A comparison of three methods of determining geomagnetic paleointensities

Paleointensity studies by the methods of the Thelliers, Wilson, and van Zijl were compared for adjacent specimens from each of five basaltic lava flows. For three of the flows, the actual paleointensity is independently known within ±6%. All specimens had Curie temperatures above 500°C, and those from four of the flows had lower Curie temperatures as well, indicating they probably underwent varying degrees of high-temperature oxidation and disproportionation during initial cooling. These four suffered irreversible changes in their blocking temperature spectra when heated in air above 400 to 500°C. Nonetheless, the paleointensities derived by the Thelliers' method below these temperatures agree with the actual values within the uncertainties. The values obtained by the other methods are not as accurate or reliable. In four out of five lavas, A. F. cleaning of 100 to 200 oe prior to each measurement of remanence in the Thelliers' method degraded the results for determination of paleointensity.