The timescale over which magma rises to the surface during a volcanic eruption is a critical parameter that controls eruption style but is challenging to determine. Melt embayments, which are pockets of magma trapped within crystals but are still open to the host magma, tend to develop volatile (e.g., H2O, CO2) concentration gradients during ascent through the process of diffusion. These gradients are increasingly used as output constraints for diffusion models to calculate total ascent time. However, there are two main assumptions associated with the method that have yet to be properly evaluated. First, the diffusion models tend to be carried out in 1D, which may be less accurate than 3D models if the embayment narrows near the opening. This geometry has more complicated volatile flux pathways that 1D models cannot account for. Second, the models typically assume that volatile contents follow equilibrium saturation trends defined by depth below the surface, which may not be accurate for high silica magmas. For these compositions, a significant supersaturation of volatiles may be needed for volatile exsolution to start, depending on the availability of suitable crystal phases for bubble nucleation. This nucleation lag delays the start of diffusion as well and could influence modeled timescales. We developed 3D diffusion models to evaluate the degree of inaccuracy in calculated timescales introduced by these assumptions for high silica magmas. We built synthetic embayments with variable 3D geometries and imposed both equilibrium and disequilibrium ascent conditions in our models. Our results indicate that the inaccuracy introduced by embayment geometry in 1D diffusion models increases as the diameter of the embayment opening decreases. This degree of inaccuracy is also tied to diffusion duration – as it increases, so does the relative error in timescales retrieved by 1D models fitting 3D-generated profiles. Once the diffusion front extends beyond the narrow, “necked” region of the embayment, relative error increases with increasing diffusion time. However, volatile disequilibrium behavior does not appear to significantly impact modeled timescales. These results indicate that embayment geometry is a critical variable that must be accounted for whereas delayed nucleation is of secondary influence.

After 35 years of fairly steady lava effusion from the Pu’u ‘O’o vent along Kilauea’s mid portion of the East Rift Zone, a drastic change in the volcano’s behavior was initiated in May 2018. A series of new fissures opened between May 3rd and May 9th along the eastern end of the subaerial ERZ ( >40 km from the summit caldera), feeding a few short lived, relatively small volume flows. The products from the early phase of the eruption were dominated by plagioclase and clinopyroxene phenocrysts, gabbroic glomerocrysts and other crystal clots likely picked up prior to eruption. The initial activity transitioned between May 12th and around May 28th to steadier fountaining, higher effusion rate activity, leading to the formation of larger volume lava flows and the ensuing destruction of hundreds of houses and structures within the Puna area. This period marked the arrival of olivine-bearing Pu’u ‘O’o-like magma to the eruption site. Since the end of May, activity has focused on a unique vent (Fissure 8). This contribution examines the crystal cargo from the different magmas involved in the initial four weeks of the eruption, and their history through chemical zoning patterns of crystals. We discuss the magmatic origin of mineral assemblages from the evolved basalt end-member, and present initial results on timescales of magma interactions prior to their arrival at the surface from diffusion modeling in olivine.

Understanding the physical and chemical dynamics of caldera magma chambers prior to VEI 7+ eruptions is important for eruption forecasting and societal preparedness. The 1.26 Ma, 400 km3 Tshirege Member of the Bandelier Tuff is the later of two zoned rhyolitic ignimbrites erupted from the Valles caldera in the Jemez Mountains volcanic field, New Mexico, USA. To avoid effects of crystal-glass sorting, lithic contamination, and post-emplacement thermal alteration of glasses and mineral phases, we sampled whole pumices from non-welded tuff, including zones of otherwise welded tuff quenched against cold paleovalley walls. The tuff is broadly systematically chemically zoned from early-erupted high-silica rhyolite enriched in incompatible trace elements to late-erupted low-silica rhyolite. Whole-pumices indicate the upper portions of the Tshirege magma system were unaffected by convective stirring prior to the eruption, while some mixing and overturn is reflected by pumices from deeper in the system (i.e. stratigraphically higher in tuff sheet), likely closer to recharge sources. Analyses of well-preserved minerals, glasses and melt inclusions, and application of mineral thermometers and barometers, show a vertical stratification of temperature (710-840 °C), pressures of ~0.16 GPa, and variable [H2O] (~2.0-3.5 wt. %). Whole-pumice and melt inclusion chemistry supports crystal accumulation and subsequent melting and remobilization of the cumulate pile as a major contributor to the overall compositional zoning. 2-D thermal models indicate that the creation of the temperature gradient over ~10ka from mush development to mobilization would require volumetrically unreasonable influx of recharge magma (flux rate >30 km3/yr), suggesting that recharge heating is likely only affecting the lowest portions of the chamber. As a result, the thermal gradient likely existed prior to mobilization of the mush. Heating alone cannot affect thermal loosening of a mechanically locked mush to an eruptible crystallinity in reasonable time frames, and so input of volatiles such as H2O or CO2 from a second boiling of recharge magmas is required to melt and remobilize the crystal mush pile. The 1.26 Ma eruption was thus triggered by a final recharge event recorded as dacite pumice clasts distributed throughout the tuff.