High Crystal Number Densities From Mechanical Damage

Laboratory experiments investigating syn-eruptive crystallization are fundamental for interpreting crystal and vesicle textures in pyroclasts. Previous have advanced our understanding by varying decompression cooling pathways, volatile components, melt composition. However, they largely failed to produce the high number densities seen many cryptodome dome samples. This is feasibly due relatively simple pathways employed experimental studies. In this study, we approach problem exploring non-linear pathways. We present two series of experiments: (1) from low initial starting pressure (2) a compression-and-release step after decompression. The purpose each was simulate magma that stalls during ascent cycling occurs non-erupted episodic explosive activity. were carried out on synthetic rhyodacite (SiO 2 = 69 wt%) held initially at 50 MPa 885°C then decompressed rates 0.026 0.05 s −1 10 A subset subjected compression 110 followed near-instantaneous release back MPa. substantial volume fraction dendritic microlites ( ϕ xtl 0.27–0.32, N 4.79 × 3 mm –2 ) formed hold MPa; additional subsequent ≥ minimal, as evidenced only small increases crystallinity 0.28–0.33) comparable (4.11–7.81 ). Samples underwent recompression second showed no increase but did show extensive disruption dendritic, box-work microlite structures produced 43.5–87.2 individual crystals. driven combination rapid vesiculation, expansion resulting shear along capsule walls. From these results, suggest may be signature deformation occurring stalling subsurface, perhaps related accompanying changes vesicularity repeated shallow-sourced explosions. compare pyroclasts shallow intrusions preceded 18 May 1980 eruption Mount St Helens. These erupted both prior May, precursory activity, lateral blast. pattern activity indicates multiple episodes pressurization (prior events) (during use illustrate significance results.