島弧火山下のマッシュ状マグマ溜り : 赤城火山湯ノ口軽石の例

Phenocrysts in pre-caldera pyroclastic deposits of Akagi volcano were examined in detail in order to reconstruct thermal and compositional structures of a magma chamber. The eruption of the Yunokuchi pumice fall deposit (UP), followed by the effusion of the Garan pyroclastic flow, took place at ca. 45 ka just prior to the formation of the summit caldera with a dimension of 2 by 4 km. Both orthopyroxene (opx) and clinopyroxene (cpx) show reversed zoning in Mg^# (100×Mg/ (Mg + Fe)) and can be divided into two types on the basis of zoning patterns in the cores and further subdivided into three types based on those in the rims. Type a pyroxenes have uniform cores with Mg^# of 61-65 in opx and 69-72 in cpx. Type b pyroxenes have oscillatory zoned cores showing repeated resorption. Type I pyroxenes have narrow rims (100-150μm) with high Mg^#s of 70-74 in opx and 75-78 in cpx. Type II pyroxenes have wide rims (500-600μm) with a distinct compositional gap between the core and rim. Type III pyroxenes have broad rims (400-500μm) and the boundaries between the rim and the core are obscured. Plagioclase phenocrysts are divided into Type A and B based on An content of the cores. Type A plagioclase has the core composition of An_75-85. Type B plagioclase has the core of An_92-94. All of plagioclase phenocrysts have normally zoned rims with a plateau of An_82-85 in the innermost rims. The equilibrium temperatures of coexisting ortho- and clinopyroxene are 990-1060℃ at the maximum for the rim-rim pairs and 900-920℃ for the core-core pairs. In order to explain the heterogeneities in the zoning patterns in the rims of pyroxenes and plagioclase, we propose a magma chamber filled with mush consisting of crystals and melts. Before 45 ka the magma chamber was filled with a homogeneous crystal mush which was formed after mixing of a larger amount of magma containing cores of Type a pyroxenes and Type A plagioclase with a small amount of magma with Type b pyroxene cores. Supply of more mafic, hot magma into the magma chamber formed a melt-enriched pocket in the mush. Mushy magma adjacent to the path of the injected magma was mixed with the supplied magma, resulted in resorption and reverse zoning of pyroxenes. The pyroxenes facing the melt pocket also grew reversely zoned rims. Pyroxenes experienced such repeated suppliance of magma formed multiple reverse zoning in the rims, giving rise to Type II pyroxenes. Because the melt-enriched pocket migrated in the mushy chamber due to balance of densities and strength of the mush and the supplied magma, some pyroxenes with a multiple reverse zoning were not incorporated into the melt-pocket, but were left in the mush adjacent to the hot pocket. Fe-Mg inter-diffusion was accelerated in such pyroxenes, yielding Type III pyroxenes. On the contrary, Type I pyroxene crystals were embedded deep in the crystal mush near the rind of the chamber without experience of renewal of the interstitial melt. At least several tens of years prior to the eruption of UP, injection of primitive magma with anorthositic plagioclase (Type B) caused extensive mixing of the supplied magma with the mush, resulted in the reverse zoning in the outermost rim of the preexisting Type A plagioclase, orthopyroxene and clinopyroxene. Tapping the chamber first expelled a less viscous, crystal-poor mixed magma with a higher proportion of Type B plagioclase and Type II and III pyroxenes. As eruption proceeded, increasing amount of the mushy magma with Type A plagioclase and Type I pyroxenes were incorporated into the effusing magma.