Search

Understanding the pre-eruptive volatile contents in magmas is critical to charactering the magmatic plumbying systems that feed acative volcanoes, and is key to volcano monitoring and volcanic hazard assessment. Silicate melt inclusions (MIs) hosted in primitive minerals are a powerful tool to definite parental melt volatile contents, and to track the volatile degassing path upon magma ascent and decompression.Here, we apply different analyses (Raman Spectroscopy, Nano SIMS, Electron microprobe, Laser Ablation ICPMS) for the characterisation of major and trace elements and volatiles in silicate melt inclusions entrapped in minerals from recently erupted tephra by Fogo Volcano in Cape Verde archipelago, one of the most active intraplate volcanic systems on Earth.Our aims are to (i) characterise the pressure-dependent magma compositional changes taking place during magma storage and ascent, (ii) model magmatic degassing and (iii) constrain the magmatic source, and the rates/modes of magma ascent prior and during eruption.Seventeen MIs hosted in twelve olivine phenocrysts (Fo79-85) were examined from tephra samples of two distinct periods of the last 10 ky of activity of the volcano. In detail, we studied basanitic (SiO2 ~42 wt.%, MgO ~4.8 wt. %) and alkali-rich (Na2O + K2O = 6.9 wt.%) tephra samples of São Jorge (early Holocene activity, ~10 ka) and of the most recent eruptions (1951 and 2014/15).Results reveal high concentrations of incompatible trace elements (e.g.,˜70 ppm Nb) and dissolved volatiles ( ˜2.1 wt.% H2O and ≥1 wt.% CO2) in the parental (un-degassed) magma. We use different H2O-CO2 solubility models to estimate MI entrapment pressures along the magma plumbing system. The deepest entrapment pressures of ˜1000-1400 MPa (corresponding to ˜ 30-46 km) are recorded in Holocene products, while the inclusions from the recent eruptions indicate shallower entrapment pressures of ˜ 350-1100 MPa (˜ 11-35km). These entrapment pressure data, combined with previous independent barometric results, demonstrate a relatively deep (30-40 km) magma source for Fogo eruptions. Our results are the first to unambiguously demonstrate the CO2-rich nature of alkali-rich mafic melts feeding intraplate volcanism at Cape Verde.

Experimental homogenization of olivine-hosted melt inclusions representative of near-primary basic and ultrabasic magmas is a powerful approach to investigate the nature of their source regions and the melting conditions in Earth's mantle. There is growing evidence that the total CO2 contents of olivine-hosted melt inclusions may reach values of the order of a single to several weight percent, especially in intraplate continental basalts. To be able to homogenize melt inclusions with such high CO2 contents, we developed a technique allowing for heat treating of the melt inclusions under hydrostatic pressures up to 3–4 GPa in a piston cylinder, using thick-walled Au80–Pd20 containers and molten NaCl as the surrounding medium for the inclusion-bearing olivines. We applied this technique to olivine phenocrysts from Thueyts basanite, Bas-Vivarais volcanic province, French Massif Central. Thueyts melt inclusions were chosen because of their high CO2 contents, as indicated by up to 1.19 wt % dissolved CO2 in the glasses and by the presence of shrinkage bubbles containing abundant carbonate microcrystals in addition to a CO2 fluid phase. The homogenization experiments were conducted at pressures of 1.5 to 2.5 GPa, temperatures of 1275 and 1300 ∘C, and run durations of 30 min. In all the melt inclusions treated at 2.5 GPa–1300 ∘C and half of those treated at 2 GPa–1300 ∘C, we were able to completely homogenize the inclusions, as indicated by the disappearance of the starting bubbles, and we obtained total CO2 contents ranging from 3.2 wt % to 4.3 wt % (3.7 wt % on average). In all the other melt inclusions (equilibrated at 1.5 or 2 GPa and 1300 ∘C or at 2.5 GPa–1275 ∘C), we obtained lower and more variable total CO2 contents (1.4 wt % to 2.9 wt %). In the inclusions with the highest total CO2 contents, the size of the shrinkage bubble was in most cases small (

International audience; Although explosivity is linked with high decompression rates induced by magma ascent, the quantitative relationships between decompression rate and eruption energy have yet to be properly assessed, especially for open-conduit basaltic volcanoes, where ordinary weak activity can rapidly evolve into more intense eruptions. Here, we selected three eruptions of different explosivity from Mt. Etna's recent activity to study the relationships between the observed explosive intensities and decompression rates determined through diffusion chronometry, which is based on modeling volatile diffusion along olivine-hosted melt embayments. The approach used in this study has provided important indications on differences in the timescales of decompression-driven degassing for magmas emitted with markedly distinct eruptive dynamics, starting from similar physical and chemical conditions of the magmas involved in the three eruptions. The intense paroxysmal activity at Voragine Crater on December 3, 2015, was fostered by high decompression rate (∼0.36-0.74 MPa/s), slightly higher than in the less energetic paroxysm that occurred on February 19, 2013, at New South-East Crater (NSEC) (∼0.14-0.29 MPa/s). Decompression rates of magmas emitted during lava fountaining are one order of magnitude greater than values obtained for the mild flank eruption that occurred in December 2018 (∼0.045-0.094 MPa/s). Our results indicate that degassing kinetics controlled the intensity of activity at Mt. Etna, thus suggesting that the explosivity does not depend exclusively on the degree of overpressurization of the shallowest reservoir due to injection of gas from the deepest levels of the plumbing system.

