Magnetic properties of Kurokami pumices from Mt. Sakurajima, Japan

Both chromite and low-Ti titanomagnetite are carriers of natural remanent magnetization (NRM) in Kurokami andesitic pumices of Mt. Sakurajima, Japan. Thermal demagnetization of the NRM and of a laboratory isothermal remanence indicate unblocking of chromite remanence between 200 and 260°C and of titanomagnetite remanence between 400 and 520°C. Electron microprobe analyses support the compositions Fe1.9Cr1.11O4 and Fe2.9Ti0.1O4 indicated by the respective Curie temperatures of 260°C and 520–530°C. Chromite is thermally unstable and converts to magnetite during thermal demagnetization, leading to a self-reversal of NRM between 500 and 580°C. The chromite contributes about 25% of the NRM intensity of the sample.