S.-H. Shim

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

Significance We studied high-pressure and high-temperature phase equilibria using state-of-the-art microanalytical techniques to measure iron partitioning and valence in iron-bearing lower-mantle rocks. Our data allow us to reconcile the discrepancies observed in previous reports, and to propose a comprehensive model of the evolution of iron partitioning in the lower mantle. We reveal an intricate interplay between spin and valence states. Our results suggest a mineral physics basis for rheological variations in the deep lower mantle as a process for stabilizing large thermochemical piles and large low-shear-velocity provinces. 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.

Multiple seismic reflectors in Earth’s lowermost mantle

Significance Deep in the Earth’s interior, the region just above the core–mantle boundary exerts control on mantle convection and heat loss from the core. It has long been thought that the so-called D″ region is separated from a more uniform mantle above by a single interface, often attributed to a phase transition in Mg perovskite. Systematic deep-mantle exploration with massive seismic waveforms now yields evidence for multiple reflectors up to at least 600 km above the core–mantle boundary. Some of the newly discovered interfaces can be explained by postperovskite transitions in differentiated oceanic slab materials, transported from Earth’s surface through deep subduction and convection. The lowermost mantle appears more complex than hitherto thought, and this complexity is not confined to the canonical D″ region. The modern view of Earth’s lowermost mantle considers a D″ region of enhanced (seismologically inferred) heterogeneity bounded by the core–mantle boundary and an interface some 150–300 km above it, with the latter often attributed to the postperovskite phase transition (in MgSiO3). Seismic exploration of Earth’s deep interior suggests, however, that this view needs modification. So-called ScS and SKKS waves, which probe the lowermost mantle from above and below, respectively, reveal multiple reflectors beneath Central America and East Asia, two areas known for subduction of oceanic plates deep into Earth’s mantle. This observation is inconsistent with expectations from a thermal response of a single isochemical postperovskite transition, but some of the newly observed structures can be explained with postperovskite transitions in differentiated slab materials. Our results imply that the lowermost mantle is more complex than hitherto thought and that interfaces and compositional heterogeneity occur beyond the D″ region sensu stricto.