The effect of ferric iron concentration on lower mantle phase assemblage: Implications for mantle convection

Abstract:

Earth’s mantle connects the atmosphere with the deep interior through convection thus setting up a gradient of oxidation: currently the atmosphere is ~20% oxygen, while the core is mostly metallic iron. As the atmosphere has changed composition throughout Earth history, the redox evolution has proven to be critical to Earth’s evolution and the origin of life. Despite its importance, the redox state of Earth’s mantle is largely an enigma and its effect on composition, phase relations and the physicochemical properties of the mantle phases is controversial. It is expected that oxygen fugacity of the mantle decreases with increasing depth. However, some experiments show an increase in the ferric iron (Fe³⁺) concentration in Mg-silicate perovskite (bridgmanite)—the most abundant mineral in the Earth—with accompanying metallic Fe, and suggest that the trend in iron oxidation state is opposite. In this study, we take two samples of nearly identical bulk composition [enstatite chondrite composition (Javoy, 1995: Mg/Si ~0.77, Fe/Si ~0.26)] that only differ in the relative abundance of Fe³⁺ to the same synthesis conditions in a laser-heated diamond-anvil cell (LHDAC). We find that despite similar starting bulk compositions and synthesis conditions (lower mantle conditions: ~30-90 GPa, 1900-2300 K), different assemblages are synthesized due to differing ferric iron content. For a more reduced (lower Fe³⁺/ΣFe) composition, we find Al₂O₃ forms its own phase separate from the dominant Mg-silicate perovskite phase, in contrast to a more Al-rich Mg-silicate perovskite-dominated assemblage for a more oxidized starting composition. The density of the reduced composition is found to be 1-1.5% denser than the more oxidized sample. This implies that as a single composition is taken deeper in to the mantle, where it is more reducing, may become denser than if the redox state remained constant. This redox-induced density difference could potentially cause sluggish convection due to the increased density in Earth’s lower mantle and lead to a more oxidized upper mantle and more reduced lower mantle.