Fig. 3: Magnetic Reynolds number and magnetic field strength.

a The magnetic Reynolds number (red) and Dynamo number (blue) computed from our electrical conductivity results and thermal evolution model with shading indicating the difference between low-spin (top of range) and high-spin (bottom of range) results compared with the respective critical values of R_m = 40 and D = 100 (thick horizontal lines). b Magnetic field strength computed from our model (red line) compared with paleomagnetic data: diamonds from the PINT database⁴⁰, and circles (Thellier–Coe method) and squares (565C method) from zircons⁴¹. We note that the evidence of a field prior to the oldest whole-rock PINT data at 3.45 Ga relies on the single-crystal zircon results, which have been questioned as not deriving from a primary magnetic carrier⁴².