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There is a broad consensus that magma eruptibility—the ability of magma stored in the subsurface to erupt onto Earth's surface—is strongly controlled by viscosity. Related to this, a critical parameter that controls viscosity is crystallinity. However, there is uncertainty in the crystallinities that distinguish eruptible from non-eruptible magmas, and whether highly crystalline magmas (>60 vol.%) could be erupted in some conditions. An underutilized but important source of information for understanding this relationship is the observed crystallinities in erupted volcanic materials, which by definition represent a set of eruptible magmas. Here we present a compilation of reported crystallinities for nearly 1000 volcanic samples of differing composition, tectonic setting, and eruption style, which provides valuable insight into the fundamental mechanisms which drive eruptions. Overall, the 95th percentile crystallinity value of our full dataset is 57 vol.%, and \>99 % of all non-dome samples have crystallinity ≤53 vol.%. This suggests that 50–60 vol.% crystallinity represents a fundamental limit for eruptibility for most volcanic rocks. Some dome samples are clear exceptions to this and are erupted with considerably higher crystallinities. There is also a significant correlation between crystallinity and whole rock SiO2 content as observed previously, but a shallow slope suggests whole rock and melt silica content have less impact on critical crystallinity for erupted magma than previously thought. Melt viscosity (as a function of SiO2, temperature, and H2O content) and crystallinity both play important roles on increasing effective viscosity, where melt viscosity plays a more important role at low crystal fractions, and crystallinity plays a more important role at crystallinities greater than ~40 vol.%.

 

Abstract Ignimbrite flare-ups are rare periods of intense silicic volcanism during which the pyroclastic volume and eruptive frequency is more than an order of magnitude higher than background activity. Investigating the compositional differences between flare-up and steady-state magmas provides critical constraints on the petrogenetic causes for the event and can offer unique opportunities to investigate the role of large-scale tectonic or geodynamic processes in arc magmatism. In this study, we focus on the bimodal Deschutes Formation ignimbrite flare-up of Central Oregon, which erupted unusually high volumes of pyroclastic material 6.25–5.45 Ma from a new axis of volcanism in the Cascades arc. This episode is marked by increased eruption rates and eruption of more silicic compositions relative to the Quaternary Cascade arc, which rarely erupts rhyolites. Ignimbrites are crystal-poor (<10%) dacite to rhyolites (mostly 65–77 wt.% SiO2) with anhydrous mineral assemblages and higher FeO/MgO, Y, Eu/Eu*, MREE and Zr/Sr, indicating drier magmatic evolution compared to the Quaternary arc, and are more similar to those from the rear-arc High Lava Plains (HLP) province that lies to the east. Magnetite-ilmenite oxybarometry indicates that Deschutes Formation felsic magmas tend to be hotter and more reduced (NNO-1 to NNO) than the Quaternary arc (NNO to NNO + 1.5). Rhyolite-MELTS geobarometry suggests complex storage of diverse Deschutes Formation magmas within the shallow crust (50–250 MPa), and the common co-eruption of multiple plagioclase populations, pumice compositions, and compositionally banded pumice suggest variable degrees of mixing and mingling of distinct magmas. Deschutes magmas also have low δ18Oplagioclase values that indicate partial melting and assimilation of hydrothermally altered shallow crust. Trace element systematics and rhyolite-MELTS modeling suggests that felsic pumice cannot be produced by simple fractionation of co-erupted mafic pumice or basaltic lavas, and requires a crustal melting origin, and trace elements and Pb isotopes suggest that young mafic crust may have been the primary protolith. We suggest that partial melting produced low-Si rhyolite melt (~72 wt.%) that acted as both a parent for the most evolved rhyolites, and as a mixing endmember to create the dacite to rhyodacite magmas with heterogenous plagioclase populations. Unlike the predominantly calc-alkaline basalts erupted in the Quaternary Cascade arc, Deschutes Formation primary basalts are mostly low-K tholeiites, indicative of decompression melting. These are similar to the compositions erupted during a contemporaneous pulse of low-K tholeiite volcanism across the whole HLP that reached into the Cascades rear-arc. We suggest that intra-arc extension focused decompression melts from the back-arc into the arc and that tensional stresses allowed this high flux of hot-dry-reduced basalt throughout the crustal column, causing partial melting of mafic protoliths and the production of hot-dry-reduced rhyolite melts. Depletion of incompatible elements in successive rhyolites implies progressive depletion in fertility of the protolith. Extension also allowed for the establishment of a robust hydrothermal system, and assimilation of hydrothermally-altered rocks by magmas residing in a shallow, complex storage network lead to low δ18O melts. Our findings suggest the integral role that extensional tectonics played in producing an unusual ignimbrite flare-up of hot-dry-reduced rhyolite magmas that are atypical of the Cascades arc and may be an important contributor to flare-ups at arcs worldwide. 

