AN IMPROVED METHODOLOGY FOR THELLIER-TYPE PALEOINTENSITY DETERMINATION IN IGNEOUS ROCKS AND ITS USEFULNESS FOR VERIFYING PRIMARY THERMOREMANENCE

We propose a new methodology for Thellier-type paleointensity experime nts. In standard paleointensity experiments the blocking temperatures of most stable remanences are often not reached before thermal alterat ion of magnetic mineralogy begins, and subsequent data have to be disc arded. We emphasize that when alteration begins at low temperature and continues throughout the experiment (which then fails in standard ana lyses), the resulting unblocking temperature (T-ub) spectrum of the al teration product does not necessarily overlay, with the whole T-ub spe ctrum of natural remanent magnetization (NRM), because T-ub depends on physical characteristics of the new grains, not the temperatures at w hich they were formed. NRM may survive in an uncontaminated higher T-u b window. If alteration remanence only has low T-ub then partial therm oremanent magnetization (pTRM) checks at temperature step T-j-1 can be used to correct for alteration that has occurred at T-j, and furtherm ore, thermal demagnetization of a full ?RM acquired in the laboratory at the end of the progressive Thellier-Thellier experiment should reve al the true, uncontaminated T-ub spectrum of the NRM in higher tempera ture intervals. We propose a new experimental sequence which allows mo nitoring and correction of alteration and provides two semi-independen t estimates of palaeointensity in the circumstances described above. W e illustrate the issues involved through new experimental work on igne ous rocks from a Paleozoic charnockitic syenite, where we unravel TRM from multidomain isothermal remanence despite massive alteration durin g laboratory heating. Finally we point out the value of these experime nts for verifying primary TRM in igneous rocks, indicating properties that distinguish TRM from other remanence.