The distribution of ultrahigh-pressure metamorphism (UHP) at the scale of a mountain belt is of prime importance for deciphering its past subduction history. In the Western Alps, coesite has been recognized in the southern Dora-Maira massif, in the lens-shaped Brossasco-Isasca Unit, but has not been found up to now in the other parts of the massif. We report the discovery of a new UHP unit in the northern Dora-Maira Massif (Western Alps), named Chasteiran Unit (Manzotti et al. 2022). It is only a few tens of metres thick and consists of garnet-chloritoid micaschists. Garnet inclusions (chloritoid, rutile) and its growth zoning allow to precisely model the P–T evolution. Coesite crystals, which are pristine or partially transformed to palisade quartz occur as inclusions in the garnet outer cores. According to thermodynamic modelling, garnet displays a continuous record of growth during the prograde increase in P and T (25–27 kbar 470–500 °C) (stage 1), up to the coesite stability field (27–28 kbar 520–530 °C) (stage 2), as well as sub-isothermal decompression of about 10 kbar (down to 15 kbar 500–515 °C) (stage 3). The main regional, composite, foliation, marked by chloritoid and rutile, began to develop during this stage, and was then overprinted by chlorite-ilmenite (stage 4). The Chasteiran Unit is discontinuously exposed in the immediate hangingwall of the Pinerolo Unit, and it is located far away from, and without physical links to the classic UHP Brossasco-Isasca Unit. Moreover, it records a different, much colder, P–T evolution, showing that different slices were detached from the downgoing subduction slab. The Chasteiran Unit is the fourth and the coldest Alpine UHP unit known so far in the entire Alpine belt. Its P–T conditions are comparable to the ones of the Tian Shan coesite-chloritoid-bearing rocks. Manzotti, P., Schiavi, F., Nosenzo, F., Pitra, P., Ballèvre, M. (2022). A journey towards the forbidden zone: a new, cold, UHP unit in the Dora-Maira Massif (Western Alps). Contributions to Mineralogy and Petrology, in press.

BackgroundTungurahua volcano (Ecuador) intermittently emitted ash between 1999 and 2016, enduringly affecting the surrounding rural area and its population, but its health impact remains poorly documented.ObjectivesWe aim at assessing the respiratory health hazard posed by the 16-17 August 2006 most intense eruptive phase of Tungurahua.MethodsBased on detailed field surveys and grain size analyses, we mapped the spatial distribution of the health-relevant ash size fractions produced by the eruption in the area impacted by ash fallout. We used Scanning Electron Microscopy and Raman Spectroscopy to quantify the mineralogy, composition, surface texture and morphology of a respirable ash sample isolated by aerodynamic separation. The cytotoxicity and pro-inflammatory potential of this respirable ash towards lung tissues was assessedin-vitrousing A549 alveolar epithelial cells, by Electron Microscopy and biochemical assays (LDH assay, RT-qPCR, multiplex immunoassays).ResultsThe eruption produced a high amount of inhalable and respirable ash (12.0-0.04 kg/m2of sub-10 µm and 5.3-0.02 kg/m2of sub-4 µm ash deposited). Their abundance and proportion vary greatly across the deposit within the first 20 km from the volcano. The respirable ash is characteristic of an andesitic magma and no crystalline silica is detected. Morphological features and surface textures are complex and highly variable, with few fibres observed.In-vitroexperiments show that respirable volcanic ash are internalized by A549 cells and processed in the endosomal pathway, causing little cell damage, but some changes in cell morphology and membrane texture. The ash trigger a weak pro-inflammatory response.DiscussionThese data provide the first understanding of the respirable ash hazard near Tungurahua, and the extent to which it varies spatially in a fallout deposit. Given the long exposure duration of the surrounding population, the chronic effects of this inhalable, weakly bio-reactive ash on health could be further investigated.

International audience; Olivine-hosted melt inclusions (MIs) from tephra of the recent 2013–2018 activity at Mt. Etna were investigated for assessing the chemical evolution of magmas and quantifying their pre-eruptive volatile budget. Microanalyses revealed two types of MIs present in all investigated eruptions; the inclusions, particularly the less evolved ones, appear to have experienced water loss coupled with SiO2 depletion. Restoration ofthe original SiO2-H2O concentrations provides consistency with the thermodynamic modelling of magma evolution. The two types of MIs developed during crystallization of olivine plus clinopyroxene between 200 and 100 MPa, where magmas also experienced CO2 flushing. Degassing processes at these levels are responsible for water depletion in the melt and diffusive water loss from inclusions. Our data suggest that initial water budget is unchanged all over the last 20 years, reflecting therefore a potential in triggering highly explosive eruptions depending on degassing dynamics under open versus closed system conditions at shallow levels.

co-auteur étranger; International audience; Transition metal isotopes are particularly useful for understanding the conditions under which magmatic processes occur. Moreover, those with more than one oxidation state (e.g. Cr2+, Cr3+ and Cr6+) may also provide powerful constraints on the evolution of the redox state of the mantle over time. By investigating the Cr isotopic compositions in both magnesiochromite and silicate melts during experiments performed at 1300 °C and under controlled redox conditions (−12 −6) are much lighter than those of melts equilibrated with magnesiochromite at lower oxygen fugacity. The observed variations can be explained by changes in bonding environment for Cr under oxidised conditions in the silicate melts and/or in the magnesiochromite grains. Similarly, the second set of experiments designed to study fractional crystallisation (series B) suggest that Cr isotope fractionation is larger under oxidising conditions.

