Valence and spin states of iron are invisible in Earth\u2019s lower mantle

Jiachao Liu,Jie Li,Yuming Xiao,E Ercan Alp,Yonggang Wang,Susannah M Dorfman,Feng Zhu,Dongzhou Zhang,Wenli Bi

doi:10.1038/s41467-018-03671-5

Abstract

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 (Mg0.46Fe3+0.53)(Si0.49Fe3+0.51)O3 Bdg that Fe3+ 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.

Highlights

Heterogeneity in Earth’s mantle is a record of chemical and dynamic processes over Earth’s history
Were identified as Purely ferric Bdg woirththoFreh3+omebviecnlGy ddFiestOri3b-utytepde between the A- and B-sites is ideal for studying the spin transition of Fe3+ because variations of its density, spin moment, hyperfine parameters, and electrical conductivity with respect to pressure are not influenced by Fe2+ or cation exchange
These values are consistent with the hyperfine parameters for Fe3+ of Bdg derived from synchrotron-based energy-domain Mössbauer spectroscopy[34,35]

Summary

Introduction

Heterogeneity in Earth’s mantle is a record of chemical and dynamic processes over Earth’s history. The compositions of Bdg synthesized in laser heated diamond anvil cells (DACs) are, in general, not well-controlled due to unknown oxygen fugacity, inhomogeneity in micron-scale starting materials, and cation migration by Soret diffusion at high temperatures Such uncertainties in chemistry hamper the investigation of the effects of Fe3+/∑Fe on thermoelastic and electrical properties of Bdg. Pressure-driven electronic spin-pairing transitions of iron could further distinguish oxidized from reduced Bdg. Highpressure experimental and theoretical studies have concluded that Fe3+ in the octahedral B-site of Bdg undergoes a high spin (HS) to low spin (LS) transition under lower mantle pressure–temperature (P–T) conditions Highpressure experimental and theoretical studies have concluded that Fe3+ in the octahedral B-site of Bdg undergoes a high spin (HS) to low spin (LS) transition under lower mantle pressure–temperature (P–T) conditions The spin transition is only likely to influence the thermoelastic and transport properties of Bdg with Fe3+ in the B-site

Methods

Results

Conclusion