KINETICS OF COOLING-INDUCED SANIDINE CRYSTALLIZATION IN RHYOLITES FROM VALLES CALDERA, NM

Rhyolites domes that erupted from Valles Caldera following the Bandelier Tuff, are crystal-rich, effusively erupted, yet lack significant microlite crystallization. Sanidine crystals within these rhyolites make up ~5-14% of the sample and are relatively large (0.25-2mm), blocky, unzoned, and euhedral. The sizes and textures of the sanidine crystals are suggestive of cooling-induced crystallization. We test conditions that favor the growth of large crystals in a series of isobaric (125 MPa) dynamic cooling experiments, using an obsidian from Valles Caldera annealed at 1475°C. We annealed experiments at either 10°C above the sanidine-in curve (“super-liquidus”) or 10°C below the sanidine-in curve (“sub-liquidus”) for periods (5 min to 2h) prior to cooling the experiments to run temperatures (700 and 750°C), where they were held for 72-168h. Cooling rates varied from 4.5 to 280°C/h. We also seeded starting materials using 0, 1, 5 and 11 wt% natural obsidian. We collected BSE images of run products with a JEOL FEI SEM and processed them using FIJI. All experiments annealed for 5 min, retain hydrothermal crystallization. Sub-liquidus experiments (>1h anneal) have 20-25% sanidine, whereas super-liquidus experiments have 1-15% sanidine, where the most crystalline samples have the greatest amount of added natural sample. Sub-liquidus experiments have similar nucleation densities (6.5-7.7x10³sites/mm²), independent of the mass of added natural sample. Super-liquidus experiments have lower nucleation densities (1.3-3.6x10³ sites/mm²), where the nucleation densities increase as a function of the mass of added natural sample. Cooling rate does not appear to affect the run products at a resolution greater than image analysis affords. We find the largest crystals (~40-60µm) in the sub-liquidus experiments that also have the greatest degree of sanidine crystallization (~25%). Super-liquidus experiments all have crystals with maximum lengths of ~10-12µm, however, these experiments have 1-15% sanidine and do not reach equilibrium crystallinity. It appears that the super-liquidus experiments have the potential to produce the largest crystals over longer hold durations (>168h), as they continuously crystallize (i.e., grow) sanidine on fewer nucleation sites towards equilibrium abundances; forthcoming experiments test this hypothesis.