Mafic alkali-rich magmas, such as those which form the Canary Islands, Spain, have been proposed as being CO2-rich due to low-degree partial melting and the presence of recycled oceanic crust in the mantle source region. A CO2-rich mantle source of Canary magmas has been suggested from melt inclusions study of the 2011 submarine El Hierro eruption, but this has not been verified yet by directly measuring magmatic CO2 emissions during a subaerial eruption as the last such event in the archipelago, in 1971, occurred before the advent of modern gas sensing tools. Here we report on the first results for gas emissions from the2021 eruption of Cumbre Vieja on La Palma island. We determined the chemical composition and mass flux of magmatic degassing during both effusive and explosive activities by combining direct plume measurements with MultiGas sensors from the ground, UAV and helicopter, OP-FTIR remote sensing and satellite-based (TROPOMI) SO2 flux quantification based on back-trajectory modelling. Degassing mass budgets and the magma volatile concentrations were then derived from microprobe analysis of olivine-hosted melt inclusions and comparing our gas results with best estimates of the magma extrusion rates during both explosive and effusive activities. Based on this approach we obtain a direct quantification of the initial CO2 content of the magma and of the exsolved pre-eruptive CO2 gas phase that fed the Cumbre Vieja eruption.We find unprecedentedly high CO2 content in the mantle source of La Palma magma, consistent with high volatile contents predicted from petrological analyses. Eruptions of oceanic island alkali-rich magmas produce disproportionately high CO2 emissions, highlighting the key role mantle heterogeneity plays in determining the impact of intraplate volcanism.

Mineral-hosted melt inclusions have become an important source of information on magmatic processes. As the number of melt inclusion studies increases, so does the need to establish recommended practice guidelines for collecting and reporting melt inclusion data. These guidelines are intended to ensure certain quality criteria are met and to achieve consistency among published melt inclusion data in order to maximize their utility in the future. Indeed, with the improvement of analytical techniques, new processes affecting melt inclusions are identified. It is thus critical to be able to reprocess any previously published data, such that reporting the raw data is one of the first “recommended practices” for authors and a publication-criteria that reviewers should be sensitive to. Our guidelines start with melt inclusion selection, which is a critical first step, and then continue on to melt inclusion preparation and analysis, covering the entire field of methods applicable to melt inclusions. Dedication In March of 2000, a melt inclusion workshop was held at the Chateau de Sassenage in Grenoble and a companion issue of Chemical Geology entitled “Melt Inclusions at the Millennium” was published. Erik Hauri was heavily involved with the meeting and contributed two landmark papers to the topical issue of Chemical Geology on the use of secondary ion mass spectrometry to analyze volatiles in melt inclusions. When the melt inclusion community re-convened at Woods Hole Oceanographic Institution (WHOI) in August of 2018, we were saddened that Erik was unable to join us due to his failing health. Less than a month later came the devastating news of his passing at only 52 years of age. In recognition of his incredible contributions to science in general and to the in situ analysis of melt inclusions in particular, the participants and organizers of the WHOI melt inclusion workshop dedicate this collegial paper to Erik Hauri, our colleague, mentor and friend. Thank you Erik.

Lawsonite is a strongly hydrated (12 weight % H2O) Ca- and Al-rich silicate, exclusively stable along low P/T gradients, typical of subduction zones. The distribution and preservation of lawsonite at the scale of a subduction/collision belt reflect the occurrence of rocks with favourable chemical composition (mainly hydrothermally altered metabasalts and marly limestones (i.e. calcschists), two lithologies especially common in the oceanic units) and their pressure-temperature-fluid history (with preservation favoured by decreasing T during decompression).The distribution of lawsonite in the Western Alps has been investigated since several decades. In the blueschist-facies units from the South-Western Alps (Queyras, Ubaye), lawsonite is well preserved in the external domain, at the contact with the Briançonnais domain, but is largely pseudomorphed in the more internal domain, at the contact with the Viso Unit. Further North, neither lawsonite nor lawsonite pseudomorphs have been reported in the supposedly blueschist-facies Combin Zone, taken by most studies as an equivalent of the Queyras-Ubaye units. This constitutes a paradox with respect to the overall metamorphic structure of the Alpine belt.This study documents for the first time several occurrences of lawsonite and garnet in the calcschists from the Combin Zone. Field and metamorphic data (thermodynamic modelling and Raman spectroscopy on carbonaceous material) point to the occurrence of two tectonometamorphic units within the Combin Zone, characterised by distinct geometry, lithological content and Alpine P-T conditions.In the higher grade unit, lawsonite and garnet were stable at peak P-T conditions (~14-16 kbar and ~460-490 °C) at very low X(CO2) values. Although lawsonite is systematically pseudomorphed, we have been able to recognize hourglass zoning in lawsonite or preservation of an internal fabric associated with the prograde ductile deformation.The lower grade unit (~8 ± 1 kbar ~370-400 °C) is discontinuously exposed along the western base of the Dent Blanche nappe and records Alpine P-T conditions similar to those reached by the Dent Blanche nappe (Manzotti et al. 2020).Our data show that lawsonite is not missing in the Combin Zone, and resolve a paradox about the large-scale metamorphic structure of the Alps. Manzotti, P., Ballèvre, M., Pitra, P., Müntener, O., Putlitz, B., Robyr, M. (2020). Journal of Petrology, egaa044, https://doi.org/10.1093/petrology/egaa044.

