Magma mixing

In order to increase our understanding of magma mixing processes and their impact on the geochemical evolution of silicate melt we present in the following works, the first set of experiments performed using natural basaltic and rhyolitic melts. In particular, we investigate the interplay of physical dynamics and chemical exchanges between these two melts using time-series mixing experiments performed under controlled, chaotic, dynamical conditions. The variation of major and trace elements is studied in detail by electron microprobe (EMPA) and Laser Ablation ICP-MS (LA-ICP-MS) and the time-evolution of chemical exchanges during mixing is investigated. Using the concentration variance as a proxy to measure the rate of chemical element homogenization in time, a model to quantify chemical element mobility during chaotic mixing of natural silicate melts is proposed. The morphology of mixing patterns at different times is quantified by measuring their fractal dimension and an empirical relationship between mixing time and morphological complexity is derived. The complexity of mixing patterns is also compared to the degree of homogenization of chemical elements during mixing and empirical relationships are established between the fractal dimension and the variation of concentration variance of chemical elements in time. Finally we discuss the petrological and volcanological implications of this work.Um unser Verständnis über die Prozesse bei der Vermischung von Magmen und dessen Auswirkungen auf die geochemische Entwicklung von Silikatschmelzen zu verbessern, werden in dieser Arbeit erstmalig eine Reihe von „Magma Mixing“ Experimenten vorgestellt, in der natürliche Basalte und Rhyolite verwendet werden. In dynamischen Zeitreihen-Mischungsexperimenten, die unter kontrollierten, chaotischen, dynamischen, Bedingungen abliefen, wurde vor allem das Zusammenspiel der physikalischen Prozessen durch das mechanische Vermengen zweier Schmelzen und der resultierenden chemischen Austauschreaktionen durch Diffusion an den Grenzflächen zwischen diesen beiden Schmelzen untersucht. Die Variation von Haupt- und Spurenelementen wurde mittels Elektronen-Mikrosonde (EMPA) und Laser Ablation ICP-MS (LA-ICP-MS) im Detail untersucht und zusätzlich konnte die zeitliche Entwicklung des chemischen Austauschs während des Mischungsvorgang dargestellt werden. Die Varianz der Konzentration einzelner Elemente über die Grenzflächen zwischen Basalt und Rhyolit hinweg wurde als Proxy verwendet, um die Homogenisierungsrate der chemischen Elemente bezogen auf die Zeit zu bestimmen. Dies wird als Modell vorgeschlagen, mit dem die Mobilität chemischer Elemente während chaotischem Mischens von natürlichen Silikatschmelzen quantifiziert werden kann. Im Weiteren wurde mit Hilfe von Fraktalanalyse die Morphologie der Mischungsmustern zu unterschiedlichen Zeiten quantifiziert und eine empirische Beziehung zwischen Mischzeit und morphologischer Komplexität abgeleitet. Die Komplexität der Mischungsmuster wurde zudem mit dem Grad der Homogenisierung der chemischen Elemente während des Mischens verglichen. Dadurch konnten empirische Beziehungen zwischen der fraktalen Dimension von Mischungsmorphologien und der zeitlichen Variation der Konzentration von chemischen Elementen abgeleitet werden. Im Laufe dieser Arbeit werden zudem die petrologischen und vulkanologischen Auswirkungen diskutiert

volcanic ash aggregation enhanced by seawater interaction the case of the secche di lazzaro phreatomagmatic deposit stromboli

