This project will advance understanding of our planet's early evolution by obtaining high-quality data on the strength (paleointensity) of ancient Earth's magnetic field. Today, the Earth's magnetic field is generated by convection of molten iron in the planet's outer core. This process, called the geodynamo, is powered by density settling and heat of fusion of the crystallizing inner core. However, according to the most models, the inner solid core is relatively young and may have existed only for the last 500 million years of the ~4.5 billion years of the geological history. At the same time, it has been established that the geomagnetic field has existed since at least 3.5 billion years ago. Hence, before the inner core formation, the geodynamo mechanisms should have been different. In particular, a recent hypothesis suggests that before ~2.45 billion years ago, a strong field was produced within a basal-mantle magma ocean, a layer of molten silicate mantle above the core-mantle boundary. After the mantle completely solidified, a weaker and less stable magnetic field was produced within the entirely liquid core ("the Proterozoic Dipole Low"). The modern style geodynamo producing a strong magnetic field started with the formation of solid inner core at about 400 million years ago. The goal of this research is to test this hypothesis by obtaining high-quality paleointensity values (measurements of the ancient magnetic field strength) for the time period (between 1.98 and 2.41 billion years ago) immediately following the demise of the proposed basal mantle ocean geodynamo. The paleointensity experiments will be conducted on several suites of quickly-cooled intrusive Paleoproterozoic rocks in India, Canada, and Australia using a novel technique based on investigation of single silicate crystals. The study will provide important insights into the mechanism that generates Earth's magnetic field and the evolution of the Earth's deep interior (the core and the mantle). Broader implications of the study include a better understanding of the link between evolution of Earth's magnetic field and evolution of biosphere and atmosphere. The project will involve Michigan Tech undergraduate and graduate students, thus training the next generation of scientists. In order to increase the general public awareness of Earth science, the results will be disseminated through a series of science exploration sessions.
Data on the long-term evolution of Earth's magnetic field strength (paleointensity) in the Precambrian are crucial for understanding the nature of early geodynamo. These data may also provide insights into important processes within the Earth's interior, such as the formation and growth of the solid inner core, or long-term changes in mantle convection affecting the forcing of the geodynamo. However, our knowledge of the Precambrian paleointensity remains very limited. Conventional paleointensity methods often have low success rate when applied to Precambrian rocks due to heating-induced alteration of samples. In order to circumvent this problem, a novel technique has been developed that uses individual rock-forming silicate crystals to measure paleointensity. Such crystals often contain single-domain to pseudosingle-domain ferromagnetic inclusions protected from natural alteration by the silicate host and stable with respect to the experimental alteration. The research objective of this proposal is to investigate the strength of the Proterozoic geodynamo by paleointensity analyses of plagioclase separated from three mafic dike swarms in India (the ~2.37 Ga Bangalore dikes, the ~2.18 Ga Mahhubnagar dikes in the Dharwar craton, and the ~1.98 Ga dikes in the Bundelkhand craton), and from the ~2.12 Ga and ~2.10 Ga Marathon dikes in the Superior Craton (Canada). In addition, single crystal paleointensity determinations will be conducted for a methodological investigation on samples from the ~2.41 Ga Widgiemooltha dikes for which reliable bulk rock paleointensity determinations have been obtained. The suitability of the crystals from the proposed dike suites for paleointensity experiments has been demonstrated by pilot rock magnetic and paleointensity investigations. This project will increase the number of high-quality paleointensity determinations for the Precambrian providing insights into the mechanisms of the geodynamo and the long-term evolution of Earth. Importantly, the age of selected dike suites will allow us to test the hypothesis of a transition from a strong field geodynamo produced within a basal-mantle magma ocean before ~2.45 Ga to a weak field geodynamo produced within the liquid core without the solid inner core ("the Proterozoic Dipole Low"). The proposed comparison of rock magnetic properties and paleointensities from the bulk rock and single crystal samples derived from the same rock unit will advance our understanding of the processes of rock and magnetic mineral formation and alteration, which may affect the fidelity of rocks and crystals as paleointensity recorders. The project will support a graduate (Ph.D.) student and several undergraduate research assistants.
