CO2-crystal wettability in potassic magmas. Implications for eruptive dynamics in light of experimental evidence for heterogeneous nucleation.

The volatile content in magmas is fundamental for the triggering and style of volcanic eruptions. Carbon dioxide, the second most abundant volatile component in magmas after H2O, is the first to reach saturation upon ascent and depressurization. We investigate experimentally CO2-bubble nucleation in trachybasalt and trachyte melts at high temperature and high pressure (HT and HP) through wetting-angle measurements on different (sialic, mafic or oxide) phenocryst phases. The presence of crystals lowers the supersaturation required for CO2- bubble nucleation up to 37 per cent (heterogeneous nucleation, HeN), with a minor role of mineral chemistry. Different from H2O-rich systems, feldspar crystals are effective in reducing required supersaturation for bubble nucleation. Our data suggest that leucite, the dominant liquidus phase in ultrapotassic systems at shallow depth (i.e. <100 MPa), facilitates late-stage, extensive magma vesiculation through CO2 HeN, which may explain the shifting of CO2-rich eruptive systems towards an apparently anomalous explosive behaviour

A bedform phase diagram for dense granular currents

Pyroclastic density currents (PDCs) are a life-threatening volcanic hazard. Our understanding and hazard assessments of these flows rely on interpretations of their deposits. The occurrence of stratified layers, cross-stratification, and bedforms in these deposits has been assumed as indicative of dilute, turbulent, supercritical flows causing traction-dominated deposition. Here we show, through analogue experiments, that a variety of bedforms can be produced by denser, aerated, granular currents, including backset bedforms that are formed in waning flows by an upstream-propagating granular bore. We are able to, for the first time, define phase fields for the formation of bedforms in PDC deposits. We examine how our findings impact the understanding of bedform features in outcrop, using the example of the Pozzolane Rosse ignimbrite of the Colli Albani volcano, Italy, and thus highlight that interpretations of the formative mechanisms of these features observed in the field must be reconsidered

Reconciling complex stratigraphic frameworks reveals temporally and geographically variable depositional patterns of the Campanian Ignimbrite

The 39.8-ka Campanian Ignimbrite was emplaced during a large caldera-forming eruption of Campi Flegrei near Naples, Italy. The ignimbrite is found up to 80 km from the caldera, and co-ignimbrite ash-fall deposits occur 3200 km away. The proximal and distal stratigraphy of the Campanian Ignimbrite has not been definitively correlated due to the dissimilar appearance of the proximal and distal deposits, a lack of medial exposures, and the inconsistency and heterogeneity of the proximal stratigraphy. Here, we document the major-element glass-shard chemistry, matrix componentry, and lithic componentry of the proximal and distal stratigraphic sequences of the ignimbrite to attempt to correlate the units. The results of these disparate observations taken together suggest that the established stratigraphic units cannot be directly and uniquely correlated between the proximal and distal regions and that neither the proximal nor distal stratigraphy provides a record of the entire eruptive sequence. However, the characteristics studied can be used to demarcate eruptive phases that are connected to some of the defined units in the proximal and distal stratigraphy

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The Magnitude of the 39.8 ka Campanian Ignimbrite Eruption, Italy: Method, Uncertainties and Errors

The calculation of the magnitude of an eruption needs the accurate estimate of its deposit volume. This is particularly critical for ignimbrites as no methods for their volume calculations and associated errors and uncertainties are consolidated in the literature, although invariably the largest magnitude eruptions on Earth are made of ignimbrites. The 39.8 ka Campanian Ignimbrite (CI) eruption is the largest of the Campi Flegrei caldera (Italy). The global cooling following the CI eruption and its widespread tephra affected the paleoenvironment and the migration of hominids in Europe at that time. Despite the large number of studies, the estimates of the Dense Rock Equivalent volume of the CI range between 60 and 300 km3 , because of the lack of clear and reproducible methods for its calculation. Here we present a new calculation of the volume of the CI, grounded on a clear and reproducible method that can be applied universally and which provides an accurate estimation of the volume of the deposits on ground and their uncertainties and errors, allowing a strong base for further estimates of the amount of deposits eroded, covered, elutriated, which are essential for the final computation of the eruption magnitude. In order to calculate the CI volume, we reconstructed the first total isopach map of the pyroclastic density current deposit preserved on land, developed through a method that reconstructs the paleotopography during the eruption, which is reproducible for all topographically controlled ignimbrites and allows the calculation of well-defined uncertainties in the on-land ignimbrite deposits. The preserved total extra-caldera bulk volume of the ignimbrite is estimated at 68.2 ± 6.6 km3 . The total pyroclastic density current deposit volume is then corrected for erosion, ash elutriation, the intracaldera deposit volume, and the volume of tephra deposited in the sea, whereas volumes of the basal fallout deposits are taken from other studies. The total Dense Rock Equivalent volume of the eruption is 181–265 km3 , whose range accounts for errors and uncertainties. This value corresponds to a mass of 4.7–6.9 × 1014 kg, a magnitude (M) of 7.7–7.8 and a volcanic Explosivity Index (VEI) of 7

