EFFECT OF COOLING RATE ON INTENSITY OF THERMOREMANENT MAGNETIZATION - EXPERIMENTAL-STUDY AND THEORETICAL IMPLICATIONS

The experiments reported here compare intensity and thermal resistance of two thermal remanent magnetizations (TRM), successively acquired by the same sample under a magnetic field, but at two different cooling speeds. The samples (one with a base of CrO2 powder, and the others derived from oceanic basalts) were representative of the different types of structure domains characteristic of magnetic grains. The experimental procedure established aimed at limiting the various factors of uncertainty: mineralogical and structural modifications related to heating; inhomogeneity of the sample temperature at crucial stages of each experiment; effects of undesirable magnetizations acquired concurrently with the TRM. The main conclusion of this study is that the TRM intensity increases as a function of time of cooling, independently of the types of structure domains carried by the magnetic grains. The greatest effects is with the CrO2 monodomain: the TRM obtained between 130 and 100-degrees-C during a cooling period of 120 h was 19% superior to the one obtained during a passive cooling period of 30 min for the same temperature limits, the measurements having been taken at 50-degrees-C. Both types of oceanic samples analyzed (pseudo-monodomains and multidomains) showed a similar relative variation, but that much weaker. For example, the TRM induced on cooling from 200 to 50-degrees-C (120 h) provided values only 6% higher than the one obtained during passive cooling (periods of 30-40 min). The theoretical implications of these results are discussed.