[SIT22-P39] A variety of Thellier behaviors from vertical sections of an obsidian-rhyolite complex
Keywords:paleointensity, FORC, obsidian
From three boreholes of the Takanoobane obsidian-rhyolite complex in Aso volcano, Japan, obsidian samples were collected from one upper and three lower sections. Glassy to cloudy obsidians contain superparamagnetic (SP) to single-domain (SD) grains as revealed by room- or low-temperature frequency dependence of susceptibility. Glassy obsidians exhibit finer superparamagnetic grain size than cloudy obsidians. First order reversal curve (FORC) measurement results support the mixture of SP and SD grains as evidenced by the horizontal ridges that spread in low coercivity range and continued to the vertical reversible ridges. However, some obsidians showed higher coercivities reaching 100 mT and high interacting field. Rhyolite samples, which were collected from the central part of the complex, yielded typical multidomain FORC diagrams.
Thellier paleointensity measurements were performed using a fully automated magnetometer-furnace system Tspin for 34 specimens by applying 45 microT. Some of the obsidian samples exhibited high and narrow unblocking temperatures within 50 deg.C below the Curie temperature of magnetite (580 deg.C) and represented straight lines on Arai diagrams showing about 45 microT. This kind of samples correspond to high coercivity and interacting FORC diagrams. On the other hand, other glassy obsidians exhibited high-slope straight lines of about 100 microT and cloudy obsidians showed sigmoid curves on the Arai diagrams. Rhyolite samples have typical multidomain-type upward concave curves.
A variety of Thellier behaviors were observed for the Takanoobane obsidian-rhyolite complex in accordance with the vertical position. Coarse-grained rhyolite and cloudy obsidians did not give straight segments on Arai diagrams, therefore are not suitable for Thellier experiments. Glassy obsidians containing SP-SD grains can produce straight lines but not yield consistent paleointensities.