EARTH'S HADEAN TO EOARCHEAN MAGNETIC FIELD RECORDED BY DETRITAL ZIRCONS

Single crystal paleointensity methods (Tarduno et al., Rev. Geophys., 2006) applied to Hadean to Eoarchean detrital zircons provide a means to examine Earth's most ancient dynamo (Tarduno et al., Science, 2015). Keys to this analysis include extensive screening methods to isolate the best-preserved zircons, CO₂ laser demagnetization which affords heating on time scales an order of magnitude shorter than those using standard paleomagnetic ovens, and measurements with an ultrasensitive WSGI 3-component SQUID magnetometer. Select Jack Hills zircons (Western Australia) preserve primary magnetite inclusions and such zircons record a field as old as 4.2 billion-years-old. Moreover, the paleointensity data outline a distinctive history incompatible with magnetic resetting. Specifically, these data record strong magnetic field strengths in the Late Hadean followed by weaker fields in the Eoarchean. The strong Late Hadean fields may record early chemical precipitation in the core, whereas the drop to lower field strengths may record solidification of a basal mantle magma ocean (Tarduno et al., PNAS, 2020). We will discuss our ongoing efforts to examine this history at other worldwide sites yielding Eoarchean-Hadean detrital zircons, as well as the implications of these data for evolution of the core, mantle and crust.