Neoarchean to Early Proterozoic evolution of Earth's core: Paleomagnetic tests using dikes and sills of the Zimbabwe craton

University Of Rochester, Rochester NY

Investigators

Abstract

The history of Earth's magnetic field (paleomagnetism) recorded by magnetic minerals when rocks form provides a way to probe conditions in Earth's core in the past. Earth's magnetic field also shields the atmosphere from erosion by energetic particles streaming from the Sun (the solar wind), and thus may have played an important role in the evolution of the atmosphere. We will test two recent hypotheses concerning the development of Earth's core and atmosphere by sampling a magnificent set of igneous rocks (dikes and sills) preserved in Zimbabwe. The first hypothesis suggests that the onset of growth of Earth's solid inner commenced more than 2 billion years ago. By sampling the dikes and sills and investigating their paleomagnetic signature, we will test whether they record evidence for initial growth of Earth's inner core. Earth's atmosphere was somehow transformed about 2.3 billion years ago, from mildly reducing to oxidizing conditions. The second hypothesis suggests that this change was aided by removal of hydrogen from the atmosphere by the solar wind. We will test this hypothesis by gauging the past intensity of Earth's magnetic field (and hence its atmospheric shielding capacity) through paleomagnetic analyses. Our work could lead to a transformative change in how we relate deep Earth processes and evolution of the atmosphere. The research will be integrated with educational efforts, involving graduate and undergraduate students who will receive training in the field and laboratory. We will also undertake outreach activities to communicate our results to the local Rochester community and to the wider public. Two recent hypotheses relate the nature of the geomagnetic field to fundamental aspects of core and atmosphere evolution. In the first hypothesis, inner core growth is postulated to occur prior to 2 billion years ago, as recorded by a lower quadrupole family contribution to Archean geomagnetic secular variation. This hypothesis in turn favors a small Phanerozoic core-mantle boundary heat flow. The second hypothesis relies on new paleointensity data and solar wind estimates for the Archean. Intense solar wind from the rapidly rotating young Sun is envisioned as stripping H from Earth's atmosphere, contributing to the transformation from mildly reducing to oxidizing conditions, potentially contributing to the ∼2.3 billion-year-old Great Oxidation Event. We will examine these ideas through paleomagnetic and paleointensity studies of a magnificent record of mafic dikes and sills exposed on the Zimbabwe craton. To test prior inferences on the nature of the paleosecular variation and its potential relationship to inner core growth, we will collect paleomagnetic directional data from these units, following a major U-Pb regional dating effort by our collaborators. We focus of three time windows spanning the Great Oxidation Event: 1.89-1.88, 2.51-2.41 and 2.58 billion-years ago. To examine the hypothesis of H-loss from the atmosphere, we will conduct paleointensity analyses using single silicate minerals; these values combined with estimates of solar winds will allow us to calculate magnetopause standoff distances that are needed to evaluate atmospheric effects. Our study will address fundamental issues of broad interest to the scientific community interested in deep Earth processes and evolution of the atmosphere. The paleomagnetic approach we outline is one of the few probes we have to gauge inner core growth, the onset of which is an essential element of thermal models for Earth. Moreover, determining whether external forcing (i.e. solar wind) had a role in atmosphere evolution could lead to transformative changes in how we view long-term Earth history. A key part of our work is the integration of research and educational efforts, including graduate and undergraduate education. The work will contribute to at least one Ph.D. thesis and will involve several undergraduates, who will receive training in the field and laboratory. We will also undertake a small number of K-12 activities, integrating our graduate and undergraduate teaching efforts with the Rochester community, as well as outreach efforts to disseminate the results of our study through the media and museums.