Japan Geoscience Union Meeting 2018

Presentation information

[EE] Poster

S (Solid Earth Sciences) » S-IT Science of the Earth's Interior & Tectonophysics

[S-IT22] Interaction and Coevolution of the Core and Mantle in the Earth and Planets

Tue. May 22, 2018 10:45 AM - 12:15 PM Poster Hall (International Exhibition Hall7, Makuhari Messe)

convener:Tsuyoshi Iizuka(University of Tokyo), Hidetoshi Shibuya(Department of Earth and Environmental Sciences, Faculty of Advanced Science and Technology, Kumamoto University), Taku Tsuchiya(愛媛大学地球深部ダイナミクス研究センター, 共同), Kenji Ohta(Department of Earth and Planetary Sciences, Tokyo Institute of Technology)

[SIT22-P39] A variety of Thellier behaviors from vertical sections of an obsidian-rhyolite complex

*Koji Fukuma1 (1.Department of Environmental System Science, Faculty of Science and Engineering, Doshisha University)

Keywords:paleointensity, FORC, obsidian

Obsidian, glassy phase of rapidly cooled rhyolitic magma, contains fine grained Ti-poor titanomagnetites that are expected to preserve reliable records of ancient geomagnetic field intensities. Varying lithologies from glassy to cloudy obsidian or even to crystalline rhyolite generally appear in a vertical section of an obsidian-rhyolite complex controlled by different cooling rate within the complex. During Thellier experiments for determining paleointensities, rock specimens exhibit different behaviors depending on the grain size. Therefore vertical sections of obsidian-rhyolite complexes should provide great opportunities for testing natural examples for grain size dependence of Thellier behaviors.
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.