The viscosity of magma exerts control on all aspects of its migration through the crust to eruption. This was particularly true for the 2021 eruption of Cumbre Vieja (La Palma), which produced ...exceptionally fast and fluid lava at high discharge rates. We have performed concentric cylinder experiments to determine the effective viscosities of the Cumbre Vieja magma, while accounting for its chemistry, crystallinity, and temperature. Here we show that this event produced a nepheline-normative basanite with the lowest viscosity of historical basaltic eruptions, exhibiting values of less than 10 to about 160 Pa s within eruption temperatures of ~1200 to ~1150 °C. The magma's low viscosity was responsible for many eruptive phenomena that lead to particularly impactful events, including high-Reynolds number turbulent flow and supercritical states. Increases in viscosity due to crystallization-induced melt differentiation were subdued in this eruption, due in part to subtle degrees of silica enrichment in alkaline magma.
The effect of F and Cl on the liquidus temperature of a hydrous (~3.5–4 wt% H
O) trachytic melt (~66 wt% SiO
) at 925 to 990 °C and at 100 MPa has been experimentally investigated. We employed a ...novel disequilibrium approach involving diffusion couple experiments with the two-diffusion couple end-members differing solely in halogen concentrations. A shift of the liquidus temperature by ~50 °C was observed between a halogen-poor and halogen-enriched melt. Each experiment spanned the entire range of F and Cl concentrations between the two end-member compositions. We determined the halogen concentrations at the transition from crystal-bearing to crystal-free melt. These concentrations correspond to the liquidus halogen concentrations of the melt at each experimental temperature. We demonstrate that there is a limiting halogen concentration (~0.19–0.52 wt% F; ~0.07–0.24 wt% Cl), below which the melt crystallizes spherulitic clinopyroxene during heating to the run temperature. At high temperatures, upon diffusion of F and Cl into the halogen-poor melt, those crystals dissolve, leaving behind a dissolution front parallel to the diffusion interface. We propose that the dissolution is a consequence of F and Cl complexing with some of the main cationic components of clinopyroxene (Mg, Fe, Ca), thereby destabilizing this phase. Thus, the experimental dissolution of clinopyroxene is a manifestation of a liquidus depression caused by increased halogen content. Our results show that the liquidus shifts at a rate of ~1575(379) K/mol% of F and Cl in the melt, which is a minimum estimate, assuming both halogens equally drive dissolution. This liquidus depression is valid for a range of halogen concentrations (~0.06–0.87 wt% F; ~0.06–0.36 wt% Cl) and the experimental temperatures. Our findings illustrate that the degassing of halogens during or prior to an eruption can enhance crystallization in the melt and therefore influence magma physical properties that may ultimately affect eruption style.
Magmatic intrusions and volcanic eruptions are intimately related phenomena. Shallow magma intrusion builds subsurface reservoirs that are drained by volcanic eruptions. Thus, the long-held view is ...that intrusions must precede and feed eruptions. Here we show that explosive eruptions can also cause magma intrusion. We provide an account of a rapidly emplaced laccolith during the 2011 rhyolite eruption of Cordón Caulle, Chile. Remote sensing indicates that an intrusion began after eruption onset and caused severe (>200 m) uplift over 1 month. Digital terrain models resolve a laccolith-shaped body ∼0.8 km
. Deformation and conduit flow models indicate laccolith depths of only ∼20-200 m and overpressures (∼1-10 MPa) that likely stemmed from conduit blockage. Our results show that explosive eruptions may rapidly force significant quantities of magma in the crust to build laccoliths. These iconic intrusions can thus be interpreted as eruptive features that pose unique and previously unrecognized volcanic hazards.
•Bubbles nucleate more efficiently on Fe-Ti oxides than on other mineral type.•Number density of oxide microlites is more relevant than volume fraction.•Bubble coalescence is enhanced by phenocrysts.
...Silicic magmas span a wide range of eruptive styles between explosive and effusive, and transitions between these styles are commonplace. Yet the triggers of switches in eruptive style remain poorly understood. Eruptions are mostly driven by degassing of magmatic water and their eruption style - effusive or explosive - is likely governed by the efficiency of outgassing as well as magma ascent rate. Microlites and phenocrysts are often purported to promote heterogeneous bubble nucleation and outgassing, both key variables in the degassing dynamics that become crucial in controlling the eruptive style. Here, in order to shed light on the role of nature, size and abundance of crystals on degassing of silicic magma, we experimentally investigate (heating-induced) vesiculation in a multiphase (microlite- and phenocryst-bearing), low-water content, and bubble-free, natural rhyolite. The experiments were conducted at magmatic temperature (∼900-1100°C) and atmospheric pressure in an optical dilatometer. Our results indicate that microlites exert a large influence on bubble nucleation while the effect of phenocrysts is subordinate. Amongst the microlite phases, bubbles nucleate more easily on Fe-Ti oxides than other mineral phases. Bubble coalescence and connectivity are, in contrast, enhanced by the phenocrysts, more than microlites, in low-crystallinity magma. Comparing the bubble textures of the post-experimental samples with those produced in a phenocryst-free and microlite-poor silicic magma, we observe that the phenocryst- and microlite-bearing magma exhibits significantly more bubble coalescence and connectivity, as well as higher bubble number densities. These findings help to constrain the roles that pre- and syn-eruptive crystalline phases may play in degassing processes during ascent of silicic magma.
