Spin and valence dependence of iron partitioning in Earth’s deep mantle

Hélène Piet (1) , James Badro (1, 2) , Farhang Nabiei (1) , Teresa Dennenwaldt (1) , Sang-Heon Shim (3) , Marco Cantoni (1) , Cécile Hébert (1) , Philippe Gillet (1)

1 EPFL - Ecole Polytechnique Fédérale de Lausanne
2 IPGP - Institut de Physique du Globe de Paris
3 ASU - Arizona State University [Tempe]

We performed laser-heated diamond anvil cell experiments combined with state-of-the-art electron microanalysis (focused ion beam and aberration-corrected transmission electron microscopy) to study the distribution and valence of iron in Earth's lower mantle as a function of depth and composition. Our data reconcile the apparently discrepant existing dataset, by clarifying the effects of spin (high/low) and valence (ferrous/ferric) states on iron partitioning in the deep mantle. In aluminum-bearing compositions relevant to Earth's mantle, iron concentration in silicates drops above 70 GPa before increasing up to 110 GPa with a minimum at 85 GPa; it then dramatically drops in the postperovskite stability field above 116 GPa. This compositional variation should strengthen the lowermost mantle between 1,800 km depth and 2,000 km depth, and weaken it between 2,000 km depth and the D" layer. The succession of layers could dynamically decouple the mantle above 2,000 km from the lowermost mantle, and provide a rheological basis for the stabilization and nonentrainment of large low-shear-velocity provinces below that depth.

Keywords: iron partitioning; lower mantle; spin state; | valence state; viscosity

Domains: Geochemistry

Hélène Piet, James Badro, Farhang Nabiei, Teresa Dennenwaldt, Sang-Heon Shim, et al.. Spin and valence dependence of iron partitioning in Earth’s deep mantle. Proceedings of the National Academy of Sciences of the United States of America, 2016, 113 (40), pp.11127-11130. ⟨10.1073/pnas.1605290113⟩. ⟨insu-02135706⟩