Rates of plagioclase growth, nanolite nucleation, and viscosity increase during Vulcanian activity of Sakurajima Volcano, Japan

This study investigated the products of successive Vulcanian eruptions in 1976 at the Minamidake summit crater of the Sakurajima Volcano, namely pumice clasts from an explosion on May 13 and dense juvenile fragments from May 17. Effective undercooling (ΔT eff ) on May 11–13 and 13–17 were estimated at 53–86 and 33–45 °C, respectively. Plagioclase phenocrysts exhibited broken surfaces with regrowth rims, which were attributed to decompression by the immediately preceding explosions. Based on the regrowth rim widths, the average growth rates were estimated to be 0.4–1.7 × 10 −11 and 1.7–4.7 × 10 −12 m s −1 on May 11–13 and 13–17, respectively, suggesting silicon diffusion-controlled growth. These growth rates were consistent with those from previous decompression experiments at similar ΔT eff under the same conditions (i.e., temperature and melt SiO 2 and H 2 O content) as Sakurajima. Assuming that plagioclase nanolites and fine-grained microlites with lengths shorter than the regrowth rims widths nucleated in the corresponding periods, the nucleation rate measured on May 11–13 agreed well with the recalculated experimental rates. In contrast, the nucleation rate measured on May 13–17 was higher by at least 1.9 orders of magnitude, suggesting nucleation acceleration at a pressure of <40 MPa. The high nucleation rates increased the groundmass total crystallinity by a factor of 2–5 compared with the experiments at similar ΔT eff , enabling the groundmass viscosity to reach the stress fragmentation threshold for a Vulcanian explosion. A precise understanding of these shallow magma crystallization kinetics thus paves the way for prediction of eruptive styles. • Successively erupted Sakurajima samples were petrographically analyzed. • The plagioclase growth rate was potentially controlled by Si diffusion in melt. • The nucleation rate accelerated at <40 MPa compared with decompression experiments. • The high nucleation rate led to increased total crystallinity and melt viscosity. • The stress fragmentation threshold for an explosion was reached by crystallization.