First experimental observations on melting and chemical modification of volcanic ash during lightning interaction

Electrification in volcanic ash plumes often leads to syn-eruptive lightning discharges. High temperatures in and around lightning plasma channels have the potential to chemically alter, re-melt, and possibly volatilize ash fragments in the eruption cloud. In this study, we experimentally simulate temperature conditions of volcanic lightning in the laboratory, and systematically investigate the effects of rapid melting on the morphology and chemical composition of ash. Samples of different size and composition are ejected towards an artificially generated electrical arc. Post-experiment ash morphologies include fully melted spheres, partially melted particles, agglomerates, and vesiculated particles. High-speed imaging reveals various processes occurring during the short lightning-ash interactions, such as particle melting and rounding, foaming, and explosive particle fragmentation. Chemical analyses of the flash-melted particles reveal considerable bulk loss of Cl, S, P and Na through thermal vaporization. Element distribution patterns suggest convection as a key process of element transport from the interior of the melt droplet to rim where volatiles are lost. Modeling the degree of sodium loss delivers maximum melt temperatures between 3290 and 3490 K. Our results imply that natural lighting strikes may be an important agent of syn-eruptive morphological and chemical processing of volcanic ash

Pyroclastic eruptions in mid-ocean ridge settings - insights from axial seamount, Juan de Fuca ridge and analogue experiments

This investigation addresses the origin and nature of explosive volcanism in deep submarine environments. Volcaniclastic deposits were sampled as sediment cores from Axial Seamount, a basaltic caldera volcano situated on the Juan de Fuca mid-ocean ridge in the northeast Pacific Ocean. These prominent volcaniclastic deposits resemble those found on other mid-ocean ridge sites. The deposits are interpreted as pyroclastic reflecting explosive eruptions. On Axial Seamount, the volcaniclastic material comprises mainly basaltic glass fragments displaying a range of morphologies, including limu o Pele and Pele's hair. In the present study, glass fragments from several core sections were examined for their morphological characteristics, as well as for major, trace and volatile element compositions. Plagioclase-hosted melt inclusions recovered from selected sections of the volcaniclastic sediments were also analysed for their major, trace and volatile element budgets. The glass fragments show MgO of 9.4–6.5 wt. %, and [La/Yb]N generally between 0.95 and 1.1, with the exception of one sample group showing [La/Yb]N of 0.6–0.85. CO2 concentrations in the melt inclusions were found to vary greatly due to decompression degassing, with concentrations ranging from 260 ppm to 9160 ppm. Cooling histories of glass fragments were estimated by differential scanning calorimetry (DSC), indicating extremely rapid quench rates of up to 10^6 K s^-1 during eruptions. The physical properties of the fragments, i.e., their morphologies and cooling rates, as well as the CO2 budget of the magmatic system, are consistent with explosive activity primarily driven by high levels of magmatic CO2.In order to better understand eruption behaviour and magma fragmentation in high-pressure environments such as the deep ocean, a series of decompression experiments were performed using gum rosin-acetone mixtures as an analogue for magma-volatile systems. These experiments demonstrate that ascent rates and fragmentation behaviour of magma are influenced by various extrinsic and intrinsic parameters such as pre-eruptive bubble content and variations in conduit diameter. These factors can facilitate explosive eruptions in high-pressure environments.Ce travail aborde l'origine et la nature du volcanisme explosif des environnements sous-marins profonds. Des carottes de dépôts volcano-clastiques ont été échantillonnées au volcan Axial, une caldeira basaltique sous-marine située sur la ride médio-océanique Juan de Fuca dans le nord-est du Pacifique. Ces dépôts volcano-clastiques proéminents ressemblent à ceux trouvés sur d'autres sites de ride médio-océanique. Les dépôts sont interprétés comme étant d'origine pyroclastique, donc ayant été formés au cours d'éruptions explosives. Au volcan Axial, le matériel volcano-clastique est composé principalement de fragments de verre basaltique de morphologies diverses, incluant des limu o Pele et des cheveux de Pele. Au cours de cette étude, les caractéristiques morphologiques et la composition en éléments majeurs, traces et volatiles de fragments de verre provenant de plusieurs sections de carottes ont été déterminées. Des inclusions de verre contenues dans des cristaux de plagioclase ont également été analysées afin d'évaluer leur budget en éléments majeurs, traces et volatiles.Les fragments de verre ont une teneur en MgO de 9.4–6.5 %m et des rapports [La/Yb]N généralement entre 0.95 and 1.1, à l'exception d'un groupe d'échantillons ayant des ratios [La/Yb]N de 0.6–0.85. La teneur en CO2 des inclusions de verre fluctue grandement dû au dégazage de décompression avec des mesures variant de 260 à 9160 ppm. L'historique de refroidissement des fragments de verre a été estimé par calorimétrie différentielle à balayage et les résultats indiquent des taux de refroidissement extrêmement rapide de plus de 10^6 K s^-1 durant les éruptions. Les propriétés physiques des fragments, i.e. leurs morphologies et leurs taux de refroidissement, ainsi que le budget du système magmatique en CO2 suggèrent une activité volcanique explosive essentiellement entrainée par les teneurs élevées en CO2 magmatique.Afin de mieux comprendre le comportement éruptif et la fragmentation du magma dans des environnements de haute pression tels les fonds marins, des expériences de décompression ont été menées en utilisant des mélanges de colophane et d'acétone en tant qu'analogue des systèmes magma-gaz. Ces expériences démontrent que les rythmes de montée du magma et sa fragmentation sont influencés par plusieurs paramètres extrinsèques et intrinsèques tels que la concentration de bulles dans le magma avant l'éruption et le diamètre du conduit magmatique. Ces facteurs peuvent faciliter le volcanisme explosif en milieu sous-marin à haute pression

