Earth Planets Space, Vol. 59 (No. 7), pp. 863-870, 2007
Takeshi Saito1 and Naoto Ishikawa2
1Institute for Geothermal Sciences, Kyoto University, Noguchibaru, Beppu, Oita
2Graduate School of Human and Environmental Studies, Kyoto University, Yoshida-Nihon-Matsu, Sakyo-ku, Kyoto 606-8501, Japan
(Received September 30, 2006; Revised March 16, 2007; Accepted March 23, 2007; Online published July 20, 2007)
Magnetic petrologic analyses were carried out on lava samples from the lava dome of the 1991-1995 eruption of Unzen volcano, Japan. As a result, three kinds of titanomagnetites with different Curie temperatures were identified and the difference between the samples from the lava dome which grew exogenously and endogenously were revealed. Titanomagnetites with Tc of 460-500°C and 380-400°C are the predominant magnetic minerals. In addition, a titanomagnetite with Tc of ∼540°C, but in low concentration, is also present. The exogenous lava samples contain more titanomagnetite with Tc of 380-400°C and less microphenocrysts of titanomagnetites than the endogenous samples. The equilibrium temperature of 900-920°C, 780-820°C and 720°C is estimated from titanomagnetite with Tc of 380-400°C, 460-500°C and 540°C, respectively. We suggest that these titanomagnetites are probably formed by magma mixing and eruption processes. Titanomagnetite with Tc of 460-500°C was derived from felsic magma (780-820°C) in a shallow chamber. Titanomagnetite with Tc of 380-400°C was produced from the mixed magma (900-920°C). After being squeezed out from the reservoir, a small amount of titanomagnetite with Tc of ∼540°C was crystallized.
Key words: Unzen volcano, lava dome, iron-titanium oxide, magnetic petrology, magma mixing.