International audience; Remanent magnetization and active magnetic fields have been detected for several telluric planetary bodies in the solar system (Earth, Mercury, Moon, Mars) suggesting the presence of core dynamos active at the early stages of the planet formation and variable lifetimes. Among the factors controlling the possibility of core dynamos generation, the dynamics of the surrounding silicate mantle and its associated thermal properties are crucial. The mantle governs the heat evacuation from the core and as a consequence the likeliness of an early thermally driven dynamo. In the case of planets with a thick mantle (associated with supercritical Rayleigh numbers), the core heat is efficiently removed by mantle convection and early thermally-driven dynamos are likely. At the opposite, planets with a thin mantle (associated with subcritical Rayleigh numbers) might evacuate their inner heat by diffusion only, making early thermally-driven dynamos difficult. Within the Solar System, Mercury is a potential example of such a regime. Its small mantle thickness over the planet radius ratio might be inherent to its small orbital semi-axis and hence, might be ubiquitous among the terrestrial objects formed close to their star. To constrain the likeliness of a thermally driven dynamo on “Mercury-like” planets (i.e. with large Rc/R), we present new thermal diffusivity measurements of various solid, glassy and molten samples. We applied the Angstrom method on cylindrical samples during multi-anvil apparatus experiments at pressures of 2 GPa and temperatures up to 1700 K. Thermal diffusivities and conductivities were estimated for solid and partially molten peridotites, with various melt fractions, and for basaltic and rhyolitic glasses and melts. Our study demonstrates that melts have similar thermal properties despite a broad range of composition investigated. The melts reveal much lower thermal conductivities than the solids with almost an order of magnitude of decrease: 1.70 (±0.19) to 2.29 (±0.26) W/m/K against 0.18 (±0.01) to 0.41 (±0.03) W/m/K for peridotites at high temperatures and various melts respectively. Partially molten samples lie in between and several predictive laws are proposed as a function of the melt fraction and solid/melt texture. Using our results into forward calculations of heat fluxes for dynamo generation for Mercury-like planets, we quantify the effect of mantle melting on the occurrence of thermally driven dynamos. The presence of a mushy mantle and partial melting could significantly reduce the ability of the mantle to evacuate the heat from the core and can prevent, shut or affect the presence of a planetary magnetic field. The buoyancy and fate of molten material in such bodies can thus influence the magnetic history of the planet. Future observations of Mercury-like planets accreted near their star and the detections of their magnetic signatures could provide constraints on their inner state and partial melting histories.

Co-auteur étranger; International audience; We have coupled three independent methods to investigate the time evolution of eruptive intensity during the sub-Plinian andPlinian phases of the 3600-year BP Late Bronze-Age eruption of Santorini Volcano: (1) mass eruption rate based on new lithicisopleth maps for multiple layers of the fall deposit, (2) magma decompression rate calculated from vesicle number densities, and(3) magma decompression rate calculated from H2O gradients in melt reentrants, with methods 2 and 3 measured on the samesuite of pyroclasts. Mass eruption rate increased by two orders of magnitude, reaching 210 × 106kg s−1at the peak of the Plinianphase (plume height 28.4 ± 1.0 km); it then declined in the final stage of fallout emplacement following the first generation ofpyroclastic surges. Decompression rates from melt reentrants (0.008 to 0.25 MPa s−1) are two to three orders of magnitude lowerthan those from vesicle number densities, assuming heterogeneous vesicle nucleation (2 to 19 MPa s−1). Melt reentrants arethought to record slow decompression in the deep feeder conduit, whereas vesicles record much higher rates of decompression inthe shallow conduit due to the steep, nonlinear pressure gradients associated with magma vesiculation and fragmentation.Upwardly converging flow from a dike-like, deep conduit to a more cylindrical, shallow conduit may also have played a rolein causing upwardly accelerating flow. Variations in deep decompression rate recorded by melt reentrants are decoupled frommass eruption rate, whereas those recorded by vesicles lie in between. Taken with the transition from unsteady to steady Plinianeruption, this may reflect the existence of transient flow conditions in the conduit system due to widening and lengthening of adeep feeder dike as Plinian eruption progressed. As the mass eruption rate rose to its peak value, the fragmentation level fell in theconduit due to increasing rates of magma strain and decompression.

