Search for: All records

Abstract Two distinct types of rare crystal-rich mafic enclaves have been identified in the rhyolite lava flow from the 2011–12 Cordón Caulle eruption (Southern Andean Volcanic Zone, SVZ). The majority of mafic enclaves are coarsely crystalline with interlocking olivine-clinopyroxene-plagioclase textures and irregular shaped vesicles filling the crystal framework. These enclaves are interpreted as pieces of crystal-rich magma mush underlying a crystal-poor rhyolitic magma body that has fed recent silicic eruptions at Cordón Caulle. A second type of porphyritic enclaves, with restricted mineral chemistry and spherical vesicles, represents small-volume injections into the rhyolite magma. Both types of enclaves are basaltic end-members (up to 9.3 wt% MgO and 50–53 wt% SiO 2 ) in comparison to enclaves erupted globally. The Cordón Caulle enclaves also have one of the largest compositional gaps on record between the basaltic enclaves and the rhyolite host at 17 wt% SiO 2 . Interstitial melt in the coarsely-crystalline enclaves is compositionally identical to their rhyolitic host, suggesting that the crystal-poor rhyolite magma was derived directly from the underlying basaltic magma mush through efficient melt extraction. We suggest the 2011–12 rhyolitic eruption was generated from a primitive basaltic crystal-rich mush that short-circuited the typical full range of magmatic differentiation in a single step. 

Magma from Plinian volcanic eruptions contains an extraordinarily large numbers of bubbles. Nucleation of those bubbles occurs because pressure decreases as magma rises to the surface. As a consequence, dissolved magmatic volatiles, such as water, become supersaturated and cause bubbles to nucleate. At the same time, diffusion of volatiles into existing bubbles reduces supersaturation, resulting in a dynamical feedback between rates of nucleation due to magma decompression and volatile diffusion. Because nucleation rate increases with supersaturation, bubble number density (BND) provides a proxy record of decompression rate, and hence the intensity of eruption dynamics. Using numerical modeling of bubble nucleation, we reconcile a long-standing discrepancy in decompression rate estimated from BND and independent geospeedometers. We demonstrate that BND provides a record of the time-averaged decompression rate that is consistent with independent geospeedometers, if bubble nucleation is heterogeneous and facilitated by magnetite crystals.

 

Bubble nucleation is the critical first step during magma degassing. The resultant number density of bubbles provides a record of nucleation kinetics and underlying eruptive conditions. The rate of bubble nucleation is strongly dependent on the surface free energy associated with nucleus formation, making the use of bubble number density for the interpretation of eruptive conditions contingent upon a sound understanding of surface tension. Based on a suite of nucleation experiments with up to >10¹⁶bubbles per unit volume of melt, and using numerical simulations of bubble nucleation and growth during each experiment, we provide a new formulation for surface tension during homogeneous nucleation of H₂O bubbles in rhyolitic melt. It is based on the Tolman correction with a Tolman length ofδ = 0.32 nm, which implies an increase in surface tension of bubbles with decreasing nucleus size. Our model results indicate that experiments encompass two distinct nucleation regimes, distinguishable by the ratio of the characteristic diffusion time of water,τ_diff, to the decompression time,t_d. Experiments with >10¹³ m⁻³bubbles are characterized byτ_diff/t_d≪ 1, wherein the nucleation rate predominantly depends on the interplay between decompression and diffusion rates. Nucleation occurs over a short time interval with nucleation rate peaks at high values. For experiments with comparatively low bubble number density the average distance between adjacent bubbles and the diffusion timescale is large. Consequently,τ_diff/t_d≫ 1 and nucleation is nearly unaffected by diffusion and independent of decompression rate, with bubbles nucleating at an approximately constant rate until the sample is quenched.