Claire I O Nichols , Ben P Weiss, Athena Eyster, Craig R Martin, Adam C Maloof, Nigel M Kelly, Mike J Zawaski, E. Bruce Watson, Stephen J Mojzsis, Daniele J Cherniak

We present paleomagnetic field tests that hint that a record of Earth’s 3.7-billion-year (Ga) old magnetic field may be preserved as a chemical remanent magnetization acquired during amphibolite-grade metamorphism in the banded iron formation from the northeastern Isua Supracrustal Belt. Multiple petrological and geochronological lines of evidence indicate that the northern most part of Isua has not experienced metamorphic temperatures exceeding 350◦C since the Eoarchean, suggesting the rocks have not been significantly heated since magnetization was acquired. We use a ‘pseudo’ baked contact test to assess paleodirections in the banded iron formation that pre-date the intrusion of the 3.26-3.5 Ga Ameralik dyke swarm. We demonstrate that specimens that pass this test also go on to pass a fold test and may also pass a reversal test. We recover what appears to be the oldest known whole rock record of the geomagnetic field, and oldest known records of reversals suggesting that Earth’s magnetic field behaviour in the Eoarchean may have been similar to that observed today.