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Abstract Plagioclase microlites in a magma nucleate and grow in response to melt supersaturation (Δϕplag). The resultant frozen plagioclase crystal size distribution (CSD) preserves the history of decompression pathways (dP/dt). SNGPlag is a numerical model that calculates the equilibrium composition of a decompressing magma and nucleates and grows plagioclase in response to an imposed Δϕplag. Here, we test a new version of SNGPlag calibrated for use with basaltic andesite magmas and modeldP/dtfor the ca. 12.6 ka Curacautín eruption of Llaima volcano, Chile. Instantaneous nucleation (Nplag) and growth (Gplag) rates of plagioclase were computed using the experimental results of Shea and Hammer (J Volcanol Geotherm Res 260:127–145, 10.1016/j.jvolgeores.2013.04.018, 2013) and used for SNGPlag modeling of basaltic andesite composition. MaximumNplagof 6.1 × 105 cm h−1is achieved at a Δϕplagof 44% and the maximumGplagof 27.4 μm h−1is achieved at a Δϕplagof 29%. Our modeled logdP/dtavgrange from 2.69 ± 0.09 to 6.89 ± 0.96 MPa h−1(1σ) with an average duration of decompression from 0.87 ± 0.25 to 16.13 ± 0.29 h assuming a starting pressurePiof 110–150 MPa. These rates are similar to those derived from mafic decompression experiments for other explosive eruptions. Using assumptions for lithostatic pressure gradients (dP/dz), we calculate ascent rates of < 1–6 m s−1. We conducted a second set of Monte Carlo simulations usingPiof 15–30 MPa to investigate the influence of shallower decompression, resulting in logdP/dtavgfrom 2.86 ± 0.49 to 6.00 ± 0.86 MPa h−1. ThedP/dtmodeled here is two orders of magnitude lower than those calculated by Valdivia et al. (Bull Volcanol, 10.1007/s00445-021-01514-8, 2022) for the same eruption using a bubble number density meter, and suggests homogeneous nucleation raisesdP/dtby orders of magnitude in the shallow conduit. Our modeling further supports the rapid-ascent hypothesis for driving highly explosive mafic eruptions.
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The Influence of Phenocrysts on Degassing in Crystal‐Bearing Magmas With Rhyolitic Groundmass Melts
Abstract The porosity at which a magma becomes permeable (i.e., the percolation threshold;ϕc) is important for magma degassing; it is also poorly constrained in crystal‐bearing systems. To address this, we conduct high pressure‐temperature decompression experiments on water‐saturated rhyolitic melts with variable crystal contents. We find that crystal‐bearing run products become permeable at ~55‐vol.% vesicularity (crystal free), a value that is similar to that found in decompression‐crystallization experiments using basaltic andesite compositions. Our results provide insight into controls on the eruption styles of hydrous, crystal‐bearing magmas in general and controls on pulsatory Vulcanian behavior, in particular.
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- Award ID(s):
- 1650185
- PAR ID:
- 10459506
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 46
- Issue:
- 10
- ISSN:
- 0094-8276
- Page Range / eLocation ID:
- p. 5127-5136
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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