The high-pressure electronic spin transition in iron: Potential impacts upon mantle mixing

Abstract

[1] At the Rayleigh number appropriate to Earth's mantle, radial heat transport is dominated by solid state thermal convection. Because of the large number of physical properties required to determine the Rayleigh number, and because these properties are expected to be (perhaps strong) functions of pressure and temperature (P-T), laboratory measurements of them under the high pressure and temperature conditions that occur in the deep Earth are of fundamental importance. Recent experimental data demonstrate that an electronic spin transition in iron that occurs at midmantle depths results in significant changes in the physical properties of the ferropericlase component of mantle mineralogy. Additional recent results suggest that it may also exist in the dominant perovskite component. Using control volume based numerical models we investigate the impacts on mantle mixing of this spin transition through its influence on the most important subset of these physical properties, namely density, thermal expansivity, bulk modulus and heat capacity. Our numerical model results demonstrate that this electronic transition enhances mixing in the lower regions of the lower mantle by enhancing the vigor of rising plumes. The lowermost region of the mantle is slightly warmed and the upper mantle slightly cooled by spin-induced effects. However, the spin crossover in the lower mantle appears not to significantly influence mantle layering. Due to the competition that could exist between the strength of the spin-induced thermodynamic properties of ferropericlase and perovskite, cold descending thermal anomalies could stagnate at middle-to-lower mantle depths and lead to the occurrence of "mid mantle avalanches."

Iron-spin transition controls structure and stability of LLSVPs in the lower mantle

“…These signatures have been widely reported in previous seismic tomography studies such as the disruption of P wave image below some hot spots (Zhao, 2007), the global disruption of P wave structure (van der Hilst and Kárason, 1999) at ∼1700 km depth, anti-correlation between shear velocity and bulk sound velocity around middle lower mantle (Simmons et al, 2010), and discrepancy between P and S wave model at the origin depth of thermal plumes (Boschi et al, 2007). Here we further found that iron-spin transition may control the formation of LLSVPs, a signature of the spin transition from geodynamic aspect.…”

“…T is temperature perturbation; p is dynamic pressure; K T is isothermal bulk modulus. When iron-spin transition occurs, the new ρ can be written in the form that is similar to Shahnas et al (2011):…”

“…The anomalous thermodynamics properties of ferropericlase caused by spin transition ) inspire investigations of their effects on mantle convection. It has been shown that iron-spin transition enhances the vigor of plumes in numerical simulations of purely thermal mantle convection (Bower et al, 2009;Shahnas et al, 2011). However, the influence of iron-spin transition on thermochemical convection has not been discussed yet.…”

Spin crossover in ferropericlase and velocity heterogeneities in the lower mantle

“…For comparison, the spin crossover in Fp increases the density of a chemically homogeneous pyrolitic mantle by ∼0.7%. It is a smooth and broad feature that should enhance convection in a chemically uniform mantle (22)(23)(24), especially in the spin crossover zone, which spans most of the lower mantle. This broad spin crossover also seems consistent with the lack of seismological evidence of a compositional transition and associated thermal boundary layers (39,40) in the mid-lower mantle.…”

“…Therefore, its effects on aggregates are more elusive and indirect. For instance, the associated density anomaly can invigorate convection, as demonstrated by geodynamics simulations in a homogeneous mantle (22)(23)(24). The bulk modulus anomaly may decrease creep activation parameters and lower mantle viscosity (10, 24, 25) promoting mantle homogenization in the spin crossover region (24), and anomalies in elastic coefficients can enhance anisotropy in the lower mantle (16).…”