Theory of CRM acquired by grain growth, and its implications for TRM discrimination and palaeointensity determination in igneous rocks

Abstract

The behaviour of grain-growth CRM in SD grains can be predicted using Néel's (1949) theory for the acquisition of TRM. This theoretical approach suggests that the ratio of CRM to TRM will not be constant throughout the grain-size range for either magnetite or haematite. Hence the blocking-temperature spectra for CRM and TRM in an identical set of magnetic grains will be different, and grain-growth CRM can be identified by non-linear palaeointensity plots over certain temperature intervals. It is shown that on thermal demagnetization both CRM and TRM should unblock at the same temperature, Tb, but their magnitudes will be different, because the net fractional alignment for CRM is controlled by the blocking volume and the reaction temperature, while that for TRM is controlled by the final volume and the blocking temperature. CRM/TRM ratios for magnetite and haematite are calculated using the standard relaxation-time equation, and experimental values of spontaneous magnetization as a function of temperature. Calculation of CRM/TRM ratios for actual examples of Tb spectra suggest that grain-growth CRM in both magnetite and haematite can be distinguished from TRM on the basis of a Thellier-Thellier palaeointensity experiment for data that span a temperature interval from room temperature up to at least 450 °C, or for smaller, high-temperature intervals. However, grain-growth CRM cannot be distinguished from TRM if pTRM checks fail below about 400 °C, as the CRM/TRM ratio is close to 1 below this temperature. Single-domain CRM grown over laboratory time-scales should always be smaller than a laboratory TRM according to this model, while natural CRM formed over much longer times than available in the laboratory may be as much as twice as strong as TRM. Multidomain grain-growth CRM may always be larger than TRM, due to the difficulty of nucleating domain walls during low-temperature crystal growth.