Paleomagnetic records of a mobile lithosphere and dipolar geodynamo by the Paleoarchean

Abstract

Plate tectonics dominate the geodynamics of the modern Earth’s surface, segmenting the surface into mobile plates whose differential motions are responsible for the physiography of our planet and for constructing the lithosphere. Likewise, thermochemical convection of Earth’s core produces the geodynamo, a strong, relatively stable dipolar magnetic field that envelops the planet and dictates many interactions of the upper atmosphere with space. Paleomagnetism represents the most important quantitative archive of these processes in deep geologic time. By measuring the ancient local magnetic field direction and intensity preserved by ferrimagnetic mineral phases in rocks, paleomagnetism can track lithospheric blocks as tectonic motions carry them through Earth’s magnetic field, and directly record geodynamo processes such as reversals. However, evidence of these phenomena becomes sparse the further back in geologic time one looks, given the paucity of the early rock record. Very few paleomagnetic data exist for the Archean and Hadean Eons in particular (>2.5 billion years ago or Ga), rocks from which represent a mere 5% of Earth’s present-day surface. Further complicating the scarcity of the early paleomagnetic record, the most ancient rocks have experienced complex histories of billions of years of alteration, metamorphism, and weathering that can overprint magnetizations. Yet, Earth’s early history is exactly where constraints on geodynamics are most needed. Geochemical, petrological, field, and modeling studies have suggested – and not without vigorous ongoing debate – that the early Earth may have experienced a different geodynamic regime. For instance, Earth’s geodynamics could have resembled those of most other terrestrial planets, with a so-called “stagnant-lid” in which the lithosphere was not segmented into plates and experienced limited or no horizontal motion. This would have profound effects on local geologic processes and Earth’s global thermal evolution. And in Earth’s core, the probable lack of an inner core this far back in time has led to competing proposals for how to power the early geodynamo, including exotic dynamos hosted by a basal magma ocean. Thus, despite the Archean and Hadean seeing the emergence and earliest evolution of life, few constraints have been made to-date on the early Earth’s underlying geodynamics. This dissertation aims to develop new constraints on geodynamic processes and their rates on the early Earth. This is based on paleomagnetic measurements of mafic volcanic rocks dating to the late Paleo- to early Mesoarchean (~3.3-3.2 Ga) from the Pilbara Craton, Western Australia. In Chapter 2, I present new paleomagnetic data from the ~3.18 Ga Honeyeater Basalt. Comparison with an existing paleomagnetic pole from the ~3.34 Ga Euro Basalt demonstrated the earliest resolvable horizontal motion of the lithosphere, in which the East Pilbara Craton moved in latitude by ≥2.5 cm/yr over a 160 million-year (Myr) interval, compatible with plate tectonics. Chapter 3 presents further paleomagnetic data from the ~3.25 Ga Kunagunarrina Formation. Coupled with the poles measured and discussed in Chapter 2, this result enabled the oldest time-resolved lithospheric motion reconstruction, revealing that the East Pilbara Craton experienced 95 Myr of latitudinal motion at 6.1 cm/yr followed by 65 Myr of rotation at 0.55°/Myr. The rates, durations, and time-variability of these motions are all compatible with plate tectonic motions, yet are incompatible with the limited motions that are possible in a stagnant-lid. Further, this dataset included a symmetric 3.25 Ga geomagnetic reversal, the oldest ever documented, the geometry of which requires that the geodynamo was dominantly dipolar and generated in the core. Finally, Chapter 4 details how the magnetizations preserved in the Kunagunarrina Formation originated via hydrothermal alteration of the formation following its eruption onto the seafloor. Using a uniquely-detailed combination of petrographic observations, magnetic microscopy, and geochronology, I link the magnetization-hosting phases to iron-mobilizing reactions within a set of well-documented volcanic-hosted massive sulfide hydrothermal systems in the region. This demonstrates the promise of hydrothermal systems as potential targets for future paleomagnetic work in Archean rocks. Taken together, these studies paint a fuller picture of geodynamics on Earth when it was about 1.3 billion years old. The evidence developed herein portrays a geodynamically-mature early Earth, with a tectonically-mobile lithosphere and a stable core-generated dynamo. While these are not necessarily indications of truly “modern” geodynamic processes sensu stricto, they nevertheless are consistent with a uniformitarian interpretation of the rock record. Finally, this work demonstrates the most detailed understanding yet of how many Archean rocks acquired magnetizations, paving the way for future paleomagnetic studies of the early Earth and of complexly-altered rocks in general.