The Secche di Lazzaro formation (ca. 6.2-7 kys BP) is a phreatomagmatic deposit situated in the southwestern part of the island of Stromboli (Aeolian Archipelago, Italy). The volcanic sequence is comprised of three main units. In the lower unit accretionary lapilli are particularly abundant and are characterized by strong cementation between the particles and an uncommon resistance to breakage. To understand the processes behind the formation of the Secche di Lazzaro (SdL) accretionary lapilli a multi-analytical approach was used on the lapilli Aggregate Tuff (AT), and on single Accretionary Lapilli (AL). We carried out granulometric analysis, Field Emission – Scanning Electron Microscopy (FE-SEM), Electron Microprobe Analysis (EMPA), X-ray powder diffraction (XRPD) and 3D imaging by X-ray micro-tomography (X-mCT). The granulometric data show that most particles in the AT have a diameter equal to Φ -1 corresponding to 2 mm. The EMPA, FE-SEM and XRPD analyses reveal the presence of different mineral phases, mainly plagioclase, K-feldspar, halite, and clinopyroxene, together with volcanic glass. From the X-mCT analysis, we constrained the particle distribution and estimated the porosity of AL. The results of the FE-SEM images provided the chemical distribution within individual lapilli allowing the identification of rim and core zoning as well as the presence of halite located both on the border of single lapilli and on the juncture between different lapilli. Moreover, halite occurs among different aggregates in single AL, thus acting as a binding agent, as well as within rim pores. The results of this work shed new light into the formation of accretionary lapilli in phreatomagmatic eruption a t volcanic island involving marine water

Water-enhanced interdiffusion of major elements between natural shoshonite and high-K rhyolite melts

The interdiffusion of six major elements (Si, Ti, Fe, Mg, Ca, K) between natural shoshonite and a high-K calc-alkaline rhyolite (Vulcano island, Aeolian archipelago, Italy) has been experimentally measured by the diffusion couple technique at 1200{\deg}C, pressures from 50 to 500 MPa and water contents from 0.3 (nominally dry) to 2 wt%. The experiments were carried out in an internally heated pressure vessel, and major element profiles were later acquired by electron probe microanalysis. The concentration-distance profiles are evaluated using a concentration-dependent diffusivity approach. Effective binary diffusion coefficients for four intermediate silica contents are obtained by the Sauer-Freise modified Boltzmann-Matano method. At the experimental temperature and pressures, the diffusivity of all studied elements notably increases with dissolved H2O content. Particularly, diffusion is up to 1.4 orders of magnitude faster in a melt containing 2 wt.% H2O than in nominally dry melts. This effect is slightly enhanced in the more mafic compositions. Uphill diffusion was observed for Al, while all other elements can be described by the concept of effective binary interdiffusion. Ti is the slowest diffusing element through all experimental conditions and compositions, followed by Si. Fe, Mg, Ca and K diffuse at similar rates but always more rapidly than Si and Ti. This trend suggests a strong coupling between melt components. Since effects of composition (including water content) are dominant, a pressure effect on diffusion cannot be clearly resolved in the experimental pressure range

Exponential Decay Of Concentration Variance During Magma Mixing: Robustness Of A Volcanic Chronometer And Implications For The Homogenization Of Chemical Heterogeneities In Magmatic Systems

The mixing of magmas is a fundamental process in the Earth system causing extreme compositional variations in igneous rocks. This process can develop with different intensities both in space and time, making the interpretation of compositional patterns in igneous rocks a petrological challenge. As a time-dependent process, magma mixing has been suggested to preserve information about the time elapsed between the injection of a new magma into sub-volcanic magma chambers and eruptions. This allowed the use of magma mixing as an additional volcanological tool to infer the mixing-to-eruption timescales. In spite of the potential of magma mixing processes to provide information about the timing of volcanic eruptions its statistical robustness is not yet established. This represents a prerequisite to apply reliably this conceptual model. Here, new chaotic magma mixing experiments were performed at different times using natural melts. The degree of reproducibility of experimental results was tested repeating one experiment at the same starting conditions and comparing the compositional variability. We further tested the robustness of the statistical analysis by randomly removing from the analysed dataset a progressively increasing number of samples. Results highlight the robustness of the method to derive empirical relationships linking the efficiency of chemical exchanges and mixing time. These empirical relationships remain valid by removing up to 80% of the analytical determinations. Experimental results were applied to constrain the homogenization time of chemical heterogeneities in natural magmatic system during mixing. The calculations show that, when the mixing dynamics generate millimetre thick filaments, homogenization timescales of the order of a few minutes are to be expected

Diffusive exchange of trace elements between alkaline melts: implications for element fractionation and timescale estimations during magma mixing

