This Early Grant for Exploratory Research (EaGER) will support a collaborative study between mineral physicists and seismologists to try a new approach of combining mineral physics and seismology to provide new perspectives of the deep Earth. Advances in seismic imaging of the Earth's deep interior, from global to local scales, are providing structural information about convective and thermal patterns in the lower mantle. Development of first principles methodologies to tackle key mineral physics problems, e.g., thermoelastic properties, spin crossover in iron in lower mantle minerals, and anharmonic thermal properties, are greatly expanding characterization of mineral properties at deep mantle conditions.
The goal and primary intellectual merit of the proposed work is to advance understanding of deep mantle structure, temperature, and composition with a transformative multidisciplinary effort to directly link seismological imaging with modeling approaches based on realistic mineral properties. The team will address an unresolved central issue in investigations of deep mantle temperature and composition: the subtle effects of spin state changes in iron in lower mantle minerals and potential seismological detection of this fundamental transition and corresponding implications for bulk chemistry of the lower mantle. Clarification of the seismic signature of spin state crossovers is a major hurdle to be overcome in mineralogical interpretations of seismological data of the deep Earth. The proposed search for spin transition signatures in the deep mantle is not without risks, but this unprecedented joint seismology/mineral-physics enterprise will pave the way for future studies of numerous fascinating structures holding keys to the nature of the deep mantle. It will open a needed first-hand dialogue between these communities and enable elasticity data to be accessible online for use by the seismology community for modeling purposes. The PIs of this study have diverse expertise that will help the team to foster a multi-disciplinary education experience at the interface between seismology and mineral physics.
This 1-year seed project involved collaboration between faculty and graduate students at the University of California Santa Cruz, Arizona State University, and the University of Minnesota. The focus of our collaborative research is to use seismological and mineral physics methods to establish whether iron (Fe) in lower mantle minerals (Mg0.9Fe0.1SiO3 perovskite or Mg0.9Fe0.1O ferropericlase) undergoes detectable transition from high-spin state to low-spin state as pressure increases across the lower mantle. Spin-transition of Fe has important implications for both the elasticity of the minerals (low-spin Fe is more compact than high-spin and thus the mineral forms would have different elastic properties if there is a high-spin to low-spin transition; this is predicted to affect the speed at which seismic waves travel through the mantle) and transport properties such as thermal conduction, electric conductivity, and viscosity. Theoretical calculations at the University of Minnesota and separate laboratory experiments for lower mantle minerals under high pressure and high temperature predict that high-spin to low-spin transition will occur in both perovskite and ferropericlase, but at different depth (pressure) ranges, and that the existence of intermediate states and sluggishness of the transition will cause the properties to change slowly over significant depth ranges rather than as abrupt discontinuous changes. This makes seismological detection of associated effects challenging, as gradients of structure with depth must be resolved without having localized reflections, triplications or other indicators of structural variation being produced by the spin transition. At UCSC we have explored global tomographic seismic model comparisons to bound the possible elastic manifestations of Fe spin transition at the depth ranges targeted by the mineral physics results, but find that existing models are not reliable enough for relative behavior of P-wave and S-wave velocity to provide a confident diagnostic. The ab initio mineral physics calculations at the Unversity of Minnesota indicate that the relative behavior of P-wave and S-wave velocity may be the best diagnostic of the presence of the spin transition. However, many global seismic tomography models that determine both P-wave and S-wave velocity appear to have assumptions about their relative behavior (thermal derivatives, or other assumptions that couple the two models in order to stabilize the tomography inversions). This can easily suppress the effects of any actual spin-transition in the mantle for specific regions of the global tomoraphy models. This motivates a large data collection process to refine localized depth-gradients of P-wave and S-wave velocity velocity structure (being conducted at Arizona State University), with the goal of improving confidence in depth-variations across the lower mantle within relatively uniform thermal regimes. At UCSC research also included development of mantle thermal models from first-principles theory incorporating spin-transitions. The results were assessed in terms of the weak deviations from adiabaticity associated with seismic characteristics of the lower mantle. The results hold the prospect that based on a coupled thermal/seismic argument, it should be possible to bound the amount of ferropericlase in the lower mantle, although there are trade-offs with the mildly uncertain iron content of ferropericlase in the deep mantle. The progress in understanding this problem guides us to future collaborative work, with specific new seismological measurements that have better prospect of resolving the gradients in structure that might be affected by spin-transition, and then using improved thermal-model prediction of the elasticity effects to bound volumes of material that may undergo spin-transition with depth.
