Valence and spin states of iron are invisible in Earth’s lower mantle

• Published in: Nature communications Vol. 9; no. 1; pp. 1284 - 9
• Main Authors: Liu, Jiachao, Dorfman, Susannah M., Zhu, Feng, Li, Jie, Wang, Yonggang, Zhang, Dongzhou, Xiao, Yuming, Bi, Wenli, Alp, E. Ercan
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 29.03.2018; Nature Publishing Group; Nature Portfolio
• Subjects: 147/28; 704/2151/2809; 704/2151/330; Acoustic velocity; Bulk density; Compressibility; Conductivity; Density; Earth mantle; Electrical conductivity; Electrical resistivity; Environmental science; Geophysics; GEOSCIENCES; Heterogeneity; Humanities and Social Sciences; Iron; Laboratories; Lower mantle; Mineralogy; multidisciplinary; Phase transitions; Physical properties; Science; Science (multidisciplinary); Spin transition; Viscosity
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/s41467-018-03671-5

Heterogeneity in Earth’s mantle is a record of chemical and dynamic processes over Earth’s history. The geophysical signatures of heterogeneity can only be interpreted with quantitative constraints on effects of major elements such as iron on physical properties including density, compressibility, and electrical conductivity. However, deconvolution of the effects of multiple valence and spin states of iron in bridgmanite (Bdg), the most abundant mineral in the lower mantle, has been challenging. Here we show through a study of a ferric-iron-only (Mg 0.46 Fe 3+ 0.53 )(Si 0.49 Fe 3+ 0.51 )O 3 Bdg that Fe 3+ in the octahedral site undergoes a spin transition between 43 and 53 GPa at 300 K. The resolved effects of the spin transition on density, bulk sound velocity, and electrical conductivity are smaller than previous estimations, consistent with the smooth depth profiles from geophysical observations. For likely mantle compositions, the valence state of iron has minor effects on density and sound velocities relative to major cation composition. Bridgmanite is the most abundant mineral in the lower mantle and therefore is crucial to interpreting geophysical observations and models. Here, the authors show that ferric-iron-only bridgmanite Fe 3+ undergoes a spin transition at 43–53 GPa at 300 K and therefore has implications for mantle structure and dynamics.