In this study, we investigate the metamorphic history of the Assynt and Gruinard blocks of the Archean Lewisian Complex, northwest Scotland, which are considered by some to represent discrete crustal ...terranes. For samples of mafic and intermediate rocks, phase diagrams were constructed in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 (NCKFMASHTO) system using whole‐rock compositions. Our results indicate that all samples equilibrated at similar peak metamorphic conditions of ~8–10 kbar and ~900–1,000°C, consistent with field evidence for in situ partial melting and the classic interpretation of the central region of the Lewisian Complex as representing a single crustal block. Melt‐reintegration modelling was employed in order to estimate probable protolith compositions. Phase equilibria calculated for these modelled undepleted precursors match well with those determined for a subsolidus amphibolite from Gairloch in the southern region of the Lewisian Complex. Both subsolidus lithologies exhibit similar phase relations and potential melt fertility, with both expected to produce orthopyroxene‐bearing hornblende granulites, with or without garnet, at the conditions inferred for the Badcallian metamorphic peak. For fully hydrated protoliths, prograde melting is predicted to first occur at ~620°C and ~9.5 kbar, with up to 45% partial melt predicted to form at peak conditions in a closed‐system environment. Partial melts calculated for both compositions between 610 and 1,050°C are mostly trondhjemitic. Although the melt‐reintegrated granulite is predicted to produce more potassic (granitic) melts at ~700–900°C, the modelled melts are consistent with the measured compositions of felsic sheets from the central region Lewisian Complex.
Chemical diffusion of F and Cl has been experimentally determined in a rhyodacitic melt obtained from remelting a sample of Hekla pumice (Iceland). Diffusion couple experiments were conducted in a ...vertical tube furnace over a temperature range of 750-950°C and in air for durations of 1 to 35 days. Concentration profiles of F and Cl were obtained for the quenched samples using an electron microprobe. Fluorine and chlorine exhibit Arrhenian behavior over the range of temperature investigated here. The pre-exponential factors of F and Cl are D0(F)=4.3×10-4 and D0(Cl)=1.6×10-5 m2/s. Fluorine diffusion coefficients vary in the order of 1×0-15 to 1×10-13 m2/s, whereas Cl diffusivity is up to two orders of magnitude slower. The activation energies for F and Cl diffusivities are equal within error at 223±31 and 229±52 kJ/mol, respectively. The difference in diffusivity between F and Cl is particularly pronounced in the melt of our study, compared to results obtained for other magmatic melt compositions. This means that the potential for diffusive fractionation exists and may occur especially under conditions of magma ascent and bubble growth, as this would favor partitioning of the relatively fast-diffusing halogens into growing bubbles, due to H2O exsolution. A dependence of diffusivity on atomic radius observed here is enhanced over that observed in more basic, less viscous melts, indicating that diffusive fractionation is more likely to be pronounced in more silicic, more viscous systems. A proper parameterization and modeling of diffusive fractionation of halogens in actively degassing volcanic systems thus holds the potential of serving as a tool for quantifying the processes responsible for volcanic unrest.
CO2 flux measurements are often used to monitor volcanic systems, understand the cause of volcanic unrest, and map sub-surface structures. Currently, such measurements are incomplete at Tarawera (New ...Zealand), which erupted with little warning in 1886 and produced a ∼17 km long fissure. We combine new soil CO2 flux and C isotope measurements of Tarawera with previous data from Rotomahana and Waimangu (regions also along the 1886 fissure) to fingerprint the CO2 source, understand the current pathways for degassing, quantify the CO2 released along the entire fissure, and provide a baseline survey. The total CO2 emissions from the fissure are 1227 t⋅d–1 (742–3398 t⋅d–1 90 % confidence interval), similar to other regions in the Taupō Volcanic Zone. The CO2 flux from Waimangu and Rotomahana is far higher than from Tarawera (>549 vs. ∼4 t⋅d–1 CO2), likely influenced by a shallow silicic body at depth and Okataina caldera rim faults increasing permeability at the southern end of the fissure. Highly localized regions of elevated CO2 flux occur along the fissure and are likely caused by cross-cutting faults that focus the flow. One of these areas occurs on Tarawera, which is emitting ∼1 t⋅d–1 CO2 with a δ13CO2 of −5.5 ± 0.5 ‰, and comparison with previous observations shows that activity is declining over time. This region highlights the spatial and temporal complexity of degassing pathways at volcanoes and that sub-surface structures exert a primary control on the magnitude of CO2 flux in comparison to the surface mechanism (i.e., CO2 released through the soil or lake surface).
