Modeling decompression paths in a basaltic andesite magma using the nucleation and growth of plagioclase microlites.

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 model dP/dt for 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. Maximum N_plag of 6.1 × 10⁵ cm h⁻¹ is achieved at a Δϕ_plag of 44% and the maximum G_plag of 27.4 μm h⁻¹ is achieved at a Δϕ_plag of 29%. Our modeled log dP/dt_avg range 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 pressure P_i of 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 using P_i of 15–30 MPa to investigate the influence of shallower decompression, resulting in log dP/dt_avg from 2.86 ± 0.49 to 6.00 ± 0.86 MPa h⁻¹. The dP/dt modeled 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 raises dP/dt by orders of magnitude in the shallow conduit. Our modeling further supports the rapid-ascent hypothesis for driving highly explosive mafic eruptions. [ABSTRACT FROM AUTHOR]