10:15 〜 10:30
[SEM16-06] Microscopic observation of iron oxide grains in basalts from the Agulhas Plateau drilled during IODP Expedition 392
Basaltic units drilled at the Agulhas Plateau during IODP Expedition 392 are expected to record the absolute paleointensity at the time of the Cretaceous Normal Superchron. We have been conducting paleointensity experiments by the Tsunakawa-Shaw method on samples taken from three of the six basaltic sills from Site U1580. However, the rate of samples that passed the standard acceptance criteria for reliable paleointensity estimate is only about 25%, even though the samples used in the experiments were preselected based on paleomagnetic and rock-magnetic analyses such as alternating field demagnetization (AFD) of the natural remanent magnetization (NRM) by the on-board superconducting rock magnetometer (SRM), magnetic hysteresis measurements, and thermomagnetic analysis. Therefore, we performed scanning electron microscope (SEM) and reflection microscope observations of polished sections of the sister samples used in paleointensity experiments, in order to clarify the characteristics of magnetic minerals in samples that passed or were rejected and to obtain a guideline for sample selection. In the rejected samples, euhedral or dendritic iron oxides with grain sizes ranging from a few hundred micrometers to over 1 mm were observed, and many of them showed Ti-rich fine lamellae. On the other hand, the passed samples did not show such coarse-grained iron oxides; instead they contained fine particles with grain sizes less than 10 micrometers. These results indicate that cooling rate is a key factor controlling the stability of the magnetic carriers. Indeed, all of the passed samples were collected within 80 cm from the upper or lower margin of a sill, while most of the rejected samples were taken from the middle of a sill. Therefore, we conclude that rapidly cooled areas near the margin of a sill should be selected for paleointensity experiments.