Search for: All records

Abstract Plagioclase microlites in a magma nucleate and grow in response to melt supersaturation (Δϕ_plag). 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 Δϕ_plag. 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 (N_plag) and growth (G_plag) 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. MaximumN_plagof 6.1 × 10⁵ cm h⁻¹is achieved at a Δϕ_plagof 44% and the maximumG_plagof 27.4 μm h⁻¹is achieved at a Δϕ_plagof 29%. Our modeled logdP/dt_avgrange 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 pressureP_iof 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 usingP_iof 15–30 MPa to investigate the influence of shallower decompression, resulting in logdP/dt_avgfrom 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.