The diffusive exchange of 30 trace elements during the interaction of natural mafic and silicic alkaline melts was experimentally studied at conditions relevant to shallow magmatic systems. In detail, a set of 12 diffusion couple experiments have been performed between natural shoshonitic and rhyolitic melts from the Vulcano Island (Aeolian archipelago, Italy) at a temperature of 1200 {\deg}C, pressures from 50 to 500 MPa, and water contents ranging from nominally dry to ca. 2 wt. %. Concentration-distance profiles, measured by Laser Ablation ICP-MS, highlight different behaviours, and trace elements were divided into two groups: (1) elements with normal diffusion profiles (13 elements, mainly low field strength and transition elements), and (2) elements showing uphill diffusion (17 elements including Y, Zr, Nb, Pb and rare earth elements, except Eu). For the elements showing normal diffusion profiles, chemical diffusion coefficients were estimated using a concentration-dependent evaluation method, and values are given at four intermediate compositions (SiO2 equal to 58, 62, 66 and 70 wt. %, respectively). A general coupling of diffusion coefficients to silica diffusivity is observed, and variations in systematics are observed between mafic and silicic compositions. Results show that water plays a decisive role on diffusive rates in the studied conditions, producing an enhancement between 0.4 and 0.7 log units per 1 wt.% of added H2O. Particularly notable is the behaviour of the trivalent-only REEs (La to Nd and Gd to Lu), with strong uphill diffusion minima, diminishing from light to heavy REEs. Modelling of REE profiles by a modified effective binary diffusion model indicates that activity gradients induced by the SiO2 concentration contrast are responsible for their development, inducing a transient partitioning of REEs towards the shoshonitic melt.Comment: 57 pages, 12 figures, 5 table

Calibrating Carbonization Temperatures of Wood Fragments Embedded within Pyroclastic Density Currents through Raman Spectroscopy

The Grant of Excellence Departments, MIUR (ARTICOLO 1, COMMI 314–337 LEGGE 232/2016) is gratefully acknowledged. MathWorks is acknowledged for providing the Matlab license for this work. A version of the script for calculating temperatures from Raman spectra can be provided by the authors upon request. This research was funded by an MIUR Roma Tre post-doctoral grant (2017–20) No. REP. 22-PROT. 219 of 26 January 2017.Peer reviewedPublisher PD

Decoupling the volcano infrasound source from the crater acoustic response

Volcano infrasound is an important component of multi-disciplinary volcano geophysics and has proven utility for tracking eruptive activity and quantifying eruption dynamics. Unfortunately, a major limitation in our interpretation of volcano infrasound is that it is critically affected by the morphology of the volcanic crater, which can transform potentially simple source-time functions occurring within the crater into a signal that is substantially more complex. If infrasound waveforms are to be used to recover important physical parameters about an eruption source, then a robust understanding of the acoustic response of the crater is required. In many cases, and especially for large deep craters, the acoustic response function acts as a severe filter. For example, at Cotopaxi Volcano (Ecuador) infrasound ‘tornillos’ with an impulsive onset and peaked spectra at 0.2 Hz decaying for more than 90 s are part of the source response due to the crater’s steep-walled, deep crater. We analyze broadband infrasound data from open-vent volcanoes with a wide variety of crater geometries and jointly calculate their crater acoustic response using 1-D (axisymmetric) and 3-D morphologies derived from structure-from-motion digital terrain models. We analyze both explosion and lava lake infrasound from Villarrica (Chile), Stromboli (Italy), and Nyiragongo (Democratic Republic of the Congo) to demonstrate a broad spectrum of volcano infrasound, whose attributes are heavily influenced by crater shape. We demonstrate how some differences between simulations and recorded explosion are influenced by sourcetime functions, which may range from brief and impulsive to complicated or extended in time. Numerical modeling shows that each volcanic crater has a unique impulse response and that deconvolving this acoustic response is vital for estimating important eruption parameters including the size of volcanic explosions.PublishedNapoli (Italia)5V. Processi eruttivi e post-eruttiv

