Multidomain behavior during Thellier paleointensity experiments: results from the 1915 Mt. Lassen flow

Paleointensity experiments using the Thellier method on samples of the 1915 Mt. Lassen dacite flow yielded concave and S-shaped NRM–TRM curves on Arai plots. Departure of these curves from the ideal straight line corresponding to the known geomagnetic intensity of 54 μT for the region in 1915 is significant. The magnetic properties of these rocks change very little during heating, and hysteresis experiments that suggest pseudo-single domain to multidomain (MD) magnetic grains are confirmed by microscopic observations. Thus, the curvature appears to be due to MD-like behavior rather than to irreversible alteration of magnetic minerals. In support of this interpretation, the slope of the line through just the initial and the final point yields an average value of 52 μT that is in good agreement with the known field intensity. Using the slope of only the lower or higher temperature points, which is never advisable for persistently curved Arai plots such as these, would lead to estimates that are 30–80% too high or too low. S-shaped NRM–TRM curves are atypical in paleointensity studies, and the usual explanation in terms of MD unblocking temperatures less than blocking temperatures cannot account for the convex curvature of the higher-temperature part. Rather, the convex curvature can be explained by the predominance of remanence with unblocking temperatures greater than blocking temperatures, the so-called tail of pTRM*, acquired in the higher-temperature part of the pTRM spectrum during the Thellier experiment.