LATERAL VARIATIONS OF H2O CONTENTS IN QUATERNARY MAGMAS OF NORTHEASTERN JAPAN

Volcanoes of the East Japan volcanic arc are divided into two groups on the basis of their phenocryst assemblages; volcanoes with lavas or pyroclastic rocks containing quartz phenocrysts and no hornblende phenocrysts (type A), and those with rocks containing hornblende phenocrysts and no quartz phenocrysts (type B). Type A volcanoes occur only in the narrow region along the volcanic front, whereas type B volcanoes are distributed in the area closer to the Sea of Japan. Recent experimental studies on calc-alkaline andesite-dacite under H2O-saturated and -undersaturated conditions indicate that the liquidus temperature (maximum thermal stability limit) of quartz decreases drastically with increasing H2O content in magma, whereas the liquidus temperatures of hornblende and biotite are relatively constant with variations in the H2O content and bulk chemical composition of the magma. It is suggested from the lateral variation of mafic phenocryst assemblages [1] and from the above result that the temperature of the parental magmas of these volcanoes increases, and their H2O contents decrease, towards the volcanic front in the East Japan volcanic arc. Such lateral variations in the H2O contents of magmas under the East Japan volcanic arc are in agreement with those of other incompatible elements (K, Rb, REE, etc.). If H2O-undersaturated partial melting of upper mantle peridotite can be represented by the univariant line (olivine, Ca-rich clinopyroxene, orthopyroxene and liquid coexist) in the system H2OMg2SiO4z.sbnd;CaMgSi2O6z.sbnd;SiO2, the decrease of H2O content in the magma suggests that the melting temperature of the peridotitic mantle may gradually increase, and so the degree of partial melting may increase, towards the volcanic front. The lateral variation of other incompatible elements can also be explained by this model. © 1979.