Explosive eruptions at mid-ocean ridges driven by CO2-rich magmas

Author Posting. © The Authors, 2011. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature Geoscience 4 (2011): 260–263, doi:10.1038/ngeo1104.The abundance of volatile compounds, and particularly 18 CO2, in the upper oceanic mantle affects the style of volcanic eruptions. At mid-ocean ridges, eruptions are generally dominated by the gentle effusion of basaltic lavas with a low volatile content. But, explosive volcanism has been documented at some ocean spreading centres1-3, indicative of abundant volatile compounds. Estimates of the initial CO2 concentration of primary magmas can be used to constrain the CO2 content of the upper oceanic mantle, but these estimates vary greatly4,5. Here we present ion microprobe measurements of the CO2 content of basaltic melt trapped in plagioclase crystals. The crystals are derived from volcanic ash deposits erupted explosively at Axial Seamount, Juan de Fuca Ridge, in the northeast Pacific Ocean. We report unusually high CO2 concentrations of up to 9,160 ppm, which indicate that the upper oceanic mantle is more enriched in carbon than previously thought. And we furthermore suggest that CO2 fluxes along mid-ocean ridges4,5 vary significantly. Our results demonstrate that elevated fluxes of CO2 from the upper oceanic mantle can drive explosive eruptions at mid-ocean ridges.The expedition and DAC were supported through a grant to MBARI from the David and Lucile Packard Foundation. C.H. was supported by R.H. Tomlinson, GEOTOP and J.W. McConnell Memorial Fellowships at McGill University. J.S. was supported by grants from the Natural Sciences and Engineering Research Council of Canada

An expanded model and application of the combined effect of crystal-size distribution and crystal shape on the relative viscosity of magmas

International audienceThis study examines the combined effect of crystal-size distributions (CSD) and crystal shape on the rheology of vesicle free magmatic suspensions and provides the first practical application of an empirical model to estimate the relative effect of crystal content and CSD's on the viscosity of magma directly from textural image analysis of natural rock samples in the form of a user-friendly texture-rheology spreadsheet calculator. We extend and apply established relationships between the maximum packing fraction ϕm of a crystal bearing suspension and both its rheological properties and the polydispersity γ of a CSD. By using analogue rotational rheometric experiments with glass fibres and glass flakes in silicone oil acting as magma equivalent, this study also provides new insights in the relationship between ϕm and the aspect ratio rp of suspended particles

Experimentally-derived F, Cl and Br fluid/melt partitioning of intermediate to silicic melts in shallow magmatic systems

