Experimental Evaluation of Remanence Carriers Using the Microcoercivity‐Unblocking Temperature Diagram

Abstract

AbstractKnowledge of remanence carriers in recording media is crucial for obtaining reliable paleomagnetic data. In this paper, we report a newly developed technique to determine remanence carriers in natural rock samples that uses anhysteretic remanent magnetization (ARM) spectra in the microcoercivity (hc) and unblocking temperature (TUB) planes. The distribution of ARM carriers was determined via systematic stepwise demagnetization experiments of ARM combining thermal demagnetization and alternating field demagnetization treatments and calculating the change in ARM intensity on the hc‐TUB plane (hc‐TUB diagram). Samples from three basaltic lavas and two welded tuffs were analyzed. Combining the hc‐TUB diagram with the change in ARM coercivity spectra, both decreases and increases in magnetic minerals due to the thermal alteration during laboratory heating could be identified. From experimental samples, ARM carrier components were identified as being original rather than produced as a result of thermal alteration. In the case of the Izu‐Oshima 1986 A lava flow (1986La) sample, it was confirmed from the hc‐TUB diagram that high‐hc components can be separated using stepwise alternating field demagnetization treatment but not stepwise thermal demagnetization treatment. Paleointensity experiments were also conducted on Izu‐Oshima 1986 A lava flow samples. Two of the three paleointensity estimates were greater than the expected field intensity, suggesting a certain rock‐magnetic property that affects the paleointensity estimation. The hc‐TUB diagrams of the Izu‐Oshima 1986 A lava flow samples suggest that the differences in the paleointensity estimates were likely caused by different degrees of the high‐temperature oxidation state.