The Determination of Paleo-Intensities of the Earth's Magnetic Field with Emphasis on Mechanisms which Could Cause Non-ideal Behavior in Thellier's Method

Abstract

95 NRM-TRM curves were determined by Thellier's method from a variety of volcanic rocks. Most of them deviate from a straight line over parts of their length, sometimes so much that not even a crude estimate of paleo-intensity can be made. Some of the many possible causes of such non-ideal behavior include the effects of the sample demagnetizing field, secondary components of magnetization, mechanisms of acquisition of TRM which violate the assumptions of Thellier's method (such as nonlinearity of TRM with field), changes in the TRM spectrum induced by heating in the laboratory, and others. Where possible these mechanisms are discussed from both a theoretical and experimental standpoint, and their effects are identified in the NRM-TRM curves. In addition, diagnostic tests designed to determine quickly the suitability of a rock for intensity studies were sought. Tests tried included the comparison of heating and cooling Js-T curves, measurement of susceptibility before and after heating, and others. None were adequate. Finally, the quicker method of simply comparing the NRM and the total TRM is compared with Thellier's method for determining paleo-intensities. The latter is clearly the more informative and reliable when dealing with individual units, but the former may be useful for deriving average values of the paleo-intensity during geologic periods from large suites of volcanic rocks of varying types.