Mesoarchean diamond-bearing sediments: implications for Archean continental roots and their surface expression

点击翻译按钮获取中文摘要

The continental crust is fundamental to the development and sustainment of life on Earth. Continental crust, and in particular ancient continental crust, produces 90% of the world’s gold and platinum deposits and nearly all of the mined diamonds. Ancient continental crust is clearly stable on Earth today and often has a much deeper mantle root than younger crustal blocks. Yet, how ancient continental crust formed and, perhaps more importantly, was stabilized, remains an outstanding question in the Geosciences. This project will help to address this question by making use of recent discoveries of 2.85 billion-year-old diamonds preserved in ancient sedimentary rock samples. Diamonds are the best available tools to sample the deep roots of continents, and the sediments they are contained within preserve a record of the elevation and surface of the continents back in Earth history. These small diamonds are likely some of the oldest diamonds on Earth, and their chemical signatures record how the earliest roots of the continental crust formed. This project will look for more of these valuable records of the deep continental root, while also investigating the sedimentary rocks where they are now found. Sedimentary rocks record the surface features at the time they were deposited, so this combined deep-and-surface study will help address questions such as: Was the early Earth an ocean world? Did the roots of continents form first before continents rose above sea level? Were ancient continents loose and malleable early in their life before they became rigid and stable? The outputs of this research will serve as input data for scientists trying to understand the composition of the ancient atmosphere, ancient biological processes, and how important mineral deposits were formed on Earth. The research team involved in this proposal recently discovered detrital diamonds preserved in ca. 2.85 billion-year-old sediments in the Canadian shield. The presence of Archean diamonds at Earth’s surface during this time interval, and their detailed geochemical signatures, contain key evidence for the formation and time-integrated stability of regions of preserved continental crust. The team will evaluate the prevalence, location, and mantle-residence ages of these ancient diamonds, as well as their geochemical signatures to test models for the timing of the formation deep lithospheric mantle roots and their significance for large scale craton formation. The team will also evaluate the sediments that now contain the ancient diamonds including their catchment size (size of the ancient watershed), catchment age distributions, and the depositional environment. These inputs will help understand if significant continental elevation was formed in the very early in Earth history, potentially due to continental root stabilization. The work in this proposal will address two major aspects of craton formation and continental stability, using a single sample set and a dedicated field campaign. The research team will use float planes to sample rocks in the remote regions of Northern Canada and will then conduct state-of-the-art chemical analyses in laboratories in the US and Canada. A full understanding of the life cycle of continental crust throughout Earth history has broad scientific implications for the geoscience community, including helping to understand how the atmosphere became rich in oxygen and how life evolved on the surface of the Earth. This proposal will also result in a large collection of ancient diamonds which can be studied in future work. Additionally, ancient cratonic blocks host a large fraction of Earths economic Pt, Au, and diamonds making them economically important geologic features. Importantly, the prevalence of detrital diamonds in ancient sediments is largely unknown, such that the results of this project may drive economic activities (exploration and resource development) in similar sediments globally. This project will fund training of a PhD who will also be exposed to many sections of the geoscience industry, including academia, governmental surveys, and economic exploration and mining as well as many types of advanced analytical techniques. Diamond research, as well as the geology of the early Earth, is a research topic that is popular with the science media and general public. Every effort will be made to engage with the popular science media to expand the influence and public engagement with the results of this project.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.