DIFFERENCE IN N2 AR RATIO OF MAGMATIC GASES FROM NORTHEAST AND SOUTHWEST JAPAN - NEW EVIDENCE FOR DIFFERENT STATES OF PLATE SUBDUCTION

Two different plates are subducting beneath the Japanese island arcs, the Pacific plate in the north and the Philippine Sea plate in the sou th. Geophysical evidence indicates that the Philippine Sea plate subdu cts without extending beneath many of the volcanoes in the southwest J apan arc. The N2/Ar ratio (424) of a common magmatic gas from geotherm al and volcanic areas in southwestern Japan is an order of magnitude l ower than the value (4250) in northeastern Japan. Because the high N2/ Ar values reflect input from subducted sediments, the large difference in the N2/Ar ratios of magmatic gases indicates that the contribution of subducted sediment to the source of magma in south-western Japan i s much lower than that in northeastern Japan. The N2/Ar ratios of magm atic gases from island arcs elsewhere may also yield information regar ding the degree of contribution of subducted sediments to arc magma so urces and differences in the states of plate subduction.