11:30 AM - 11:45 AM
[SCG56-06] Crystallization mechanism of groundmass nanolites inferred from the crystal size distribution of the Shinmoedake 2011 eruption
Keywords:nanolite, microlite, decompression, crystallization, undercooling
In the framework of classical CSD theory, in which straight CSD represents constant nucleation density (N0) and crystal growth rate (in length, G), two explanations can be applied for steepening of CSD slope: a sudden increase of undercooling and crystallization delay leading to rapid recovering. The constant G can be assumed when the undercooling is constant. If crystallization differentiation of the melt catches up with the increase of the liquidus temperature by decompression, the degree of undercooling is kept constant and thus the kink of CSD slope is not formed. Assuming that N0 and G are proportional to the degree of undercooling, the kink of CSD requires a sudden change of undercooling. The change from phenocrysts to microlites is usually caused by onset of magma ascent from a magma chamber. The increase of undercooling from the crystallization stage of microlite to that of nanolite may be caused by the rapid decrease of water solubility in melts and resulting sharp increase of the liquidus temperature. When crystallization differentiation of melt does not catch up with the increase of the liquidus temperature, the crystallization delays and the degree of undercooling increases gradually. If such magmas with large undercooling are emplaced in the shallow level, crystal nucleation is facilitated and thus the kink of CSD slope may form.
The gap of CSD requires nucleation pause for a certain period of time in the course of crystallization. This may occur when activation energy for nucleation exceeds undercooling with decreasing water content (Dowty, 1980). The observed gap in the CSD of the Shinmoedake eruption may be caused by accelerating decrease of water solubility in the magmas as they approaches to the surface.