Deciphering the Evolution of the Early Earth's Crust-Mantle System

About This Grant

The planet Earth consists of three main layers: the metallic core, the silicate mantle, and the crust. Earth’s outermost layer is a thin shell of solid rock where life flourishes. The crust under oceans, called oceanic crust, is thinner and denser, and continental crust that forms continents is thicker and lighter. The mantle lies beneath the crust and is approximately 2,900 km thick comprising 67% of the total mass of the planet and 84% of its volume. The mantle is mostly solid, but it can flow slowly over long periods of time due to high temperatures and pressures, driving plate tectonics. The Earth's innermost layer, the core, is extremely hot and dense. While the core has been a near-closed system since the time it formed, the crust and the mantle represent a very dynamic and ever-changing system constantly interacting through the continuous transfer of mass and heat. One of the most fundamental challenges in the Earth sciences is reconstruction of the chemical evolution of the crust-mantle system. Samples of some of the remaining well-preserved vestiges of the early rock record, the invaluable time capsules that may shed light on the Earth's distant geological past, have been assembled by the PI of this project over his entire scientific career; these samples will be interrogated in this project using state-of-the-art isotope and geochemical tools to help decipher the evolution of the early Earth's crust-mantle system. The work for this project will include creating a comprehensive model for the timing of formation of the continental crust and complementary chemical evolution of the mantle, significantly advancing our understanding of early Earth’s history. Therefore, it will have relevance to the long-debated question of how terrestrial planets formed and evolved and will ultimately improve our understanding of the modern Earth. This project will involve training students for their future scientific careers. This research project seeks to constrain the history of continental crustal growth and mantle depletion in the early Earth by obtaining estimates on the timing of extraction, absolute volumes, and relative proportions of continental and oceanic crust in the Archean. A novel approach of combining lithophile element isotope (Pb-Pb, 146,147Sm-142,143Nd, and 176Lu-176Hf) and trace element (e.g., rare earth elements, Nb, U, Th, Hf) abundance data obtained using the state-of-the-art analytical techniques for a global set of thirteen komatiite-basalt systems ranging in age between 3.6 and 2.7 Ga and located on five continents, will be utilized. These komatiite-basalt suites were selected because: (a) their emplacement ages have been robustly determined using several independent geochronometers, (b) a large set of high-precision short- and long-lived radiogenic isotope and lithophile trace element abundance data are available for most of them from the previous and current studies of the PI, and (c) these komatiite-basalt suites have been shown to have escaped contamination with the material of earlier-formed upper continental crust and their compositions were mostly unaffected by secondary alteration. Using the combined isotope and trace element data both available from the previous work of the PI and obtained in this study, the time-integrated Sm/Nd, Th/U, and "true" Nb/U ratios of their mantle sources will be calculated. Using these data, the absolute volumes and relative proportions of continental and oceanic crust extracted for each time frame defined by the ages of the studied komatiite-basalt systems will be estimated, and a well-constrained model of the history of continental crustal growth and complementary mantle depletion in the Archean will be generated. While working on this project, the PI will continue supporting the integration of research and education via training students, postdoctoral researchers, and visiting scientists. The PI and the undergraduates involved in this project will develop a display for the annual Maryland Day event highlighting the results from this project, presenting current ideas about early Earth history to the public and future students. This project will make a significant contribution towards building the still extremely limited high-precision radiogenic isotope and trace element database for the Earth’s earliest mafic-ultramafic rock record, and this database will be made available to the broader scientific community for the future research and development efforts. 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.