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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.

 

Bubble and crystal textures evolve during magma ascent, altering properties that control ascent such as permeability and viscosity. Eruption style results from feedbacks between ascent, bubble nucleation and growth, microlite crystallization, and gas loss, all processes recorded in pyroclasts. We show that pyroclasts of the mafic Curacautín ignimbrite of Llaima volcano, Chile, record a history of repeated autobrecciation, fusing, and crystallization. We identified pyroclasts with domains of heterogeneous vesicle textures in sharp contact with one another that are overprinted by extensive microlite crystallization. Broken crystals with long axes (l) >10 μm record fragmentation events during the eruption. A second population of unbroken microlites with l ≤10 μm overprint sutures between fused domains, suggesting the highly crystalline groundmass formed at shallow depths after autobrecciation and fusing. Nearly all pyroclasts contain plutonic and ancestral Llaima lithics as inclusions, implying that fusing occurs from a few kilometers depth to as shallow as the surface. We propose that Curacautín ignimbrite magma autobrecciated during ascent and proto-pyroclasts remained melt rich enough to fuse together. Lithics from the conduit margins were entrained into the proto-pyroclasts before fusing. Autobrecciation broke existing phenocrysts and microlites; rapid post-fusing crystallization then generated the highly crystalline groundmass. This proposed conduit process has implications for interpreting the products of mafic explosive eruptions. 

The faces of volcanic phenocrysts may be marked by imperfections occurring as holes that penetrate the crystal interior. When filled with glass these features, called embayments or reentrants, have been used to petrologically constrain magmatic ascent rate. Embayment ascent speedometry relies on the record of disequilibrium preserved as diffusion-limited volatile concentration gradients in the embayment glass. Clear, glassy embayments are carefully selected for speedometry studies. The use and subsequent descriptions of pristine embayments overrepresent their actual abundance. Here, we provide a textural analysis of the number, morphology, and filling characteristics of quartz-hosted embayments. We target a collection of large (i.e., >20 km³erupted volume) silicic eruptions, including the Bishop Tuff, Tuff of Bluff Point, Bandelier Tuff, Mesa Falls Tuff, and Huckleberry Ridge Tuff in the United States, Oruanui Tuff in New Zealand, Younger Toba Tuff in Indonesia, the Kos Plateau Tuff in Greece, and the Giant Pumice from La Primavera caldera in Mexico. For each unit, hundreds of quartz crystals were picked and the total number of embayment-hosting crystals were counted and categorized into classifications based on the vesicularity and morphology. We observed significant variability in embayment abundance, form, and vesicularity across different eruptions. Simple, cylindrical forms are the most common, as are dense glassy embayments. Increasingly complex shapes and a range of bubble textures are also common. Embayments may crosscut or deflect prominent internal cathodoluminescence banding in the host quartz, indicating that embayments form by both dissolution and growth. We propose potential additional timescales recorded by embayment disequilibrium textures, namely, faceting, bubbles, and the lack thereof. Embayment formation likely occurs tens to hundreds of years before eruption because embayment surfaces are rounded instead of faceted. Bubble textures in embayments are far from those predicted by equilibrium solubility. Homogenous nucleation conditions likely allow preservation of pressures much greater than magmastatic inside embayments. Our textural observations lend insight into embayment occurrence and formation and guide further embayment studies.