International audience

International audience

Co-auteur étranger; International audience; We present a novel application of Raman microtomography for quantitative characterisation of glass inclusion-hosted bubbles, which allows for the simultaneous identification and volumetric quantification of mineral and fluid phases filling the bubble. The combination of Raman microtomography with synchrotron XRF mapping and scanning electron microscopy provides a complete compositional and textural characterisation of the bubble. In the studied samples, minerals are systematically present on the walls of the bubbles: dominantly carbonates in samples from continental intraplate and hotspot volcanic provinces, and sulfates in the sample from subductionrelated settings. Along with fluid CO 2 , carbonates sequester 65 to 84 % of the CO 2 originally dissolved in the melt, while 18 to 60 % of the sulfur contained in the inclusion is stored in sulfides and/or sulfates. Thus, the total melt inclusion CO 2 and S contents can be underestimated (by up to ∼ 40 % and 60 %, respectively) if minerals in the bubbles are neglected. This study highlights the importance of 3D mapping of shrinkage bubbles hosted in glass inclusions for a better assessment of the bulk pre-eruptive contents of volatiles in magmas.

Co-auteur étranger; International audience; olivine-hosted melt inclusions (Mis) provide samples of magmatic liquids and their dissolved volatiles from deep within the plumbing system. Inevitable post-entrapment modifications can lead to significant compositional changes in the glass and/or any contained bubbles. Reheating is a common technique to reverse MI crystallisation; however, its effect on volatile contents has been assumed to be minor. We test this assumption using crystallised and glassy basaltic MIs, combined with Raman spectroscopy and 3D imaging, to investigate the changes in fluid and solid phases in the bubbles before and after reheating. Before reheating , the bubble contains CO 2 gas and anhydrite (caSo 4) crystallites. The rapid diffusion of major and volatile elements from the melt during reheating creates new phases within the bubble: So 2 , gypsum, Fe-sulphides. Vapour bubbles hosted in naturally glassy MIs similarly contain a plethora of solid phases (carbonates, sulphates, and sulphides) that account for up to 84% of the total MI sulphur, 80% of CO 2 , and 14% of FeO. In both reheated and naturally glassy MIs, bubbles sequester major and volatile elements that are components of the total magmatic budget and represent a "loss" from the glass. Analyses of the glass alone significantly underestimates the original magma composition and storage parameters. Olivine-hosted melt inclusions (MIs) provide insight into the nature of the magma mantle source, storage conditions , and pre-eruptive volatile contents 1. Following entrapment, MIs undergo compositional modifications due to growth of the host olivine along the MI walls, and to crystallisation of daughter minerals from the glass due to slow ascent rates, and/or cooling 1,2. Another modification is the nucleation of a vapour bubble in response to decompression during cooling and post-entrapment crystallisation 1,2 , further reducing the solubility of volatiles in the glass. Vapour, or shrinkage, bubbles produced by differential thermal contraction between the melt (glass) and the host crystal are considered to be inherent to the MI 1,2. However, pre-existing bubbles that formed externally in a vapour-saturated system may also become trapped inside MIs. Vapour bubbles may also form during MI leakage and decrepitation of the host crystal 3,4. Discriminating between various bubble types depends upon the size of the bubble relative to the total inclusion. Since the volumetric proportions of vapour bubbles depend on the cooling rate, volatile content and melt composition, cooling-related shrinkage and melt-saturated bubbles normally comprise 0.2 to 10 vol% of the inclusion 3,4. Bubbles with greater volumetric proportions are not considered inherent to the MI 3,4. Due to the strong pressure-dependency of CO 2 solubility, the contraction of the melt and the decrease of the internal pressure in response to cooling and post-entrapment crystallisation first leads to rapid CO 2 saturation of the melt, and consequently, the transfer of CO 2 gas into the bubble 5-7 , so that analyses of the glass yield erroneously low magmatic CO 2 concentrations. This poses significant problems as the CO 2 content is commonly used to infer the pressure of crystallisation and MI entrapment, as well as magmatic storage depths. By only considering the glass, these values are grossly underestimated. Many of the post-entrapment modifications that occur within olivine-hosted MIs can be corrected using well-constrained exchange coefficients and recently established methods to quantify the amount of volatiles, particularly CO 2 , sequestered by the bubble. These approaches include the use of trace element proxies, such as CO 2 / Nb, to determine the pre-eruptive CO 2 content of the undegassed melt 8 , reheating the MI to resorb the bubble

