CAREER: Testing models of early Earth crust formation and tectonics

Pennsylvania State Univ University Park, University Park PA

Investigators

Abstract

Plate tectonics is the fundamental process governing Earth’s dynamics, including natural hazards such as earthquakes and volcanoes. Plate tectonics is a result of convection in Earth’s mantle. However, why mantle convection results in plate tectonics on Earth, but not the other rocky planets in the solar system that also have convecting mantles, is not well understood. Likewise, it is also not well known when plate tectonics started on Earth. The goal of this proposal is to use numerical models of mantle convection and the formation of Earth’s early crust to constrain when plate tectonics might have started. Observations of the chemical composition of rocks formed 3-4 Gyrs ago provide key constraints on the tectonic processes operating at this time. The investigator's models will test both plate-tectonic and non-plate-tectonic scenarios for the generation of early crust against these observations, and thus assess what tectonic processes are compatible with the ancient geologic record. The results of this work will have broad significance across the geosciences. Constraining the tectonic processes that operated on the early Earth sheds light on how plate tectonics developed, and potentially why it is absent on other solar system planets. This work also further helps constrain on how Earth’s continents formed, with implications for the climate state of the early Earth and possible environments for life. The proposal also makes significant contributions to science education. It supports a graduate student who will carry out much of the proposed work, therefore furthering their research career. The project also contributes to undergraduate education through the development of a remotely taught summer short course on data analysis and visualization. This short course will be targeted at geoscience students from minority serving institutions, with the goal of helping promote diversity in the geosciences. The project also includes undergraduate research internships for students to be selected from the short course participants. These research internships will give students from underrepresented groups firsthand research experience, which is critical for progressing in their careers as geoscientists. Whether the early Earth was characterized by a “mobile lid” mode of tectonics, featuring subduction and surface plate motion, or a “stagnant lid” mode where subduction is absent, is highly debated. Geochemical observations of Hadean and Archean felsic crust provide important constraints on the tectonic processes operating at this time. However, there is still significant ambiguity, as both subduction and non-subduction, i.e. melting at the base of a thick crustal plateau, models have been proposed to explain the formation of Earth’s early felsic crust. Here, geodynamical models integrating new data provided by Hf isotopes recorded in zircons will test the subduction and plateau melting models, from the Hadean until ~ 3.5 Ga. Hf isotopes in zircons from many Archean cratons suggest that the mafic crustal source of Earth’s earliest felsic rocks (> 3.5 Ga) persisted at the surface for 100s of Myrs. Additional petrological evidence indicates that the earliest felsic crust still preserved today formed from shallow (<30 km) melting of mafic protocrust. These observations will be integrated into two-dimensional mantle dynamic models, to test plateau melting and subduction scenarios. The modeling work will also explore the fundamental issue of whether subduction was feasible on the early Earth. Global models of mantle convection including crustal buoyancy and grain size evolution will be used to assess whether crustal buoyancy prevents subduction, or drastically alters its dynamics, at early Earth conditions, and whether slab melting can occur when subduction is active. The results of the work will provide vital new constraints on the tectonic processes that operated on the early Earth, and thus shed light on when plate tectonics might have begun. This has major implications for the evolution of the Earth as a whole, including the dynamo, crust, and even the atmosphere and oceans. The proposal also involves a significant educational component, including training of a graduate student, and a numerical modeling summer short course and REU program, geared towards students from underrepresented groups in the geosciences. A lack of programming and quantitative skills, as well as research opportunities, can be a barrier for diverse students advancing in the geosciences; the educational program developed here will seek to help address this problem. 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.