We firstly present the results of a detailed palaeointensity study performed on 54 samples from 9 volcanic units of late Archaean age (2724-2772 Ma) from the Pilbara Craton, Western Australia. These results were severely affected by magnetomineralogical alteration occurring during the laboratory heating process necessitating the application of a correction procedure. The correction allowed results from three lavas to pass strict selection criteria but we deem that only one of these exhibits sufficient internal consistency to be considered moderately reliable. It yields a virtual dipole moment of 47±6 ZAm 2 which is 60% of the present-day value. We combine this determination with a filtered dataset from the updated IAGA (International Association of Geomagnetism and Aeronomy) palaeointensity database, PINT08. Directional secular variation has recently been shown to have changed fundamentally since the Archaean, probably as a consequence of inner core growth since that time. However, here we argue that it is still unclear whether this evolution was accompanied by a single long timescale change in average poloidal field intensity. While the distribution of Precambrian palaeointensity determinations as a whole is significantly lower than that for the last 300 Myr, we show that this finding largely reflects data from the Proterozoic aeon. The distribution of more ancient measurements from the late Archaean-earliest Proterozoic is indistinguishable from that of the last 300 Myr which might suggest that a 'Proterozoic dipole low' period existed between two periods of higher field intensity. Were this pattern of long-term geomagnetic intensity variation to be supported by the addition of new data in the future, then it could indicate a related three-stage evolution in core dynamics, namely: vigorous thermal convection caused by high core-mantle heat flux early in the Earth's history, weaker thermal convection later as the heat flux fell, and finally, strong compositional convection since the inner core nucleated.

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[1] A late Archaean (circa 2775-2715 Ma) succession of terrestrial continental flood basalts, mafic tuffs, felsic volcanic rocks, and clastic sedimentary rocks in the Nullagine Synclinorium (and Meentheena Centrocline) of the East Pilbara Basin, Western Australia, has been sampled for a palaeomagnetic study. Over 500 oriented, mostly basalt, drill cores were collected from the supracrustal succession and associated dykes. Thermal and alternating field demagnetization revealed two distinct components. Positive fold, conglomerate, and reversal tests confirm that the primary natural remanent magnetization (NRM) is still preserved. The secondary component is interpreted as the record of remagnetization during a major thermal event, possibly in the Early Proterozoic. Analysis of the primary NRM directions results in a magnetostratigraphy and an apparent polar wander path (APWP) for the 60 Myr interval covered by the sampled succession. Assuming a geocentric axial dipole during this time interval, the APWP shows that the Pilbara Craton was drifting during the late Archaean and that drift rates probably varied significantly. In particular, a mean 27.2°shift in palaeolatitude is recorded across an unconformity that represents a relatively short time period and that marks a significant change in basalt geochemistry. This study suggests that continents moved horizontally during the late Archaean and that the rates of movement were significantly faster than in the Phanerozoic. In addition, a reversed polarity interval, with a positive reversal test, is recorded. We argue that it documents the oldest known geomagnetic reversals of the geomagnetic field.

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