The First Billion Years of the Geodynamo

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Earth's magnetic field, generated in the liquid iron core, shields the atmosphere from particles streaming from the Sun, known as the “solar wind". Without this shield, the atmosphere would have been eroded, and water would have been lost, especially when Earth was young and solar winds were intense. Hence, knowledge of Earth's earliest magnetic field can provide unique insight into the evolution of our planet's core and atmosphere. Because water is essential to life as we know it, this knowledge also bears on the origin of Earth's habitability. The only known material that can accurately record this early magnetic history is the mineral zircon, now found as tiny grains in younger sedimentary rocks. Some of these zircons are remarkable because they contain even smaller inclusions that can preserve signals of the ancient magnetic field from billions of years ago. But because these zircons are so small - only a few times thicker than a human hair - the measurement of their magnetism is extremely challenging. The investigators have developed and applied new techniques to characterize the nature of magnetic inclusions in zircons and to retrieve their magnetic signals, using a host of highly sensitive instruments. Their measurements indicate the presence of a magnetic field at least 4.2 billion years ago. These data indicate atmospheric shielding was in place. They also place constraints on the physical conditions in the core that resulted in the generation of a strong magnetic field. The investigators will fill gaps in the magnetic field history, generating data from new zircon localities in Western Australia, India and southern Africa. They will work with a team of international collaborators spanning 6 countries in multidisciplinary laboratory studies. The team will then use these new data to test the fidelity of the magnetic history, and to further explore the implications for magnetic shielding and Earth evolution. The work will support graduate and undergraduate students who will receive broad training in the field and in multidisciplinary analyses, and will include outreach to Rochester secondary schools through programs hosted for instructors and students.Knowledge of the earliest history of the geomagnetic field can provide key insight into our understanding of the evolution of the core, atmosphere and Earth's habitability. The only known materials that can be accurately dated and that are able to record this history on the multi-hundred-of-million-year time scales required to advance our understanding of these fundamental issues of Earth evolution are Eoarchean to Hadean zircons bearing minute magnetic inclusions that are now found in younger sedimentary units. However, the magnetic measurement of zircons, and interrogations of their magnetizations to determine if they preserve primary signals, are formidable technical challenges requiring a multidisciplinary approach. The investigators have recently presented new paleomagnetic, electron microscope, geochemical, and paleointensity data that indicate the presence of primary magnetite inclusions in select zircons from the Jack Hills of Western Australia. These new data further support that select Jack Hills zircons record primary magnetic signals of the geodynamo, with a record extending back in time to ~4.2 billion years ago. These analyses thus indicate that shielding of the atmosphere by the magnetosphere was in place very early in Earth history. The new analyses also suggest a strong magnetic field in the Late Hadean at ~4 billion years ago which could be a signal that chemical precipitation in the core was powering the geodynamo. The team will use a paleomagnetic approach to further test and fill gaps in the paleointensity record building on recent discoveries of Eoarchean to Hadean detrital zircons at other global sites. Specifically, they will generate records spanning hundreds-of-millions of years from new localities in Western Australia, India and southern Africa. They will work with a team of international collaborators spanning 6 countries in multidisciplinary laboratory studies [including light and scanning electron microscopy, focused ion beam slice and view and lift-outs, transmission electron microscopy, magneto-optical Kerr effect measurements, ultra-sensitive 3-component direct current superconducting quantum interference device (SQUID) magnetometry, scanning SQUID microscope magnetometry, sensitive high-resolution ion microprobe (SHRIMP) analyses, and secondary-ion mass spectrometry (SIMS)]. The team will use these new data to test the internal fidelity of each record, test consistency between records, and further explore the implications for planetary magnetic shielding and chemical evolution of the core. The work will support graduate and undergraduate students who will receive broad training in multidisciplinary analyses. The work will contribute to Ph.D. and M.S. theses. The investigators will continue outreach to Rochester secondary schools by hosting programs for instructors and students.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.