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

The amount of ferric iron Fe3+ in the lower mantle is largely unknown and may be influenced by the disproportionation reaction of ferrous iron Fe2+ into metallic Fe and Fe3+ triggered by the formation of bridgmanite. Recent work has shown that Fe3+ 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 Fe3+ content; with one sample with more Fe3+ (oxidized: Fe3+/Sigma Fe similar to 55%) and the other with less Fe3+ (reduced: Fe3+/Sigma Fe similar to 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 similar to 1.5% at the bottom of the lower mantle and slower (bulk sound speed) by similar to 2%. Thus, heterogeneities of Fe3+ 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 Fe2+ (more reduced form), and ferric iron Fe3+ (more oxidized form); however, the amount of Fe3+ in the lower mantle is largely unknown. Recent work has shown that Fe3+ 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 Fe3+ 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 Fe3+ 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 Fe3+ 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 Fe3+ content may lead to density and seismic wave speed variations in Earth's lower mantle.