Co-auteur étranger; International audience; Understanding the links between the magma differentiation processes, the magma plumbing system and the magma composition at arc volcanoes is of paramount importance for volcanic hazard assessment. In this work we focus on the post-glacial, Holocene, historical, and recent eruption products of Ubinas volcano (Peru), which display an overall decrease in silica content from the older, plinian (VEI 3–5), rhyolitic eruptions (69–71 wt% SiO2) to the historical and recent (2006–2009, 2013–2017), vulcanian (VEI 1–2) basaltic andesitic eruptions (55–57 wt% SiO2). Based on a comprehensive study of the major and trace elements and the Sr-Nd-Pb isotopes, we conclude that this temporal pattern reflects the evolution of the Ubinas magmas in the middle-to-upper crust by a coupled Assimilation-Fractional Crystallization (AFC) process involving a cumulate composed of plagioclase, amphibole, clinopyroxene, orthopyroxene and Fe–Ti oxides, with minor amounts of olivine and biotite at the mafic and felsic end-members, respectively. Upper crustal assimilation is limited to 5–8 vol%, but the overall radiogenic Sr-Nd-Pb signature of the Ubinas magmas requires a larger crustal component, which must therefore occur at middle to lower crustal depths. The petrology of the Ubinas magmas also points to an overall increase in P-T conditions: the large Holocene dacitic and rhyolitic eruptions record temperatures ranging from 800 to 850 °C and pressures in the range of 200–400 MPa, whereas the historical and recent (2006–2009, 2013–2017) basaltic andesitic eruptions provide higher temperatures and pressures (1000 °C, >300–400 MPa). Overall, the thermo-barometry, phase equilibrium and geochemical constraints allow us to propose the existence of a middle-to-upper crust magma column composed of a highly crystalline magma mush containing batches of liquid magma, which seems to be continually recharged from deeper levels. On the basis of the petrological nature of the historical basaltic andesitic eruptions (1667 CE, 2006–2009, 2013–2017), we postulate that during the last centuries, Ubinas experienced a recharge-dominated process, with no evidence for a rejuvenation of the silica-rich reservoir that fed the large Holocene dacitic to rhyolitic eruptions. This study highlights the importance of detailed petrological studies of Holocene sequences at explosive arc volcanoes in order to constrain the magmatic processes and conditions that control large explosive eruptions.

co-auteur étranger; International audience; Iron speciation is linked to oxygen fugacity; hence the Fe3+/ΣFe ratio of glasses can reveal the oxygen fugacity of the system where they last equilibrated. Previous studies using X-ray Absorption Near Edge Structure (XANES) spectroscopy in silicate glasses and olivine-hosted melt inclusions have shown that arc basalts are more oxidized than mid-ocean ridge basalts (MORB) and oceanic island basalts (OIB). However, Cottrell et al. (2018) recently demonstrated that hydrous glasses can experience beam-induced oxidation during XANES analysis, leading to an overestimation of their Fe3+/ΣFe ratios. Here, we determined Fe3+/ΣFe ratios in olivine-hosted melt inclusions from various arc, MORB and OIB localities by Mössbauer and XANES spectroscopy. We carried out a careful evaluation of beam damage during XANES measurements that included detailed analysis of consecutive XANES spectra and collection of time series on hydrous basaltic and basanitic glasses using different radiation doses. Fe3+/ΣFe ratios obtained using both spectroscopy methods on the same sample set allowed us to confirm the oxidation state of those magmas and the reliability of our approach to avoid the effect of beam-induced oxidation during XANES analysis. Our results show that melt inclusions from MORBs, OIBs and arc basalts display mean Fe3+/ΣFe ratios of 0.10 ± 0.05 (2σ; n = 5), 0.13 ± 0.05 (2σ; n = 2) and 0.25 ± 0.15 (2σ; n = 19), respectively, while Mount Etna melt inclusions display a mean Fe3+/ΣFe ratio of 0.26 ± 0.05 (2σ; n = 7). These results confirm that arc magmas are more oxidized than those from hot spots and mid-ocean ridges.

Co-auteur étranger; International audience; Liebermannite (KAlSi3O8) is a principal mineral phase expected to be thermodynamicallystable in deeply subducted continental and oceanic crusts. The crystal structure of liebermanniteexhibits tunnels that are formed between the assemblies of double chains of edge‐sharing (Si, Al) O6octahedral units, which act as a repository for large incompatible alkali ions. In this study, we investigate theelectrical conductivity of liebermannite at 12, 15, and 24 GPa and temperature of 1500 K to track subductionpathways of continental sediments into the Earth's lower mantle. Further, we looked at whetherliebermannite could sequester incompatible H2O at deep mantle conditions. We observe that the superionicconductivity of liebermannite due to the thermally activated hopping of K+ions results in high electricalconductivity of more than 1 S/m. Infrared spectral features of hydrous liebermannite indicate the presenceof both molecular H2O and hydroxyl (OH−) groups in its crystal structure. The observed high electricalconductivity in the mantle transition zone beneath Northeastern China and the lower mantle beneath thePhilippine Sea can be attributed to the subduction pathways of continental sediments deep into the Earth'smantle. While major mineral phases in pyrolitic compositions are almost devoid of H2O under lowermantle conditions, our study demonstrates that liebermannite could be an important host of H2O in theseconditions. We propose that the relatively high H2O contents of ocean island basalts derived from deepmantle plumes are primarily related to deeply subducted continental sediments, in which liebermannite isthe principal H2O carrier.

