THEORY OF CRM ACQUIRED BY GRAIN-GROWTH, AND ITS IMPLICATIONS FOR TRM DISCRIMINATION AND PALEOINTENSITY DETERMINATION IN IGNEOUS ROCKS

The behaviour of grain-growth CRM in SD grains can be predicted using Neel's (1949) theory for the acquisition of TRM. This theoretical appr oach suggests that the ratio of CRM to TRM will not be constant throug hout the grain-size range for either magnetite or haematite. Hence the blocking-temperature spectra for CRM and TRM in an identical set of m agnetic grains will be different, and grain-growth CRM can be identifi ed by non-linear palaeointensity plots over certain temperature interv als. It is shown that on thermal demagnetization both CRM and TRM shou ld unblock at the same temperature, T-b, 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 TR M is controlled by the final volume and the blocking temperature. CRM/ TRM ratios for magnetite and haematite are calculated using the standa rd relaxation-time equation, and experimental values of spontaneous ma gnetization as a function of temperature. Calculation of CRM/TRM ratio s for actual examples of Tb spectra suggest that grain-growth CRM in b oth 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 degree s C, or for smaller, high-temperature intervals. However, grain-growth CRM cannot be distinguished from TRM if pTRM checks fail below about 400 degrees C, as the CRM/TRM ratio is close to 1 below this temperatu re. Single-domain CRM grown over laboratory time-scales should always be smaller than a laboratory TRM according to this model, while natura l CRM formed over much longer times than available in the laboratory m ay 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 dom ain walls during low-temperature crystal growth.