A nanolite record of eruption style transition

Abstract

Microlite characteristics have been successfully used as indicators of magma decompression rate. However, microlites do not record the explosive-effusive transitions in sub-Plinian eruptions, when such transitions are governed by the decrease of exsolved volatile pressure in the shallow conduit rather than by the magma ascent rate. To overcome this limitation, we studied the nanolites in the quenched products of the 2011 Shinmoedake (Japan) eruption. Nanolites are nanometer-scale components of the groundmass minerals and exhibit a steeper slope of crystal size distribution than that of microlites. In the 2011 eruption, the style of activity had undergone transformation from sub-Plinian eruption to Vulcanian explosion and intermittent effusion of lava. We found that, although the products formed by different eruptive activities have similar microlite characteristics, such products can be distinguished clearly by their mineral assemblage of nanolites. The samples of pumice of sub-Plinian eruptions and Vulcanian explosions and the dense juvenile fragments of lava (in descending order of explosivity) contained, respectively, nanolites of low-Ca pyroxene, low-Ca pyroxene + plagioclase, and low-Ca pyroxene + plagioclase + Fe-Ti oxides. Nanolites crystallize when undercooling of the magma due primarily to dehydration increases rapidly near the surface. The water contents of the interstitial glass indicate that the quenched depths did not differ greatly between eruption styles. Hence, the different nanolite assemblages of each eruption style are assumed to have resulted from differences in magma residence time near the surface. Therefore, we propose that nanolites have the potential to indicate the physicochemical conditions of magma at the transition points of eruption styles.