FLOW UNDER THE ISLAND ARC OF JAPAN AND LATERAL VARIATION OF MAGMA CHEMISTRY OF ISLAND ARC VOLCANOES

Abstract

Dislocation densities of olivine grains in peridotite nodules from Ichinomegata (25 samples), Sannomegata (18 samples), Oki-Dogo (13 samples), Hamada (3 samples) and Takashima (10 samples) were measured for estimating differential stress in the upper mantle. The density was in the range of 106-107cm-2. The differential stress is estimated as 100-300 bars using KOHLSTEDT and GOETZE'S empirical relationship (1974) between the dislocation density and the differential stress. Strain rate is inferred from the geotherm of the island arc and the flow law of olivine single crystal proposed by KOHLSTEDT and GOETZE (1974) and DURHAM and GOETZE (1977). Strain rate in the upper mantle from 30 to 100km depth is less than 10-15sec-1, but it is nearly constant around 10-13-10-12sec-1 from 100 to 200km depth.The convective flow induced by descending oceanic plate is suggested and the velocity of the return flow is estimated to be 5cm/year. The return flow of the upper mantle toward the trench causes the lateral variation of magma chemistry and the upper mantle materials, if partially molten liquid continuously flows out from the moving upper mantle. The model in this study gives a relation between the horizontal distance from the volcanic front, X, and the concentration ratio of Cf1/C1 (Cf1; concentration at volcanic front) as follows;Cf1/C1=exp(-βφX/v), in which β and v are the rate of outflow of the liquid per unit volume of the upper mantle and the mean velocity of the return flow, and φ is a constant.