Numerical investigations of multiple phase saturation and fractional crystallization differentiation were done for a basaltic melt with major element composition identical to Innomarubi lava (INM melt), which is a phenocryst-poor, relatively undifferentiated basaltic lava erupted at ca. 1.5 ka from Fuji volcano, by using "PELE" (Boudreau, 1999), a thermodynamic phase equilibrium simulator for silicate melt, to understand the origin of whole rock compositional variation observed for volcanic

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... s erupted in the last 2200 years from Fuji volcano. The results of multiple phase saturation simulation indicate that INM melt coexists with olivine, plagioclase and clinopyroxene, which are observed as phenocryst minerals in Innomarubi lava, under conditions of temperature of ca. 1160℃, pressure of ca. 580 MPa and H 2 O content in melt of ca. 0.85 wt. % with oxygen fugacity condition near Ni-NiO buffer. Fractional crystallization differentiation simulation indicates that plagioclase and clinopyroxene are major crystallized phases and, as a result, Al 2 O 3 and CaO contents in melt decrease with cooling. However, the calculated LLD (liquid line of descent) failed to explain the whole rock compositional variation observed for natural volcanic rocks from Fuji volcano. Al 2 O 3 and CaO contents are lower and TiO 2 , FeO * and K 2 O contents are higher for the calculated LLD melt than natural volcanic rocks at the same MgO content, indicating that some process other than fractional crystallization and mixing of LLD melts are required. Up to 25 wt. % addition of plagioclase component to calculated LLD melt can explain most of the compositional variation natural volcanic rocks reveal. This indicates that plagioclase accumulation is a primary process to form the observed whole rock compositional variation of natural volcanic rocks.