It currently is unknown when Earth’s dynamo magnetic field originated.  Detrital zircon crystals found in the Jack Hills of Western Australia range up to 4.38 Ga in age and so might retain the oldest known records of geomagnetism. We are currently studying the zircon crystals to establish the age of their magnetizations and the early history of the dynamo.  We have found that although the zircons' 3.0 billion year old host rocks have been pervasively remagnetized (Weiss et al. 2015), the rocks have fortunately not been heated above the Curie point of magnetite (Trail et al. 2016).