Groundmass crystallization in dacite dykes taken in Unzen Scientific Drilling Project (USDP-4)

Groundmass textural and compositional analyses of the drilled dacite dykes of the Unzen Scientific Drilling Project (USDP-4) identify the feeder dyke of the 1990–1995 eruption and elucidate the crystallization process of dykes at depth. In the drilling depth range of 1582–1996 m (“conduit zone”), four dacite dykes were recognized. The groundmasses of all but one of these dykes have textures ranging from cryptocrystalline to microcrystalline aggregate of crystals < 10 µm across forming an equigranular mosaic of plagioclase, alkali feldspar, quartz, and pyrite. The samples include a small number of coarser-grained plagioclase microlites (20 µm to 0.3 mm long). The compositions of groundmass consisting only of grains < 10 µm plotted at the lower pressure (< 50 MPa) ternary minimum in the Qzʹ–Abʹ–Orʹ system suggests that the crystallization of plagioclase, alkali feldspar, and quartz took place nearly simultaneously. The compositions of coarser plagioclase microlites and groundmass, the plagioclase microlite textures, and the phenocryst assemblages show significant differences from historical lavas exposed in the summit area. This implies the possibility that most of the dacite dykes are not feeder dykes for the lavas at the summit and remained beneath the surface, perhaps because of high viscosity associated with high SiO2. One sample C14-1-1 collected 1977 m, has a texture, composition, and phenocryst assemblage nearly identical to that of the dome lava of the 1990–1995 eruption, differing only in the presence of hydrothermal alternation. At this time we cannot definitely conclude that C14-1-1 was the feeder dyke for the 1990–1995 eruption until we can elucidate the time scale and the conditions governing hydrothermal alternation.