Collaborative Research: GLOW: CSEDI: Compositional controls on shear localization and the development of plate tectonics on Earth and rocky planets

This proposal will help to answer one of the most important questions in the geosciences: Why plate tectonics operates on Earth. By considering non-Earth-like compositions the project will also constrain the likelihood of plate tectonics developing on other planets, an important goal in astrophysics. Ultimately, this work will help to place the Earth within the broader context of rocky planets in the galaxy. This project will also provide important educational and research opportunities to graduate students at Penn State University and Washington University in St. Louis. Undergraduate students from astronomy and planetary science will study the development of plate tectonics, giving them interdisciplinary training between astronomy and geoscience that is critical for future leaders studying planets beyond our solar system. Plate tectonics requires the formation of narrow zones of weakened rock that act as plate boundaries, separating stable plate interiors. It is within these plate boundaries that most deformation associated with plate movement occurs. However, the physical mechanism(s) allowing localized weak zones to form are not well understood, nor why this behavior is only seen on Earth. One promising mechanism to explain localized deformation, based on multiple lines of evidence, is deformation-induced reduction of the mineral grain sizes and the activation of grain-size sensitive deformation mechanisms. A planet where the mantle is dominantly composed of one mineral may not be able to experience enough phase mixing during deformation for significant grain size reduction. The goal of this proposal is to test whether certain planet compositions, among those inferred for rocky planets both within our solar system and beyond, would preclude the operation of plate tectonics due to inhibited phase mixing, using a combination of laboratory experiments and theoretical models. Motivated by the potential importance of mineral phase mixing in shear localization, and the compositional variety expected for extra solar planets based on compositions of rock forming elements in stars, this project hypothesizes that planet composition, which dictates the mineral makeup of the mantle, exerts a key control on whether plate tectonics can operate. Specifically, mantles that approach 50-50 mixtures of the two dominant mineral phases may be most favorable for plate tectonics, while plate tectonics may be precluded for planets whose mantles are near monominerallic. This hypothesis will be tested with a project integrating new rock deformation experiments with new numerical models of mantle convection to determine which mantle compositions are most favorable for plate tectonics and which, if any, preclude plate tectonics. Experiments will deform relevant materials to high strains to assess the factors that control the rates of phase mixing. Specifically, these experiments will 1) determine how the relative proportions and strength contrast between the dominant mineral phases of mantle rock affects the efficiency of phase mixing and grain size reduction. Numerical models of phase mixing, benchmarked against these experiments, will be used to develop parameterizations of this process to include in mantle convection models. The mantle convection models will then be used to 2) determine how the conditions needed for plate tectonics to develop are modified when the physics of phase mixing is considered; and 3) determine how the volume fractions of, and strength contrast between, primary and secondary mineral phases, affects the conditions for plate tectonics. Finally, the modeling and experimental work will be integrated to 4) assess the likelihood of plate tectonics across the range of proposed mantle compositions for exoplanets. This work will provide new insight into the mineralogical factors that control whether plate tectonics can develop on rocky planets, helping to both explain its operation on Earth and give new constraints on its feasibility on exoplanets. In addition, this project will provide the most complete description of mineral phase mixing, and its dependence on rock composition, to date and be the first to incorporate the physics of mineral phase mixing into mantle convection models of plate tectonics on Earth and other planets.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.