Repeated magmatic eruptions of Tokachidake volcano have caused severe volcanic disasters on three occasions during the 20th century. To prepare for the next eruptive activity, understanding the structure of the magma plumbing system by using petrological analysis of juvenile materials is crucial. Here, we perform petrological analysis of juvenile materials to investigate the difference between two contrasting eruptions in 1962 and 1988–1989, respectively. All these juvenile materials are composed of mafic andesite, which were formed by mixing of olivine-bearing basaltic and pyroxene andesitic magmas. The compositional zonations of olivine phenocrysts in all of these rocks suggest that the injection of the basaltic magma into the andesitic magma occurred several months prior to the 1962 eruption and about six months before the 1988–1989 eruption. In the case of the 1962 activity, the mixed magma rapidly ascended without stagnation from the magma chamber and erupted as a sub-Plinian type. However, the juvenile materials of the 1988–1989 eruptions show distinct petrological features such as higher crystallinity of the matrix, orthopyroxene reaction rims around the olivine, and overgrowth mantle zones around Ti-magnetite phenocrysts. These features suggest that the mixed magma ascended slowly and possibly stagnated at shallower levels prior to eruption. The stagnated magma became a cap rock of the vent system and caused a series of Vulcanian eruptions. These distinct modes of magma ascent can be explained by differences in the magma supply rate. In the case of the 1962 eruption, the volume of magma that erupted in a period of less than 24 h was 7.1 × 10⁷ m³. On the contrary, 23 explosions occurred over three months of the 1988–1989 activity and generated 1 × 10⁵ m³ of ejecta including juvenile and non-juvenile materials. These large eruption rate differences can be attributed to the distinct ascent rates of the magma between the two eruptive activities.

1. [1] T. Ishikawa, I. Yokoyama, Y. Katsui, and M. Kasahara, “Tokachi-dake, its volcanic geology, history of eruption, present state of activity and prevention of disasters,” Committee for Prevention of the Natural Disasters of Hokkaido, Sapporo, 1971 (in Japanese with English abstract).
2. [2] Y. Katsui, S. Kawachi, Y. Kondo, Y. Ikeda, M. Nakagawa, Y. Gotoh, H. Yamagishi, T. Yamazaki, and M. Sumita, “The 1988-1989 explosive eruption of Tokachi-dake, central Hokkaido, its sequence and mode: Special Section: The 1988-1989 Eruption of Mt. Tokachi, Central Hokkaido,” Bull. Volcanol. Soc. Japan, Vol.35, No.2, pp. 111-129, 1990.
3. [3] Y. Ishizuka, M. Nakagawa, and S. Fujiwara, “Geological map of Tokachidake volcano, 1:30,000,” Geological Survey of Japan, p. 7, 2010.
4. [4] Japan Meteorological Agency, “10. Tokachidake,” National catalogue of the active volcanoes in Japan (the 4th edition), pp. 1-39, 2013.
5. [5] R. Tanaka, T. Hashimoto, N. Matsushima, and T. Ishido, “Permeability-control on volcanic hydrothermal system: case study for Mt. Tokachidake, Japan, based on numerical simulation and field observation,” Earth Planets Space, Vol.69, No.1, Article: 39, doi:10.1186/s40623-017-0623-5, 2017.
6. [6] R. Takahashi and M. Yahata, “Effects of subvolcanic hydrothermal systems on edifice collapses and phreatic eruptions at Tokachidake volcano, Japan,” J. Volcanol. Geotherm. Res., Vol.352, pp. 117-129, 2018.
7. [7] Y. Ikeda, Y. Katsui, M. Nakagawa, S. Kawachi, T. Watanabe, N. Fujibayashi, T. Shibata, and H. Kagami, “Petrology of the 1988-89 essential ejecta and associated glassy rocks of Tokachi-dake volcano in central Hokkaido, Japan: Special Section: The 1988-1989 Eruption of Mt. Tokachi, Central Hokkaido,” Bull. Volcanol. Soc. Japan, Vol.35, No.2, pp. 147-162, 1990.
8. [8] S. Fujiwara, M. Nakagawa, S. Hasegawa, and D. Komatsu, “Eruptive history of Tokachi-dake volcano during the last 3,300 years, Central Hokkaido, Japan,” Bull. Volcanol. Soc. Japan, Vol.52, No.5, pp. 253-271, 2007 (in Japanese with English abstract).
9. [9] A. Tomiya and E. Takahashi, “Evolution of the magma chamber beneath Usu volcano since 1663: a natural laboratory for observing changing phenocryst compositions and textures,” J. Petrol., Vol.46, No.12, pp. 2395-2426, 2005.
10. [10] A. Matsumoto and M. Nakagawa, “Formation and evolution of silicic magma plumbing system: Petrology of the volcanic rocks of Usu volcano, Hokkaido, Japan,” J. Volcanol. Geotherm. Res., Vol.196, No.3-4, pp. 185-207, 2010.
11. [11] R. Takahashi and M. Nakagawa, “Evolution and eruption processes of a highly porphyritic silicic magma system: Petrology of the historical eruptive stage of Hokkaido-Komagatake volcano, Japan,” J. Petrol., Vol.56, No.6, pp. 1089-1112, 2015.
