with less Fe 3+ ("reduced": Fe 3+ /ΣFe~11%). We heated the samples to lower mantle conditions, and the resulting assemblages were drastically different between the two sets of samples. For the reduced composition, we observed a multiphase assemblage dominated by bridgmanite and calcium perovskite. In contrast, the oxidized material yielded a single phase of Ca-bearing bridgmanite. These Al-rich pyroxenite samples show a difference in density and seismic velocities for these two redox states, where the reduced assemblage is denser than the oxidized assemblage by~1.5% at the bottom of the lower mantle and slower (bulk sound speed) by~2%. Thus, heterogeneities of Fe 3+ content may lead to density and seismic wave speed heterogeneities in Earth's lower mantle. Plain Language Summary Iron primarily exists in three oxidation states within the mantle: metallic iron (Fe 0), ferrous iron Fe 2+ (more reduced form), and ferric iron Fe 3+ (more oxidized form); however, the amount of Fe 3+ in the lower mantle is largely unknown. Recent work has shown that Fe 3+ has a strong effect on rock density and speed at which seismic waves travel through lower mantle minerals. In order to further investigate these effects, we conducted experiments on samples nearly identical in composition except for the Fe 3+ content. We compressed the samples between two diamonds and heated the sample to the conditions of Earth's lower mantle. For the reduced sample (low Fe 3+ content), we observed a complex assemblage of minerals, primarily composed of bridgmanite (the most abundant mineral in the Earth) and calcium silicate perovskite (a common secondary phase in the lower mantle). In contrast, the oxidized sample (high Fe 3+ content) yielded a single phase of these two minerals combined together as one mineral-a Ca-bearing bridgmanite. The resulting oxidized sample is lighter and seismic waves travel faster through it than the reduced sample. Thus, changes in Fe 3+ content may lead to density and seismic wave speed variations in Earth's lower mantle.

Investigating Magma Ocean Solidification on Earth Through Laser‐Heated Diamond Anvil Cell Experiments

“…In the synthesis conditions of our pyrolite glass (2050°C at IW+1.3 in a gas‐mixing aerodynamical levitation laser furnace), our initial ferric iron concentration is ∼30% (Sossi et al., 2020), intermediate between the reduced and oxidized samples studies in Creasy et al. (2020); interestingly, the Ca‐concentration we measure in Brg are also intermediate between those observed in these subsolidus experiments, providing seamless consistency between these observations. Finally, the two experiments at higher pressures (85 and 129 GPa) were carried out (Figure ) for the sole purpose of confirming this observation at all lower mantle pressures.…”

“…Our experiments adds to this corpus of observations; they are fully consistent with crystallization experiments conducted at lower pressure (Nomura et al., 2017) showing significant (between 3 and 5 mol%) CaPv solubility in Brg at 25 GPa. In a pressure range similar to ours, the subsolidus equilibrium between Brg and CaPv (Creasy et al., 2020) shows increasing CaPv dissolution in Brg as the ferric/ferrous iron ratio in bridgmanite increases. In the synthesis conditions of our pyrolite glass (2050°C at IW+1.3 in a gas‐mixing aerodynamical levitation laser furnace), our initial ferric iron concentration is ∼30% (Sossi et al., 2020), intermediate between the reduced and oxidized samples studies in Creasy et al.…”

Solid solution of CaSiO3 and MgSiO3 perovskites in the lower mantle: The role of ferrous iron

“…Thus, based on our model the presence of ferric iron and Al will likely not increase the favorability of phase mixing and our predictions above reflect conditions of maximum solubility. This effect is in contrast to Creasy et al(2020) where an increase in ferric iron was predicted to increase mixing such that no mixing was seen with ferrous iron (as we also predict under their conditions) but mixing was seen with ferric iron. Determining the exact nature of these effects is complicated in that the oxidized sample in this work also had increased total iron (which increases mixing at these low temperatures) and a larger amount of Al in its sample (as higher ferric iron allows more Al to be adsorbed) which changes the dynamics significantly.…”

“…Nevertheless, our study is consistent with the findings of Fujino et al (2004) in that iron can promote the miscibility of two perovskite phases. A comparison can also be made with Creasy et al(2020). In the presence of pure ferrous iron, mixing was not seen in a sample with 27% Ca that was placed under pressure from 35-75 GPa and This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America.…”

“…They found that the solid solubility of Mg in Davemaoite was 4% at ~30 GPa and 1930 K but increased to ~18% at 30 GPa and 1800 K when around 9% iron was added which shows clearly that iron promotes miscibility at least to some degree. In Creasy et al(2020) samples with 27% davemaoite and around 3% ferrous iron in the bridgmanite were not mixed up to 75 GPa and 2300 K which suggests that the solubility is below this point. In both studies there are not enough P, T and Fe% points to explore systematically the effect of iron on the miscibility of the two perovskites.…”