Search for the origins of the geodynamo: Paleomagnetic studies of rocks and zircon crystals from the Jack Hills, Western Australia

It is currently unknown when Earth's dynamo magnetic field originated. This timing has major implications for the thermal evolution of the interior, the physics of dynamo action, the surface cosmic ray flux, the evolution of the terrestrial atmosphere, and planetary habitability. Paleomagnetic studies of the oldest known unmetamorphosed rocks indicate that a field with intensity similar to that of the present existed at least 3.5 billion years ago (Ga). One of the very few sample suites predating this time are detrital zircon crystals found in quartz-rich siliciclastic rocks from the Jack Hills of Western Australia. With crystallization ages ranging from 3.0-4.38 Ga, they have the potential to preserve a record of the missing first billion years of Earth's magnetic field history. Over the last fourteen years, we have been studying individual Jack Hills zircon crystals and their host rocks to characterize the nature, age, and intensity of their paleomagnetism. Petrographic and rock magnetic studies suggest the zircons contain inclusions of ferromagnetic iron oxides and sulfides. Using a newly developed ultra-high sensitivity moment magnetometry technique implemented on our superconducting quantum interference device (SQUID) microscope, we have found that many zircons carry an extremely weak natural remanent magnetization (ranging from 5-10x10-14 Am2), essentially making them the most weakly magnetic samples studied in the history of paleomagnetism. They present tremendous analytical challenges associated with magnetic contamination and limitations on magnetic recording properties. We present the first magnetic field paleointensity studies of these zircons. The key unknown is the age and origin of their magnetization. In particular, the identification of >3.9 Ga (Hadean) field records requires at least establishing that the zircons have avoided post-depositional remagnetization. However, there are presently no paleomagnetic data that have confidently established the magnetization history for the Hadean zircon locality (Erawandoo Hill), thought to have been deposited at 2.65-3.05 Ga. To address this gap, we have conducted paleomagnetic conglomerate, baked contact, and fold tests in combination with geochronometry to establish the timing of the metamorphic and alteration events and the peak temperatures experienced by the zircons' host rocks. Although we have found little evidence for remagnetization by recent lightning strikes, most rocks in this area have been pervasively remagnetized to at least 330-530°C, likely from the effects of nearby Mesoproterozoic mafic intrusions. However, localized regions of the Jack Hills may have escaped complete remagnetization. We will discuss the implications of these data for the possibility that magnetization in the detrital zircons predates their deposition.