co-auteur étranger; International audience; Formation of secondary sulfate minerals during the reaction between volcanic gases and rocks modulates the compositionand flux of gaseous emanations. We report on the sub-surface formation of anhydrous alum (MIMIII(XVIO4)2with MI=-NH4+,Na+,K+;MIII=Al3+,Fe3+and XVI=S6+,Mo6+) in the 330°C fumaroles of the Lascar volcano (Chile). The alumoccurs as a few millimetres thick crust that grew internally by two-way diffusion of reaction gases and diffusive influx of rockcations within the crust. The average growth rate is estimated at ca. 0.3lm/day, based on the 19-year-long activity of thedegassing fracture hosting the crust. The growth rate is controlled by the slow migration of the rock cations and decreasestowards crust rim. The crust selectively concentrates Tl, V and Te (thousands oflg/g) and to a lesser extent Mo (hundredsoflg/g). The uptake of gaseous Tl, V and Mo is due to the possibility for these elements to enter the MI,MIIIand XVIsites ofalum, respectively. The process of Te uptake must be related to the incorporation of Tl and V with which Te tightly correlates.Extensive substitution of Tl, V and Te occurs at the surface of the crust where the supply of rock cations is the lowest. Suchsurface enrichment does not occur for Mo, because Mo substitutes for S, another element from the gas. These findings suggestthat the surface of mature alum crust has a high adsorption capacity for those gaseous metals able to compensate for the lackof rock-derived cations. Based on the composition of gases escaping from the fracture hosting the crust, it is estimated that thepartition coefficients of Tl (3.3107), V (1.1107) and Te (0.6107) between crust surface and gases are two to four ordersof magnitude higher than for other volatile metals and metalloids. It follows that gases equilibrating with anhydrous alumslose between 77 and 95% of their initial Tl content, but less than 1% of Pb. Given the Tl emission rate of Lascar volcano(5 g/day), between 17 and 104 g of toxic Tl would deposit every day if all Lascar gases were to equilibrate with anhydrousalums. This study suggests that anhydrous alums significantly immobilize Tl, V and Te in the ground of quiescent volcanoes,reducing the atmospheric emissions of these three elements.Ó2020 Elsevier Ltd. All rights reserved.

Co-auteur étranger; International audience; Antigorite is considered as the most important source of water in subduction zones, playing a key role during arc magma genesis. Although, these magmas seem more oxidized than mid-oceanic ridge basalts (MORB), the possible inherent link between the oxidation state of arc magmas and serpentinite-derived hydrous fluids is still not well established. Here, we have performed dehydration experiments of natural antigorite serpentinite containing 5 weight percent (wt%) magnetite at 3 GPa and in a temperature range from 600 to 900 °C using a multi-anvil apparatus. These experiments aim to reproduce the different stages of H 2 O release, forming chlorite, olivine and orthopyroxene and water. Our experimental set up permits to preserve the intrinsic high oxygen fugacity (fO 2) of serpentinite during dehydration. The new olivine and orthopyroxene which formed in equilibrium with antigorite, chlorite and magnetite have high X Mg numbers setting up the oxygen fugacity to high values, between 3.1 and 4.1 log units above QFM (Quartz-Fayalite-Magnetite buffer). Hematite is observed con-comitantly with high X Mg in olivine, of 0.94-0.97, generally at low temperatures, below 800 °C, in coexistence with chlorite. Once the magnetite is destabilized, upon chlorite breakdown which occurs above 800 °C, the oxygen fugacity decreases to 3.7 due to the decrease of the X Mg of silicates. This study demonstrates the highly oxidizing nature of the fluids released from antigorite dehydration. Thus, at high pressure and high temperature conditions, f O2-sensitive elements such as carbon and sulfur are expected to be mobilized under their oxidized form, providing an oxidizing context for arc magmas genesis and assuming that they are not completely reduced by their percolation through meta-gabbro, meta-basalts and meta-sediments.

Co-auteur étranger; International audience; Magma recharge into a differentiated reservoir is one of the main triggering mechanisms for explosive eruptions. Here wedescribe the petrology of the eruptive products of the last explosive eruption of Tutupaca volcano (southern Peru) in order toconstrain the pre-eruptive physical conditions (P-T-XH2O) of the Tutupaca dacitic reservoir. We demonstrate that prior to theparoxysm, magma in the Tutupaca dacitic reservoir was at low temperature and high viscosity (735 ± 23 °C), with a mineralassemblage of plagioclase, low-Al amphibole, biotite, titanite, and Fe-Ti oxides, located at 8.8 ± 1.6 km depth (233 ± 43 MPa).The phenocrysts of the Tutupaca dacites show frequent disequilibrium textures such as reverse zonation, resorption zones, andovergrowth rims. These disequilibrium textures suggest a heating process induced by the recharge of a hotter magma into thedacitic reservoir. As a result, high-Al amphibole and relatively high-Ca plagioclase phenocryst rims and microlites were formedand record high temperatures from just before the eruption (840 ± 45 °C). Based on these data, we propose that the recent eruptionof Tutupaca was triggered by the recharge of a hotter magma into a highly crystallized dacitic magma reservoir. As a result, theresident dacitic magma was reheated and remobilized by a self-mixing process. These magmatic processes induced an enhancedphase of dome growth that provoked destabilization of the NE flank, producing a debris avalanche and its accompanyingpyroclastic density currents.