Understanding the processes that initiate volcanic eruptions after periods of quiescence are of paramount importance to interpreting volcano monitoring signals and mitigating volcanic hazards. However, studies of eruption initiation mechanisms are rarely systematically applied to high-risk volcanoes. Studies of erupted materials provide important insight into eruption initiation, as they provide direct insight into the physical and chemical changes that occur in magma reservoirs prior to eruptions, but are also often underutilized. Petrologic and geochemical studies can also constrain the timing of processes involved in eruption initiation, and the time that might be expected to elapse between remote detection of increased activity and eventual eruption. A compilation and analysis of literature data suggests that there are statistical differences in the composition, volume, style and timescales between eruptions initiated by different mechanisms. Knowledge of the processes that initiate eruptions at a given volcano may thus have significant predictive power. 

Abstract Interpretation of chemical zoning within igneous minerals is critical to many petrologic studies. Zoning in minerals, however, is commonly observed in thin sections or grain mounts, which are random 2D slices of a 3D system. Use of these 2D sections to infer 3D geometries requires a set of assumptions, often not directly tested, introduces several issues, and results in partial loss of zoning information. Computed X-ray microtomography (microCT) offers a way to assess 3D zoning in minerals at high resolution. To observe 3D mineral zoning using microCT, however, requires that zoning is observable as differences in X-ray attenuation. Sanidine, with its affinity for Ba in the crystal lattice, can display large, abrupt variations in Ba that are related to various magma reservoir processes. These changes in Ba also significantly change the X-ray attenuation coefficient of sanidine, allowing for discrete mineral zones to be mapped in 3D using microCT. Here we utilize microCT to show 3D chemical zoning within natural sanidines from a suite of volcanic eruptions throughout the geologic record. We also show that changes in microCT grayscale in sanidine are largely controlled by changes in Ba. Starting with 3D mineral reconstructions, we simulate thin-section making by generating random 2D slices across a mineral zone to show that slicing orientation alone can drastically change the apparent width and slope of composition transitions between different zones. Furthermore, we find that chemical zoning in sanidine can commonly occur in more complex geometries than the commonly interpreted concentric zoning patterns. Together, these findings have important implications for methodologies that rely on the interpretation of chemical zoning within minerals and align with previously published numerical models that show how chemical gradient geometries are affected by random sectioning during common sample preparation methods (e.g., thin sections and round mounts). 

The Cascade arc has produced a remarkable diversity of volcanic rocks over the Quaternary period. The major stratovolcanoes that define the arc front are dominated by eruptions of andesitic and dacitic intermediate magmas, produced largely by fractionation, melting, assimilation, and mixing within the crust. In addition, relative to many other subduction zones, the arc has produced significant mafic volcanism. These more primitive magmas reveal complexity in mantle wedge dynamics, sources, and magma production processes, and suggest that there are significant differences along the arc in the amount of magma that enters the lower Cascade crust from the underlying mantle. 