magma mixing history and dynamics of an eruption trigger

The most violent and catastrophic volcanic eruptions on Earth have been triggered by the refilling of a felsic volcanic magma chamber by a hotter more mafic magma. Examples include Vesuvius 79 AD, Krakatau 1883, Pinatubo 1991, and Eyjafjallajokull 2010. Since the first hypothesis, plenty of evidence of magma mixing processes, in all tectonic environments, has accumulated in the literature allowing this natural process to be defined as fundamental petrological processes playing a role in triggering volcanic eruptions, and in the generation of the compositional variability of igneous rocks. Combined with petrographic, mineral chemistry and geochemical investigations, isotopic analyses on volcanic rocks have revealed compositional variations at different length scales pointing to a complex interplay of fractional crystallization, mixing/mingling and crustal contamination during the evolution of several magmatic feeding systems. But to fully understand the dynamics of mixing and mingling processes, that are impossible to observe directly, at a realistically large scale, it is necessary to resort to numerical simulations of the complex interaction dynamics between chemically different magmas

Experimental investigations on degassing behavior and related seismo-acoustic markers: the effect of complex conduit geometries

One of the main challenge in monitoring active volcanoes is understanding gas-magma dynamics in volcanic conduits and relating them with their geophysical markers at the surface. Accordingly, we combine here two main approaches used to investigate conduit dynamics via indirect observations, i.e. analogue laboratory experiments and seismo-acoustic measurements, and address a crucial, yet unexplored, subject: the irregularity (i.e. the departure from an ideal smooth cylindrical shape) of the conduit surface. To this aim, we developed a protocol to assemble epoxy conduits with different fractal dimensions (Dc; i.e. irregularity) of the internal surface, and used silicone oil as a proxy for magma. We investigated different degassing patterns, from bubbly to slug and churn-annular flow, by varying systematically: 1) injected gas flux (5 to 180x10-3 l/s); 2) analogue magma viscosity (10 to 1000 Pas); 3) fractal dimension (Dc) of the conduit surface (i.e. Dc=2, Dc=2.18 and Dc =2.99). The experiments were monitored by tracking the temporal evolution of sample expansion and outgassing periodicity through a video-camera and investigating the relative seismo-acoustic fingerprint by means of a set of dedicated sensors (i.e. microphone, piezo-films, accelerometer). Results show that viscosity strongly influences the transition among degassing patterns and the frequency of slug bursts at the surface. Furthermore, we noticed an increase of the exponent of the power law equation linking squared seismic amplitude to gas flow rate with conduit roughness; whereas the opposite trend was observed increasing the viscosity of the liquid phase. These results have fundamental implications for linking eruption source parameters such as the volume discharge rate to seismic data (i.e. volcanic tremor) at different volcanoes or for investigating their temporal evolution at a single vent.UnpublishedParma5V. Processi eruttivi e post-eruttiv

An experimental approach to investigate seismo-acoustic markers of degassing patterns

The analysis of geophysical patterns enables tracking the surface effects of sub-surface volcanic processes with great detail and provides a fundamental tool for the surveillance of active volcanoes. The full exploitation of geophysical data (in particular seismic and infrasonic) requires the capability of linking quantitatively fluid dynamics and degassing processes at depth with signals recorded at the surface. Nonetheless, the outcomes of the attempts made so far are still considered very uncertain because of volcanoes inaccessibility to direct observation on fundamental parameters such as plumbing system geometry and magma properties. This issue can be solved by integrating field measurements with laboratory experiments. To this extent, we developed a novel experimental device aimed to mimick volcano degassing processes with different regimes and gas flow rates, and allowing for the investigation of the related seismoacoustic emissions. The implemented device permitted us to (i) precisely fix and control fundamental parameters such as the geometry of the structure where the two-phase analogue mixture flows, the gas flow rate (5-180x10-3 l/s), and the fluid viscosity (10-1000 Pas); (ii) measure micro-seismic signals in fixed locations of the analog conduit by means of an array of laboratory sensors (including one microphone, two piezo-film sensors and one accelerometer); (iii) directly observe the degassing pattern through the optically clear analog magma and define the degassing regime producing the seismo-acoustic radiations.PublishedNapoli (Italia)5V. Processi eruttivi e post-eruttiv