Geomagnetic intensity between 100 million and 2500 million years ago

Abstract

The decay of the NRM of 146 specimens (including the 41 specimens reported on earlier) of igneous rocks during heating in steps of 25 or 50 °C between 200 or 300 °C and 550 °C was compared to the acquisition of TRM (H = 0.35 Oe) during subsequent cooling to 20 °C. The K-Ar ages of the specimens are distributed between 100 m.y. and 2500 m.y. The selection of results suitable for intensity determination was based on constancy of the rate of decay of the NRM and acquisition of TRM determined from at least 3 successive determinations over a temperature interval of at least 100 °C, an appreciable decay of the supposed original NRM in that temperature interval, and constancy in the NRM direction in that temperature interval. This selection yielded a total of 41 equatorial paleointensities. The intensity results obtained for several specimens collected from the same rock unit show a reasonable degree of internal consistency. The results indicate that the mean equatorial geomagnetic intensity during most of the Phanerozoic and the Precambrian eras as far back as 2.5 billion years was 0.25 Oe with a standard deviation of 0.13 Oe. The large scatter in the equatorial intensities may be due to various possible errors and/or fluctuations in geomagnetic moment of periods shorter than the experimental error (5 to 10%) of the radiometric ages of the specimens, possibly including a period as short as that suggested by the archeomagnetic results for the last five millenniums. The dipole representation of the paleomagnetic field as far back as 2500 m.y. is not contradicted by the variation of the paleointensity values with paleomagnetic latitude. Thus, several characteristics of the present geomagnetic field seem to have been present as far back as 2500 m.y. ago.