The conditions under which halogens partition in favor of an exsolved fluid relative to the coexisting melt are key for understanding many magmatic processes, including volcanic degassing, evolution of crustal melt bodies, and ore formation. We report new F, Cl, and Br fluid/melt partition coefficients for intermediate to silicic melts, for which F and Br data are particularly lacking; and for varying CO2-H2O contents to assess the effects of changing fluid composition (XH2O) on Br fluid/melt partitioning for the first time. The experiments were conducted at pressures 50–120 MPa, temperatures 800–1100 °C, and volatile compositions [molar XH2O = H2O/(H2O +CO2)] of 0.55 to 1, with redox conditions around the Nickel-Nickel Oxygen buffer (fO2 ≈ NNO). Experiments were not doped with Cl, Br, or F and were conducted on natural crystal-bearing volcanic products at conditions close to their respective pre-eruptive state. The experiments therefore provide realistic constraints on halogen partitioning at naturally occurring, brine-undersaturated conditions. Measurements of Br, Cl, and F were made by Secondary Ion Mass Spectrometry (SIMS) on 13 experimental glass products spanning andesite to rhyolitic compositions, together with their natural starting materials from Kelud volcano, Indonesia, and Quizapu volcano, Chile. Fluid compositions were constrained by mass balance. Average bulk halogen fluid/melt partition coefficients and standard deviations are: Dfluid/meltCl = 3.4 (±3.7 1 s.d.), Dfluid/meltF = 1.7 (±1.7), and Dfluid/meltBr = 7.1 (±6.4) for the Kelud starting material (bulk basaltic andesite), and Dfluid/meltCl = 11.1 (±3.5), Dfluid/meltF = 0.8 (±0.8), and Dfluid/meltBr = 31.3 (±20.9) for Quizapu starting material (bulk dacite). The large range in average partition coefficients is a product of changing XH2O, pressure and temperature. In agreement with studies on synthetic melts, our data show an exponential increase of halogen Dfluid/melt with increasing ionic radius, with partitioning behavior controlled by melt composition according to the nature of the complexes forming in the melt (e.g., SiF4, NaCl, KBr). The fundamental chemistry of the different halogens (differing ionic size and electronegativities) controls the way in which partitioning responds to changes in melt composition and other variables. Experimental results confirm that more Cl partitions into the fluid at higher bulk Cl contents, higher melt Na, higher fluid XH2O ratios, and lower temperatures. Bromine shows similar behavior, though it seems to be more sensitive to temperature and less sensitive to Na content and XH2O. In contrast, F partitioning into the fluid increases as the melt silica content decreases (from 72 to 56 wt% SiO2), which we attribute to the lower abundance of Si available to form F complexes in the melt. These new data provide more insights into the conditions and processes that control halogen degassing from magmas and may help to inform the collection and interpretation of melt inclusions and volcano gas data

Volatile dilution during magma injections and implications for volcano explosivity

Magma reservoirs underneath volcanoes grow through episodic emplacement of magma batches. These pulsed magma injections can substantially alter the physical state of the resident magma by changing its temperature, pressure, composition, and volatile content. Here we examine plagioclase phenocrysts in pumice from the 2014 Plinian eruption of Kelud (Indonesia) that record the progressive capture of small melt inclusions within concentric growth zones during crystallization inside a magma reservoir. High-spatial-resolution Raman spectroscopic measurements reveal the concentration of dissolved H2O within the melt inclusions, and provide insights into melt-volatile behavior at the single crystal scale. H2O contents within melt inclusions range from ∼0.45 to 2.27 wt% and do not correlate with melt inclusion size or distance from the crystal rim, suggesting that minimal H2O was lost via diffusion. Instead, inclusion H2O contents vary systematically with anorthite content of the host plagioclase (R2 = 0.51), whereby high anorthite content zones are associated with low H2O contents and vice versa. This relationship suggests that injections of hot and H2O-poor magma can increase the reservoir temperature, leading to the dilution of melt H2O contents. In addition to recording hot and H2O-poor conditions after these injections, plagioclase crystals also record relatively cold and H2O-rich conditions such as prior to the explosive 2014 eruption. In this case, the elevated H2O content and increased viscosity may have contributed to the high explosivity of the eruption. The point at which an eruption occurs within such repeating hot and cool cycles may therefore have important implications for explaining alternating eruptive styles