12. [12] M. Amma-Miyasaka and M. Nakagawa, “Evolution of deeper basaltic and shallower andesitic magmas during AD 1469–1983 eruptions of Miyake-jima volcano, Izu-Mariana arc: Inferences from temporal variations of mineral compositions in crystal-clots,” J. Petrol., Vol.44, No.12, pp. 2113-2138, 2003.
13. [13] Y. Suzuki, A. Yasuda, N. Hokanishi, T. Kaneko, S. Nakada, and T. Fujii, “Syneruptive deep magma transfer and shallow magma remobilization during the 2011 eruption of Shinmoe-dake, Japan – Constraints from melt inclusions and phase equilibria experiments,” J. Volcanol. Geotherm. Res., Vol.257, pp. 184-204, 2013.
14. [14] A. Tomiya, I. Miyagi, G. Saito, and N. Geshi, “Short time scales of magma-mixing processes prior to the 2011 eruption of Shinmoedake volcano, Kirishima volcanic group, Japan,” Bull. Volcanol., Vol.75, No.10, Article: 750, doi:10.1007/s00445-013-0750-1, 2013.
15. [15] S. Uesawa, “A study of the Taisho lahar generated by the 1926 eruption of Tokachidake volcano, central Hokkaido, Japan, and implications for the generation of cohesive lahars,” J. Volcanol. Geotherm. Res., Vol.270, pp. 23-34, 2014.
16. [16] F. Tada and H. Tsuya, “The eruption of the Tokachidake volcano, Hokkaido, on May 24th, 1926,” Bull. Earthq. Res. Inst. Univ. Tokyo, Vol.2, pp. 49-84, 1927 (in Japanese with English abstract).
17. [17] Y. Katsui, T. Takahashi, Y. Ôba, Y. Hirai, M. Iwanaga, T. Nishimura, T. Soya, and H. Itô, “1962 eruption of Tokachi-dake, Hokkaido,” J. Japan Assoc. Mineral. Petrol. Econ. Geol., Vol.49, pp. 213-226, 1963 (in Japanese with English abstract).
18. [18] T. W. Sisson and T. L. Grove, “Experimental investigations of the role of H₂O in calc-alkaline differentiation and subduction zone magmatism,” Contrib. Mineral. Petrol., Vol.113, No.2, pp. 143-166, 1993.
19. [19] D. Takagi, H. Sato, and M. Nakagawa, “Experimental study of a low-alkali tholeiite at 1–5 kbar: optimal condition for the crystallization of high-An plagioclase in hydrous arc tholeiite,” Contrib. Mineral. Petrol., Vol.149, No.5, pp. 527-540, 2005.
20. [20] J. Crank, “The mathematics of diffusion,” Second edition, Oxford University Press, 1975.
21. [21] S. Chakraborty, “Rates and mechanisms of Fe–Mg interdiffusion in olivine at 980^◦–1300^◦C,” J. Geophys. Res. Solid Earth, Vol.102, No.B6, pp. 12317-12331, 1997.
22. [22] R. Dohmen, H. -W. Becker, and S. Chakraborty, “Fe-Mg diffusion in olivine I: experimental determination between 700 and 1,200^◦C as a function of composition, crystal orientation and oxygen fugacity,” Phys. Chem. Minerals., Vol.34, pp. 389-407, 2007.
23. [23] L. A. Coogan, A. Hain, S. Stahl, and S. Chakraborty, “Experimental determination of the diffusion coefficient for calcium in olivine between 900^◦C and 1500^◦C,” Geochim. Cosmochim. Acta, Vol.69, No.14, pp. 3683-3694, 2005.
24. [24] A. Jambon, P. Lussiez, R. Clocchiatti, J. Weisz, and J. Hernandez, “Olivine growth rates in a tholeiitic basalt: an experimental study of melt inclusions in plagioclase,” Chem. Geol., Vol.96, No.3-4, pp. 277-287, 1992.
25. [25] P. L. Roeder and R. F. Emslie, “Olivine-liquid equilibrium,” Contrib. Mineral. Petrol., Vol.29, No.4, pp. 275-289, 1970.
26. [26] D. R. Baker and D. H. Eggler, “Compositions of anhydrous and hydrous melts coexisting with plagioclase, augite, and olivine or low-Ca pyroxene from 1 atm to 8 kbar: application to the Aleutian volcanic center of Atka,” Am. Min., Vol.72, No.1-2, pp. 12-28, 1987.
27. [27] P. Beattie, “Olivine-melt and orthopyroxene-melt equilibria,” Contrib. Mineral. Petrol., Vol.115, No.1, pp. 103-111, 1993.
28. [28] M. Nakamura, “Continuous mixing of crystal mush and replenished magma in the ongoing Unzen eruption,” Geology, Vol.23, No.9 pp. 807-810, 1995.
29. [29] R. Freer and Z. Hauptman, “An experimental study of magnetite-titanomagnetite interdiffusion,” Phys. Earth Planet. In., Vol.16, No.3, pp. 223-231, 1978.