CO2 bubble nucleation upon pressure release in potassium-rich silicate magmas.

We report the first experimental study on CO2 bubble nucleation and growth in silicate magmas in response to pressure release. The starting materials were nominally anhydrous trachyte and trachybasalt glasses that were CO2 saturated at 1200 °C and 300 MPa for at least 48 h under oxidizing conditions. These starting glasses were doped with diverse crystals relevant to the studied melt compositions: olivine, Cr-spinel, clinopyroxene, K-feldspar and leucite. Decompression experiments were performed after an initial equilibration at 1128–1156 °C and 300 MPa for time durations ranging between 15 and 45 h. Pressure was subsequently decreased to a final value of 30 MPa with a decompression rate of 4 MPa/min. Backscattered electron microphotographs of the quenched products were collected using a scanning electron microscope. In all experiments, we observed an event of homogeneous and heterogeneous bubble nucleation, testified by the formation of vesicles with diameters up to 30 μm on the crystal rims as well as in the residual melts. In the samples containing K-feldspar and leucite, another generation of bubbles with diameter up to 130 μm is present in proximity of those crystals, suggesting the occurrence of an earlier heterogeneous nucleation event. CO2 bubble nucleation and growth was investigated by the determination of the wetting angles (ϑ) between the vesicles and the crystal surfaces, which can be used as proxy for the efficiency of the crystals as nucleation sites. It was calculated that the activation energy for bubble nucleation in the studied magmas it is lowered by a factor φ up to approx. 0.64 in presence of crystals. The experimental results show several peculiarities of heterogeneous CO2 bubble nucleation: i) CO2 vesicles form indiscriminately on diverse crystals, in accordance to the small ϑ distribution range between 40° and 70°; ii) CO2 bubbles do not nucleate preferentially on capsule walls; iii) the crystal shapes seem to not to influence bubble nucleation. Our findings are compared with data on heterogeneous H2O bubble nucleation from previous studies to highlight distinctive features. Our new experimental data on crystal wettability in CO2 dominated magmas allow a first assessment of the surface tension through the classical nucleation theory approach. The new findings about CO2 bubble nucleation will be useful to better understand the onset and the intensity of the eruptions in the K-rich magmatic systems

Parameterizing multi-vent activity at Stromboli Volcano (Aeolian Islands, Italy)

The crater terrace of Stromboli Volcano (Italy) hosts several active vents which have evolved and migrated through time within three main vent areas: south-west (SW), central (C), and north-east (NE). Frequent, jet-like explosions typically take place, episodically interrupted by larger-scale paroxysms, which can substantially modify the morphology of the crater terrace and vent geometries. However, the link between the time-space evolution of vent activity and the shallow conduit system are still a matter of debate. In this work, we analyze the vent position and explosion parameters (jet duration and geometry) of 4296 events at Stromboli in five 72-h-long time-windows between 2005 and 2009, as recorded by an infrared surveillance camera. Vent locations illustrate the resilience of the shallow conduit system, which controls explosive activity at different time scales and depths. At the shallowest depth, where slugs burst, conduit branching and merging determines the evolution of simultaneous or alternating twin vents, while vent shape and slug size control local explosion parameters. These processes show variability on an hourly to daily time scale. Below the depth of the slug burst, the conduit system feeding each vent area controls which specific vent will host the explosions and also, possibly, the size of the slugs. Several observations suggest that the C and SW vent areas may be connected at this depth. The deeper conduit system, common to all vent areas, sets the overall explosion rate of the volcano and maintains a balance of this rate between the NE and the combined SWand C vent areas