IV. Geological Study

2. Petrology and Mineralogy

The years 1995-1998 have seen significant advances in petrology and mineralogy of volcanic rocks by Japanese earth scientists for varying aspects. A large number of papers were published on subduction zone volcanism as a natural consequence of Japan being a representative island arc. However, researches on plumes, oceanic island basalts and mid-oceanic ridge basalts also increased. These works focuses more on the melting processes in the mantle and dehydration processes in the subducted oceanic crust . Tatsumi et al. (1995) discussed the origin of the third volcanic chain in Kamchatka. Tatsumi and Kogiso (1997) and Kogiso et al. (1997a; 1997b) made estimates of trace element and Sr-Nd-Pb isotope compositions of the subducting basaltic oceanic crust, the hydrous peridotite layer, fluid phase, and the mixing ratio of the fluid with the mantle wedge, and discussed the origin and nature of arc magma and oceanic island basalts. Melting of subducted sediments were investigated by Shimoda et al. (1998). High pressure experiments were carried out by Takahashi et al. (1998) for investigating the origin of plumes that formed Columbia River Basalt, whereas Tatsumi made geochemical constraints for the formation of Polynesian superplumes.

Constraints from across-arc variation were made for B/Nb and B Isotopes of lavas from Kuril islands (Ishikawa and Tera, 1997), Sr, Nd isotopes and trace element compositions from Quaternary basaltic lavas in Northeastern Japan (Shibata and Nakamura, 1997), Li isotopes (Moriguti and Nakamura, 1998), and major element compositions for Quaternary volcanic rocks from Hokkaido (Nakagawa et al., 1995). Temporal variation in primary magma compositions in Northeast Japan arc was discussed by Yamashita et al. (1996). Experimental constraints on the genesis of arc magmas were also made by Kawamoto (1996), Fujinawa and Green (1997), Nakajima and Arima (1998).

General schemes of Quaternary volcanism of Japanese islands and petrological models for magma genesis is well summarized by Uto and Tatsumi (1996). Petrological studies of individual volcanoes or volcano groups in Japan include: Norikura area (Kaneko, 1995), Shirahama Group, Izu (Tamura, 1995; Tamura and Nakamura, 1996), Taradake (Ikawa and Nagao, 1996), Gassan (Inoue and Ban, 1996), Hachijojima Volcano Group (Nakano, et al., 1997), Miyake-jima (Sato et al., 1996; Amma-Miyasaka and Nakagawa, 1998), borehole core from Fuji (Togashi, et al. 1997), Kurohana-yama (Funagata) (Ohba and Ban, 1997; Ban et al., 1997), Aira caldera (Arakawa et al., 1998), Akagi (Horio and Umino, 1995; Umino and Horia, 1998), Rishiri (Kuritani, 1998), Okinawa Troughs (Shinjo, 1998).

More petrological studies were focused on Miocene or older volcanism than before including Miocene volcanism in Hokkaido (Goto et al., 1995; Ikeda, 1998), Hokuriku Province (Ishiwatari and Ohama, 1997), Oki-Dogo Island (Kobayashi and Sawada, 1998), Setouchi region (Matsuura, 1997).

Petrological works were extended outside Japan for the study of volcanism under various tectonic settings, including: Anatolia, Turkey (Notsu et al., 1995), 1991 Pinatubo eruption products (Hattori and Sato, 1996), Mesozoic intracontinental basalts from Yunnan, Southern China (Okamura et al., 1997), Tongariro Volcanic Centre, Taupo Volcanic Zone (Nakagawa et al., 1998), late Cenozoic volcanic rocks from West Java (Sunardi and Kimura, 1998)

Mineralogical studies include those of Kagi (1998), Kitamura (1995), Konishi and Akai (1995), Makino et al., (1996, 1997), Matsubara et al., (1998), Nakano and Suwa (1998).

Studies focusing on the processes of magma transport and storage in the shallow magma reservoir include Ida (1995), Nakamura (1995a; 1995b), Tomiya (1995). Toramaru (1995) investigated nucleation and growth of bubbles in viscous magmas. Tsukui and Suzuki (1995) studied vesiculation of basaltic magma to find the difference between magmatic versus phreatomagmatic eruptions. Water content of pre-eruptive magmas, an important parameter to control the physical properties of magma were studied by Miyagi and Yurimoto (1995), Miyagi et al. (1997), and Miyagi et al. (1998). Nakamura and Shimakita (1998) carried out partial dissolution experiments to reproduce melt inclusions in plagioclase during magma mixing of arc magmas.

Petrological studies on mantle materials are found in the works of Abe et al. (1995), Arai and Abe (1995), Arai and Yurimoto (1995), Arai et al., (1997), Arai and Matsukage (1998), Morishima et al. (1995), and Ozawa and Shimizu (1995). Petrological study of Horoman peridotite complexes were the main focus of mantle materials in Japan (Ozawa and Takahashi, 1995; Takahashi, 1997; Takazawa et al., 1996; Yoshida and Takahashi, 1997). Crustal xenoliths included in lavas were studied by Yasui et al. (1998) and Yokose and Yamamoto (1996).

Experimental studies and numerical modeling on melting and physical properties of mantle materials include Ando (1998), Irifune (1996), Kojitani and Akaogi (1997), Ohtani et al. (1995), Ohtani (1998), Inoue (1998), Ito et al. (1996), Iwamori et al. (1995), Iwamori (1997), Akaogi (1997), Funakoshi and Utsumi (1998), Hirose and Kawamoto (1995), Hirose (1997), Kawamoto et al. (1996), Kogiso et al. (1998), Miyajima (1998), Nakamura and Kushiro (1998), Sato et al, (1997) and Sato (1997).

Igneous rock textures were investigated from experimental and statistical approaches. Sato (1995) revealed that a slight difference in the degree of undercooling of magmas before final degassing and eruption may have caused the large difference in the population density of plagioclase of the lavas with identical composition. Statistical description of the spatial distribution of minerals in rocks were developed and applied to intrusive rocks (Morishita and Obata, 1995a; 1995b; Morishita, 1995; Morishita et al., 1998). Wada (1995) made a fractal analysis of boundary patterns of the heterogeneous ejecta from the Me-akan volcano, eastern Hokkaido in order to investigate mixing mechanism of magmas with different compositions. Other contributions include those of image analyses of rock textures (Ikeda et al., 1997; Togami et al., 1998), and modal analyses (Nishimoto, 1996).

Petrological descriptions of volcanic ashes collected at the onset of volcanic eruption contributed for volcanic hazard researches (Hatae et al., 1997; Ui et al., 1997).

(Toshiaki Hasenaka)

References of geology

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