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.
The project was aimed at improving our understanding of Earth's deep mantle in a multidisciplinary approach, specifically pursing the connection between seismology and mineral physics calculations. Our work at ASU focused on investigating the seismological capacity to bear information on important deep mantle mineralogical phenomena, especially the spin transition/crossover that occurs in iron in the deep mantle. The predictions from our mineral physics team co-workers is that there should be some seismological effects that might be detectable. Our work at ASU was devoted to establishment of algorithms that separate the different contributions to seismic anomalies (namely travel time, amplitude, and waveform broadening) -- we worked to understand the contributions from the site location where seismic sensors are placed, versus the path between the earthquake and sensor, versus effects due to structure near the earthquake source. This work was time and labor intensive. It required a collection of a massive data set (essentially all earthquakes recorded by EarthScopes Transportable Array, "TA", and for that time span, all other global seismic station data), and to process them to establish "station anomalies" and "event anomalies": this process was nearly completed. We established that it worked for a subset of the data. It demonstrated that a term for each station and each event (earthquake) can be computed that described the time, amplitude, and waveshape anomaly associated with each. Further work would establish spectral contributions from each. In the alotted time, we had not yet run the algorithms for all events. Also, we established the algorithms for the main P and S wave phases. It would be important to get it working for additional phases, as these would better populate each station and event with increased information, to more confidently establish the station and event terms. After station and event terms are established, then the data can be corrected for them, or "cleaned", and earth structure can be studied. We had established the protocol for our next steps, but the amount of person hours devoted to the project exceeded the 1 year period, and we will seek additional funds for it. We are currently investigating the correction of the data for tomographic models of mantle heterogeneity, with the goal of then combining the global data set to assess any patterns of anomalies with respect to depth, to pursue comparison with predictions for the iron spin crossover, produced by mineral physics. This work demonstrated the feasibility of seismic assessment of subtle property changes with respect to depth, and provided a clear scope of the work, the amount of work, to take it to completion. It can probably be completely in 1.5 years of Phd student time.
