Thermochemical state of the lower mantle: New insights from mineral physics

Abstract

We report recent findings in the field of high-pressure mineral physics with important implications for Earth's lower mantle. We show that the two main constituents of the lower mantle, namely, (Mg,Fe)SiO 3 -magnesium silicate perovskite-and (Mg,Fe)O-ferropericlase-undergo electronic transitions at lower mantle pressures (70 and 120 GPa), in which iron transforms from the high-spin state to the low-spin state. The transformations modify the thermochemical state of Earth's lower mantle. Minerals bearing high-spin iron have characteristic absorption lines in the near-infrared, hindering radiative conductivity at lower-mantle temperatures. These absorption lines shift to the visible (green to violet) range in the low-spin state, and their intrinsic intensities decrease; the minerals thus become increasingly transparent in the near-infrared and their radiative and total thermal conductivities rise. Thus, the heat conductivity of the lowermost mantle could be higher than previously thought. Moreover, the spin-driven partitioning of iron between the two mineral phases can explain large-scale chemical heterogeneities in the mantle that are driven by regional temperature variations. It is noteworthy that the transition pressures correspond to the bottom third of the lower mantle (70 GPa, 1700-km depth), and to the last 300 km above the core-mantle boundary (120 GPa, 2600-km depth); these regions have very special geophysical signatures, as chemical heterogeneities have been reported by seismology in the first case and the bottom 300 km of Earth's mantle constitutes the D layer. Our observations provide a mineral physics basis for these features in Earth's lower mantle.