The well-studied Cascadia subduction zone has enriched our general understanding of global subduction zones. This Elements issue explores the interconnected set of processes that link geodynamics, tectonics, and magmatism at depth and the surface expressions of these processes, which shape the landscape and give rise to natural hazards in the Cascadia region. This issue also addresses the impact of subduction zone processes on human populations using cultural records, and reviews the state of knowledge of Cascadia while highlighting some key outstanding research questions. 

Abstract The composition of clinopyroxene and clinopyroxene-liquid (Cpx-Liq) pairs are frequently used to calculate crystallization/equilibration pressures in igneous systems. While canonical uncertainties are often assigned to calculated pressures based on fits to calibration or test datasets, the sources of these uncertainties (and thus ways to reduce them) have not been rigorously assessed. We show that considerable uncertainties in calculated pressures arise from analytical error associated with Electron Probe Microanalyser (EPMA) measurements of Cpx. Specifically, low X-ray counts during analysis of elements with concentrations <1 wt% resulting from insufficient count times and/or low beam currents yield highly imprecise measurements (1σ errors of 10–40% for Na2O). Low analytical precision propagates into the calculation of pressure-sensitive mineral components such as jadeite. Using Monte Carlo approaches, we demonstrate that elemental variation resulting from analytical precision alone generates pressures spanning ~4 kbar (~15 km) for a single Cpx and ~6 kbar for a single Cpx-Liq pair using popular barometry expressions. In addition, analytical uncertainties in mineral compositions produce highly correlated arrays between pressure and temperature that have been previously attributed to transcrustal magma storage. Before invoking such geological interpretations, a more mundane origin from analytical imprecision must be ruled out. Most importantly, low analytical precision does not just affect the application of barometers to natural systems; it has also affected characterization of Cpx in experimental products used to calibrate and test barometers. The impact of poor precision on each individual measurement is often magnified by the small number of measurements made within experimental charges, meaning that low analytical precision and true variability in mineral compositions have not been sufficiently mediated by averaging multiple EPMA analyses. We compile the number of Cpx measurements performed in N = 307 experiments used to calibrate existing barometers, and N = 490 new experiments, finding ~45% of experiment charges were characterized by ≤5 individual Cpx analyses. Insufficient characterization of the true composition of experimental phases likely accounts for the fact that all Cpx-based barometers exhibit large errors (± 3 kbar) when tested using global experimental datasets. We suggest specific changes to analytical and experimental protocols, such as increased count times and/or higher beam currents when measuring low concentration elements in relatively beam resistant Cpx in experiments and natural samples. We also advocate for increasing the number of analyses per experimental charge, resolving interlaboratory analytical offsets and improving data reporting. Implementing these changes is essential to produce a more robust dataset to calibrate and test the next generation of more precise and accurate Cpx-based barometers. In turn, this will enable more rigorous investigation of magma storage geometries in a variety of tectonic settings (e.g. distinguishing true transcrustal storage vs. storage in discrete reservoirs). 

Abstract The long-term thermochemical conditions at which large bodies of silicic magma are stored in the crust is integral to our understanding of the timing, frequency, and intensity of volcanic eruptions and provides important context for interpreting volcano monitoring data. Despite this, however, individual magmatic systems may exhibit a range of time–temperature paths, or thermal histories, that are the result of many complex and, in some cases, competing processes. This complexity contributes to an incomplete understanding of the long-term thermal evolution of magma stored within the Earth’s crust. Of recent interest to the volcanology community is the length of time large volumes of rheologically eruptible and geophysically detectable magma exist within the crust prior to their eruption. Here we use a combination of diffusion chronometry, trace element, and thermodynamic modeling to quantify the long-term thermal evolution of the 2.08 Ma, 630 km3 Cerro Galán Ignimbrite (CGI) in NW Argentina; one of the largest explosive volcanic eruptions in the recent geologic record. We find that diffusion of both Mg and Sr in plagioclase indicate that erupted magmatic material only spent decades to centuries at or above temperatures (~750°C) required to maintain significant volumes of stored eruptible magma. Calculated plagioclase equilibrium compositions reveal an array of liquids that is controlled overall by fractionation of plagioclase + biotite + sanidine, although high-resolution trace element transects record a diversity of fractionation pathways. Overall, we suggest that there is compelling evidence that the magma erupted from the CGI magmatic system spent most of its upper crustal residence in a largely uneruptible state and was rapidly remobilized shortly before eruption. 

