The First Billion Years of the Geodynamo

University Of Rochester, Rochester NY

Investigators

Abstract

Knowing when Earth's magnetic field began is important for understanding the evolution of the core, atmosphere and life on Earth. The PI team has recently reported the first measurements of Earth's ancient magnetic field as recorded in zircons- natural crystals from the Jack Hills of Western Australia that are the oldest known materials on Earth, dating to a time period known as the Hadean. Some of these zircons host magnetic inclusions capable of preserving magnetic field records for billions of years. Data from zircons between 3.3 and 4.2 billion-years-old record magnetic fields that vary in strength from those typical of the present-day field, to only about 10% of that value. The earliest Hadean geomagnetic field may have been powered by chemical precipitation (of MgO) in the core. The existence of this ancient terrestrial magnetic field implies that early atmospheric evolution was regulated by dynamo behavior. These new observations provide the basis for continued work. The investigators hope to refine the history of the first billion years of the geodynamo and to test its fidelity through field and laboratory based programs. Undergraduate and graduate student training is integrated into these programs, as is international collaboration and inter-laboratory comparison and calibration efforts. Knowing when the geodynamo started is important for understanding the evolution of the core, atmosphere and life on Earth. We have recently reported the first paleointensity measurements of Archean to Hadean zircons bearing magnetic inclusions from the Jack Hills conglomerate (Western Australia) to reconstruct the early geodynamo history. Data from zircons between 3.3 and 4.2 billion-years-old record magnetic fields varying between ¡­1.0 and 0.12 times recent equatorial field strengths. A Hadean geomagnetic field may have been powered by chemical precipitation of MgO in the core. The existence of a terrestrial magnetic field prior to the Late Heavy Bombardment is supported by terrestrial N isotopic evidence and implies that early atmospheric evolution on both Earth and Mars was regulated by dynamo behavior. These new observations provide the basis for our continued work to refine the history of the first billion years of the geodynamo and to test its fidelity. We outline 8 tasks that form the basis of our work plan: 1. Rock magnetism, anisotropy studies and full Thellier-Coe analyses of Eoarchean and Hadean zircons; 2. Imaging of magnetic inclusions in zircons; 3. Analyses of zircons from new sites in Western Australia; 4. Analyses of zircons from new sites in Zimbabwe; 5. New tests of re-heating scenarios, including studies of Li zoning; 6. Inter- lab and instrument comparisons and sample distribution; 7. Studies of terrestrial-solar interactions for the early Earth and 8. Studies of the thermal evolution of Earth¡¯s deep interior informed by our findings. Ultimately, our goal is to broadly define the first billion years of geodynamo history, after the Lunar-forming giant impact to Paleoarchean times, and to evaluate the implications of this record for the evolution of the atmosphere, mantle and core. The proposed work will support graduate students and undergraduates, who will acquire valuable skills through training in field studies and in the laboratory. Salient aspects of our undergraduate education plans, and its integration into our research, include i. field excursions for sampling in Western Australia and Zimbabwe as part of an undergraduate summer research program involving rock magnetic, paleomagnetic and electron microscope analyses and ii. undergraduate research courses leading to Senior theses. The work proposed will contribute to Ph.D. and M.S. theses. We also propose a visiting scientist program to the University of Rochester paleomagnetic laboratories, providing the opportunity for community instruction on methods of single silicate crystal paleomagnetic analyses developed at Rochester over the last twenty years.