[SEM18-03] Paleomagnetic study of Holocene lava flow age at Nakadake volcano in Aso caldera, Kyushu Japan: Contribution to establish the Eruption history
Keywords:paleomagnetic secular variation curve, Aso Nakadake
The samples collected form 14 lava flows and 6 agglutinates layers (welded scoria-fall deposits) were used for paleomagnetic and rock-magnetic measurements. These samples, from visual inspection, seemed to be influenced chemical alteration of their surface by sulfides of volcanic gas. To check the effect of the chemical alternation on the lavas and agglutinates, thermomagnetic analyses were made on chip samples from the top (surface of rock) and bottom (inside of rock) of the collected paleomagnetic cores. The thermomagnetic analyses indicate that the core top and bottom are approximately the same, in spite of different color, and the carrier of magnetization of each core is either titanium rich (x is about 0.6) or poor (x is about 0.1 to 0.2) titanomagnetites. The natural remanent magnetization of each sample shows a simple, single vector component in alternating field demagnetization experiments, which well defines the primary component. Site mean directions can be categorized into three different direction groups. These data suggest that the eruption producing lava flows and/or agglutinates occurred at three different ages. Furthermore, the paleomagnetic directions of two groups are not consistent with the directions of the eruptive ages of Nakadake young edifice assigned from the present stratigraphy. Comparing these directions with the paleomagnetic secular variation curve which drew from basaltic volcanoes in Aso central cones, these direction ages assigned to the age around 5 ka, 4.5ka, 3.5 ka, respectively.