Chemical diffusion of the halogens F, Cl, Br, and I in silica-rich natural melts was experimentally investigated by the diffusion couple technique. Experiments were conducted under anhydrous ...conditions at atmospheric pressure and hydrous conditions (∼1.5 wt.% H2O) at 160 MPa, over a temperature range of 750–1000 °C and 1000–1200 °C, respectively. Quenched trachytic melt samples were analyzed using an electron microprobe (EPMA) and secondary ion mass spectrometry (SIMS).
All halogens exhibit Arrhenian behaviour during diffusion in the investigated melt compositions with F always diffusing fastest. The other halogens show progressively slower diffusion (F > Cl > Br > I) correlated to their ionic radii. In anhydrous melt a diffusivity range of 3–4 orders of magnitude is covered among the halogens with DF(1000 °C) ∼5×10-13 m2s-1 and DI(1000 °C) ∼1×10-16 m2s-1. The diffusivities of all halogens increase in hydrous melt with the largest increase being observed for the slowest-diffusing halogens, e.g., resulting in an increase of up to 2 orders of magnitude for iodine at 1000 °C compared to the anhydrous case. This behavior yields a narrower overall diffusive range of only 1–2 orders of magnitude among all halogens. Activation energies (EA) of diffusion consistently range from ∼200–390 kJ mol-1 in anhydrous melts. In hydrous melt EA generally decreases, with the highest decrease determined for F (∼131 kJ mol-1) and only slight changes for the other halogens (∼201–222 kJ mol-1).
Our diffusivity data of the anhydrous series exhibit a pronounced correlation of diffusivity with the ionic radii, however, this correlation is attenuated in hydrous melt. While in anhydrous melt, halogen diffusion is closely related to ionic porosity, in hydrous experiments, the process of ionic detachment becomes more important as a rate-limiting diffusion mechanism, e.g., comparable to the case of diffusion of divalent/trivalent.
The results of this study provide the first consistent diffusion dataset including all halogens under naturally relevant magmatic conditions and highlight the pronounced compositional effect of both, major element and dissolved H2O on halogen diffusion. The derived diffusion parameters may be readily used for modelling of diffusive fractionation in silicic melts or determining the timescales of natural silicic volcanic processes based on halogen concentration measurements. Furthermore, these data emphasize the potential of diffusive fractionation among the halogens, which may be applied as a monitoring tool for volcanic unrest on actively degassing volcanoes.
•Natural volcanic rock fulgurites record both high- and low-T processes at Earth-Atmosphere interface.•First experimental reproduction of rock fulgurite microtextures and geochemical patterns.•First ...age constraints on paleolightning on a volcanic peak.
The process of meteorological lightning-induced modification of coherent volcanic rocks is examined by geochemical, textural, and experimental analysis of fulgurites from South Sister volcano, Oregon Cascades, USA. Lightning's effects on volcanic target rocks was simulated with an arc-welding device in order to reproduce the geochemical and textural features of natural fulgurites and to constrain temperatures of melting and devolatilization behavior during lightning strikes. Melting of volcanic target rocks produces melts of exceptional compositional diversity, ranging from those with pure mineral compositions (e.g., diopside and plagioclase), resulting from congruent melting reactions, to those that are highly mixed and compositionally identical to the target rock. Geochemical and textural observations on fulgurites suggest that melting proceeds rapidly, by an in situ modal batch process whereby individual crystals melt and contribute small aliquots to a larger batch that is then mixed by viscous flow. Hydrous species remaining in the glasses are trace (OH≪0.1 wt.%), and consistent with partial equilibration to high temperature (>2000°C) supra-liquidus, 1-atm conditions. Lightning also induces physical weathering, in that the target rocks may be vapourized to form channels through the rock, representing injection of elemental components to the atmosphere. Finally, lightning acts as a multi-faceted producer of dry, chemically equivalent volcanic glass, which may be subject to long-term hydration by environmental waters at surface conditions. These re-hydration signatures contrast those of volcanic glasses, which have both magmatic and meteoric components, and are used herein to examine the post-lightning environmental history of these glasses, including age dating lightning events by diffusion chronometry. Our study of volcanic fulgurites illustrates the potential to use their hydration signatures to date paleoweather events at decadal to centuries scale. Because the lightning strike is in and of itself so effective at devolatilizing melts in an instant, the resultant fulgurites are a unique earth material that record individual weather events (i.e., a thunderstorm), and also longer-term paleoweather intervals.
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