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Magnetic petrology and its implication for magma mixing of the 1991–1995 dacite at Unzen volcano, Japan

Abstract

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 titanomag-netites 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.

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Correspondence to Takeshi Saito.

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Saito, T., Ishikawa, N. Magnetic petrology and its implication for magma mixing of the 1991–1995 dacite at Unzen volcano, Japan. Earth Planet Sp 59, 863–870 (2007). https://doi.org/10.1186/BF03352748

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Key words

  • Unzen volcano
  • lava dome
  • iron-titanium oxide
  • magnetic petrology
  • magma mixing