Name : FUJII,  Toshitsugu
            Position : Professor
            Division/Center : Division of Geodynamics
            Research Area : Magmatology, Petrology
            homepage : http://www.eri.u-tokyo.ac.jp/fujii/index.html
 
 

RESEARCH

Main research interest of my group is understanding the genesis of flood basalt magma and the behavior of aqueous fluid at high pressure. Through the petrological and geochemical study of the Deccan flood basalt, pristine basalts, which show least evidence of crustal contamination, have been discriminated. It has been demonstrated that the chemical signature of the pristine basalt cannot be derived from the partial melt of mantle peridotite. Combining the results of melting experiments of a MORB and peridotites at pressure of 3-5 GPa, it has been concluded that the Deccan pristine magma could be obtained by the mixing of two magmas, one formed by extensive melting of the basalt fragments included in the ascending peridotite plume, and the other formed by small degree of melting of the ambient peridotite. The basalt fragments should be the subducted ancient oceanic crust accumulated at the base of the upper mantle. Based on the high pressure experiments, we found that the aqueous fluid coexisting with mantle minerals, olivine and pyroxene, can dissolve more than 70 wt % of silicate components at 8 GPa. This result indicates that once we have water inside of the earth, we can expect the existence of fluid enriched in silicate without melting. Because the solubility of silicate in the aqueous fluid decreases with decreasing pressure, migration of silicate-enriched aqueous fluid toward the earth's surface may cause differentiation en route. Also the wetting of olivine against the aqueous fluid has been determined to understand the mobility of aqueous fluid at pressure, 3-8 GPa, and it has been shown that the wetting is incomplete at pressure below 3 GPa in hydrous upper mantle peridotite at convergent plate boundaries. We are now proposing that the wetting of olivine above 3 GPa may control the location of volcanic front at subduction zone.

PUBLICATIONS

Sato, H., Nakada, S., Fujii, T., Nakamura, M., and K. Suzuki-Kamata, Groundmass pargasite in the 1991-1995 dacite of Unzen volcano: phase stability experiments and volcanological implications, Jour. Volcanol. Geothermal Res., 89, 1/4, 197--212, 1999.

Mibe, K., Fujii, T. and Yasuda, A, Control of the location of the volcanic front in island arcs by aqueous fluid connectivity in the mantle wedge, Nature, 401, 6750, 259--262, 1999.

Fujii, T. and S. Nakada, The 15-September 1991 pyroclastic flows at Unzen Volcano (Japan): a flowing model, Journal of Volcanology and Geothermal Research, 89, 1/4, 159--172, 1999.