Abstract
Natural rock samples may not always be the ideal material for the Thellier-type method as they occasionally result in high paleointensities. The Kilauea 1960 lava, Hawaii, is one such example. Several previous studies have suggested that one of the possible causes for this undesirable behavior is an acquisition of thermochemical remanent magnetization (TCRM) during lava formation. In order to examine this possibility quantitatively, equilibrium temperatures of titanomagnetite grains, which are associated with samples previously subjected to Thellier experiments, are estimated by a Fe-Ti oxide geothermometer. The results show that two specimens from the rock magnetic group giving relatively ideal Thellier paleointensities have clustered equilibrium temperatures of about 800-900 and 700-800°C. In contrast, two swarmed temperatures around 300 and 700°C were observed for the specimen from a group yielding high paleointensities. Although these are semi-quantitative estimates, when the time scales of Fe-Ti interdiffusion and lava cooling are taken into consideration, the last specimen could have acquired the TCRM during its formation. For such specimens, simple calculation predicts that TCRM/TRM (thermoremanent magnetization) ratios could be 1.19-1.72 for the blocking temperature range of 400-480°C, assuming a grain-growth model. The extent of this overestimation (20-70%) is comparable to the magnitude of the observations. It is therefore suggested that attention be paid to titanomagnetite grains with well-developed ilmenite lamellae, as these could be potential sources of overestimations of Thellier paleointensities of up to a few tenths of percentage points.
