268-8 Crystallization kinetics of sanidine and anorthoclase in high-silica rhyolites: determining ascent rates for effusive rhyolites from Valles Caldera, NM, USA

Session: Old and the New, Long and the Short: Perspectives on Integration of Timescales of Magmatic Processes: Special Session Related to MGPV Awards to Madison Myers and Anita Grunder (Posters)

Presenting Author:

Authors:

Abstract:

Crystal size distributions (lengths, areas and abundances) in an erupted rock provide a record of the timescales of its storage and ascent, provided that nucleation and growth rates of mineral phases are known. We conducted two isothermal (750, 800°C) series of H₂O-saturated, single step decompression (SSD) experiments over a range of final pressures (70, 50, 30, and 10MPa) and hold times of 8-504h, where all experiments initiated from super-liquidus conditions. We also conducted two isobaric (130, 170 MPa) series of single step cooling (SSC) experiments cooled to 735°C and 750°C with dwell times ranging from 24-192h. 

Sanidine and anorthoclase cannot be distinguished in the experiments, therefore crystallization rates reflect total feldspar. Time-averaged nucleation rates range from 6.4x10^-4 to 1.2 mm^-3/s and from 2.1x10^-4 to 1.6x10^-1 mm^-3/s in SSD and SSC experiments. Time-averaged growth rates range from 7.4x10^-9 to 4.2x10^-7 mm/s and from 5.7x10^-8 to 3.7x10^-7 mm/s in SSD and SSC experiments. The highest nucleation rates occur at the lowest final pressures (largest degrees of undercooling) in both the SSD and SSC series. The fastest growth rates occur at the highest final pressures (lower degrees of undercooling) for both series. The fastest growth rates occur in the 170 MPa SSC experiments with the shortest hold times (48-72h). Crystal sizes in the SSC series are consistently larger than those in the SSD series.

We use the kinetic results to interpret time scales of ascent for two high-silica rhyolites from Valles Caldera. The Cerro Del Medio obsidian contains <1% crystals that are <50µm in length. Crystals in the obsidian likely grew from ~100 MPa to the surface within 32 days, which corresponds to 0.12 MPa/h (ascending at 4.4 m/h, using an overburden density of 2.7 g/cc). The Cerro San Luis rhyolite dome contains 19.1% feldspar, dominated by crystals >500 µm, with an absence of crystals <50µm. Growth of large crystals in the rhyolite dome occurred from 200 to 160 MPa over 521 days (i.e., equilibrium, storage conditions). The rhyolite dome must have accelerated to 2 MPa/h for the final two days of ascent (from 160 MPa to the surface) to suppress crystals <50µm. The maximum ascent rate for these effusively erupted magmas is 10.8 MPa/h (the rate that outpaces equilbrium volatile degassing).

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10475

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.