The Role of Redox on Bridgmanite Crystal Chemistry and Calcium Speciation in the Lower Mantle
Neala Creasy, Jennifer Girard, James O. Eckert, Kanani K. M. Lee
Abstract
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.