Testing determinations of Thellier paleointensities on 1962 and 1983 lava flows and scoriae in Miyakejima, Japan

Abstract

AbstractUsing a fully automated magnetometer, test measurements of paleointensity by the Thellier method were performed on basaltic lava sections, including upper and lower clinkers through the lava interior, and scoriae that erupted from Miyakejima Volcano in 1962 and 1983. Curie temperatures showed a bimodal distribution with titanium-rich and -poor titanomagnetites, and in many cases two phases coexisted in a single sample. Thermomagnetic analyses on all samples prior to the Thellier measurements helped set appropriate temperature steps during Thellier measurements for individual samples. The Thellier results were selected using a combination of TTA and curvature criteria to detect thermal alteration and multidomain effects. Two-thirds of the clinker and scoria samples passed the criteria, which is 4.3 times higher compared to the lava interior. Paleointensities at all six sites are not largely different from the expected geomagnetic field intensities, and the averaged paleointensities for the 1962 and 1983 lavas/scoriae are within 5% of the expected intensities. Pass and fail samples have distinctly different hysteresis properties with a threshold ratio of saturation remanence to saturation magnetization ($$M_{\rm r}/M_{\rm s}$$ M r / M s ) at 0.22. To obtain reliable Thellier paleointensities on volcanic rocks, rapidly cooled clinkers and scoriae are to be collected, screened for the hysteresis properties, and then heated according to the temperature steps assigned to each sample. Graphical abstract