UNLOCKING PRECAMBRIAN GEODYNAMICS, INNER CORE GROWTH AND EVOLUTION OF THE GEODYNAMO USING PALEOMAGNETIC MICROANALYSIS

Paleomagnetism represents one of the few avenues available to retrieve quantitative data on Precambrian global geodynamics and core evolution. But this endeavor is especially demanding because ubiquitous metamorphism, deformation and the presence of magnetic minerals with non-ideal recording properties leads to complex, superimposed magnetic signals of differing age. To address these challenges, one must first consider what is the appropriate scale of measurement. In a now classic work, John Geissman and his colleagues (Geissman, J.W., Harlan, S.S. and Brearley, A.J., The physical isolation and identification of carriers of geologically stable remanent magnetization: Paleomagnetic and rock magnetic microanalysis and electron microscopy, Geophysical Research Letters, v. 15, p. 479-482, 1988) discussed the benefits of paleomagnetic microanalysis whereby oriented microsamples could be used to characterize magnetic carriers and their associated magnetic directions. The subsequent decades have seen steady improvements in magnetometer sensitivity and electron microscopy instrumentation, as well as the advent of single crystal paleointensity analyses. With these advances in equipment and methods, paleomagnetic microanalysis can now be more fully employed to address fundamental questions in Precambrian geodynamics and evolution of Earth’s magnetic field. Here I will review this approach in an updated context of the relevant scale of measurement: that satisfying paleomagnetic recording limits defined by Maxwell-Boltzmann statistics and Néel theory. Salient recent advances include: i. Resolution of the paradox of widely divergent Ediacaran-age magnetization directions, and the isolation of primary directions indicating stability of the solid Earth with respect to the spin axis ii. Ultra-low field strength during the Ediacaran Period suggesting a late (ca. 565 Ma) onset of growth of the solid inner core and iii. New evidence for Hadean-age magnetizations, supporting scenarios for chemical precipitation in the earliest fluid core and that magnetic shielding of Earth’s atmosphere commenced more than 4.2 billion years ago.