We present Thermobar, a new open-source Python3 package for calculating pressures, temperatures, and melt compositions from mineral and mineral-melt equilibrium. Thermobar allows users to perform calculations with >100 popular parametrizations involving liquid, olivine-liquid, olivine-spinel, pyroxene only, pyroxene-liquid, two pyroxene, feldspar-liquid, two feldspar, amphibole only, amphibole-liquid, and garnet equilibria. Thermobar is the first open-source tool which can match up all possible pairs of phases from a given region, and apply various equilibrium tests to identify pairs from which to calculate pressures and temperatures (e.g. pyroxene-liquid, two pyroxene, feldspar-liquid, two feldspar, amphibole-liquid). Thermobar also contains functions allowing users to propagate analytical errors using Monte-Carlo methods, convert pressures to depths using different crustal density profiles, plot mineral classification and mineral-melt equilibrium diagrams, calculate liquid viscosities, and convert between oxygen fugacity values, buffer positions and Fe speciation in a silicate melt. Thermobar can be downloaded using pip and extensive documentation is available at https://thermobar.readthedocs.io/. 

Abstract The Okataina Volcanic Centre (OVC), located in the Taupo Volcanic Zone, New Zealand, is a dominantly rhyolitic magmatic system in an arc setting, where eruptions are thought to be driven by mafic recharge. Here, Sr–Pb isotopes, and compositional and textural variations in plagioclase phenocrysts from 10 rhyolitic deposits (two caldera, one immediately post-caldera, four intra-caldera, and three extra-caldera) are used to investigate the OVC magmatic system and identify the sources and assimilants within this diverse mush zone. Plagioclase interiors exhibit normal and reverse zoning, and are commonly in disequilibrium with their accompanying glass, melt inclusions, and whole-rock compositions. This indicates that the crystals nucleated in melts that differed from their carrier magma. In contrast, the outermost rims of crystals exhibit normal zoning that is compositionally consistent with growth in cooling and fractionating melts just prior to eruption. At the intra-crystal scale, the total suite of 87Sr/86Sr ratios are highly variable (0·7042–0·7065 ± 0·0004 average 2SE); however, the majority (95 %) of the crystals are internally homogeneous within error. At whole-crystal scale (where better precision is obtained), 87Sr/86Sr ratios are much more homogeneous (0·70512–0·70543 ± 0·00001 average 2SE) and overlap with their host whole-rock Sr isotopic ratios. Whole-crystal Pb isotopic ratios also largely overlap with whole-rock Pb ratios. The plagioclase and whole-rock isotopic compositions indicate significant crustal assimilation (≥20 %) of Torlesse-like metasediments (local basement rock) by a depleted mid-ocean ridge mantle magma source, and Pb isotopes require variable fluid-dominant subduction flux. The new data support previous petrogenetic models for OVC magmas that require crystal growth in compositionally and thermally distinct magmas within a complex of disconnected melt-and-mush reservoirs. These reservoirs were rejuvenated by underplating basaltic magmas that serve as an eruption trigger. However, the outermost rims of the plagioclase imply that interaction between silicic melts and eruption-triggering mafic influx is largely limited to heat and volatile transfer, and results in rapid mobilization and syn-eruption mixing of rhyolitic melts. Finally, relatively uniform isotopic compositions of plagioclase indicate balanced contributions from the crust and mantle over the lifespan of the OVC magmatic system.