EAR-PF: Geochemical Evidence for the onset of Plate Tectonics on the Early Earth from the Wyoming Province and Singhbhum Craton

Kirkpatrick, Heather Margaret, Los Angeles CA

Investigators

Abstract

Plate tectonics is the theory that the Earth’s outermost layer, which is called the crust, is divided into different plates which move across the mantle (the layer directly below the crust). This plate movement causes most volcanos and earthquakes on Earth and is theorized to be an important control on life formation on Earth. However, there is disagreement on when plate tectonics began. This work seeks to answer a crucial question in the earth sciences: When, why, and how did plate tectonics start? To understand these changes, a mineral called zircon will be used. Zircon is important because of its physical and chemical resilience, the wide variety of environments where it forms, and the rich chemical information it provides. It is also the only remnant of rock from before 4.01 billion years ago (Ga). Previous studies found evidence of a change in the source rocks of zircon between 3.8 billion Ga and approximately 3.6 Ga (Bauer et al. 2020). This variation has been argued to indicate a change in plate tectonic processes on Earth. However, more information is needed to understand whether plate tectonics was active during the first ~1 billion years of Earth’s history, including information on the extent of surface or near-surface water and variations in rock types. Through high-resolution spatial analysis of trace and rare earth elements, ages, oxygen isotopes, silicon isotopes, and mineral inclusions of Hadean (> 4.01 Ga) and Archean (2.5 Ga to 4.01 Ga) zircon from two under-investigated sites, the Wyoming Province and the Singhbhum Craton, interpretations of ~1 billion years of history related to Earth’s surface and interior will be made. These investigations will explore the spatial and temporal extent of liquid water on Earth, tectonic transitions, and crustal composition during these periods and provide further constraints on crustal processes during these early eons. This project will include mentorship of undergraduate students at Harvard, research experiences for high school students, and outreach to the greater Boston community through events at museums, Harvard and other community programs. In this project, Hadean and Archean zircons from the Wyoming Province and the Singhbhum Craton will be analyzed in order to expand people’s understanding of early Earth. Specifically, three topics will be investigated: 1) evidence for and against modern-style subduction at different periods in early Earth’s history, 2) the spatial and temporal distribution of surface or near-surface water on early Earth, and 3) the distribution of rock types on early Earth. In-situ analyses of dated zircons will include measurements of Si-O coupled isotopes, trace elements, and inclusion studies. Through this project, techniques originally designed to understand in-context zircons, or those with a very well-known geologic origin, will be applied to samples from Earth’s earliest history. Within this, assumptions related to the scalability of these techniques will be confronted. For example, can it be assumed that the physical processes which have governed Earth over the past few hundred million years, such as plate tectonics, were in effect over 4 billion years ago? If they were not, what processes operated on early Earth? 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.