Possible Eoarchean records of the geomagnetic field preserved in the Isua Supracrustal Belt, southern west Greenland

We present paleomagnetic field tests that hint that a record of Earth’s 3.7-billion-year (Ga) old magnetic field may be preserved in the banded iron formation from the northeastern Isua Supracrustal belt. Magnetite in the banded iron formation has a Pb-Pb age of 3.7 Ga (Frei et al., 1999).  The U-Pb system has a closure temperature between 150-400°C for the magnetite grain size range observed in the banded iron formation, 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 have avoided thermal overprints from subsequent igneous intrusions. We demonstrate that specimens that pass this test also go on to pass fold and reversal tests. We argue that the banded iron formation acquired a chemical remanent magnetization via magnetite growth during a multi-phase Eoarchean metamorphic event. We recover what appears to be the oldest known whole rock record of the geomagnetic field, and oldest known records of reversals and secular variation suggesting that Earth’s magnetic field behaviour in the Eoarchean may have been similar to that observed today.