Co-auteur étranger; International audience; The Gruf complex in the Lepontine Alps is one of the rare occurrences of Phanerozoic ultra-high temperature (UHT)metamorphism in the world, but its age is still a matter of debate. Herewe present LA-ICPMS dating in a petrographic context ofzircon and monazite from a UHT restitic granulite. Zircons and monazites are both included in large crystals and in retrogradesymplectites. In such restitic rocks, partial melting or fluid interactions are unlikely, precluding resetting of the monazitechronometers. Zircon cores yield Permian ages, which are interpreted as the age of charnockitization. They are sometimessurrounded by a narrow rim at 32 Ma. Monazites are strongly zoned, but all yield a 31.8 ± 0.3 Ma age interpreted as the time ofcomplete (re-)crystallization during the UHT paragenesis. We propose that the zircons dated a post-Hercynian metamorphismwhich is responsible for the widespread formation of granulites in the Southern Alps and the crust differentiation. This fluidabsentmelting event produced refractory lithologies, such as restites in charnockites. We suggest that Gruf UHT paragenesis isalpine in age and crystallized from these refractory lithologies.We conclude that the lower restitic crust produced in the Permianhad the ability to achieve UHT conditions during the fast exhumation and heating related to lithospheric thinning in Alpine time.

co-auteur étranger; International audience; New series of alumino-silicate glasses spanning a wide range of chemical compositions (basanites, tholeiitic basalts, calcalkaline andesites, peraluminous and peralkaline rhyolites) and with water contents from 0.02 to 6.70 wt% were used for improving the method of quantification of dissolved water with a highly confocal Raman micro-spectrometer. After reconsideration of previously proposed methods for spectra acquisition and post-analysis data treatment, we define the main critical steps that allow minimizing glass matrix effects. First, we carefully assess the variation of Raman band intensities, in both water (~3000–3800 cm−1) and alumino-silicate vibration (~200–1250 cm−1) regions with focus depth of the laser beam inside the sample. Our results indicate that in the first 2–10 μm depth, the intensity increase in the alumino-silicate region is twice as high as that in the water region. Optimal focus depths, where the signal of the water band is maximum and the intensity ratio of the water band to alumino-silicate band is minimum, vary with glass composition and confocal performance of the Raman spectrometer. This influences both external and internal calibration slopes. Second, this study recognizes critical parameters related with glass density, presence of ferric iron and dissolved carbonates as mainly responsible for matrix effects on the internal calibration method. (a) We provide a procedure for correcting the effect of glass density on water internal calibration based on the observation that the integrated intensity (i.e. the area) of the alumino-silicate envelope generally drops with the increase of water content and decrease of glass density. (b) In CO2-bearing glasses, the intensity of the ν1 Raman vibration of dissolved carbonate at ~1087 cm−1 has to be subtracted from the intensity of the alumino-silicate envelope before applying the density correction. (c) Using peak-fitting, the intensity of the 850–1250 cm−1 envelope of peralkaline rhyolitic glasses is corrected for the effect of the presence of four-fold coordinated Fe3+, as revealed by the strong Raman scattering of the vibrational mode at ~980 cm−1. Following this procedure, all the studied glasses define a single calibration line in spite of their compositional variability, when using either of the two classical approaches referred to as external and internal calibration methods. The linear fits of the external and internal calibrations reproduce the whole dataset within 0.13–0.11 wt% (high and standard confocality) and 0.17 wt%, respectively. The accuracy of the external calibration is evaluated based upon comparison with ion-probe measurements of water dissolved in natural glass inclusions: the relative standard deviation is ~4% (1σ) on average, and reaches ~12% (1σ) for water contents of ~0.1 wt%.

International audience; Exhumation rates for high-pressure metamorphic rocks need to be carefully estimated to decipher tectonic processes in subduction/collision belts. In the Gran Paradiso Massif (Western Alps), the Money Unit crops out as a tectonic window below the Gran Paradiso Unit. According to previous studies, the Gran Paradiso and Money Units reached peak pressure conditions at ~ 18 to 20 kbar, 480–520 °C and ~ 17 to 18 kbar, 500–550 °C, respectively. This yields a maximum difference of ~ 9 to 10 km in the subduction depth reached by these two units during the Alpine history. Thrusting of the Gran Paradiso Unit over the Money Unit led to the simultaneous development of the main foliation under the same metamorphic conditions (~ 12.5 to 14.5 kbar and 530–560 °C) in both units. The thrust contact was subsequently folded and then both units were exhumed together. The relative timing of the growth and dissolution of the accessory phases was assessed by combining thermodynamic modelling with inclusion, textural and chemical (major and trace element) data from both major and accessory phases. The age of monazite constrained the high-pressure metamorphism in both the Gran Paradiso Unit and the Money Unit at 41.5 ± 0.3 and 42.0 ± 0.6 Ma, respectively. Allanite replacing monazite in the matrix has been dated at 32.7 ± 4.2 Ma. The late growth of xenotime associated with the crystallization of biotite pseudomorphs at the expense of garnet (at about 10 kbar) was dated at 32.3 ± 1.0 Ma. Our petrochronological data indicate about 10 m.y. between the peak pressure conditions and the crystallization of xenotime leading to an exhumation rate of the order of 2.2–5 mm/year. The new ages allow to better constrain the timing of the displacement of the thrust defining the lower boundary of the extruding wedge of eclogite-facies rocks.

We present a geochemical study on olivine and clinopyroxene-hosted melt inclusions (MIs) from 2001 to 2006 Etna basaltic lavas and pyroclastites. Three MI suites are distinguished on the basis of trace element fingerprinting. Type-1 MIs (from 2001 Upper South and 2002 Northeast vents) share their trace element signature with low-K lavas erupted before 1971. Critical trace element ratios (e.g., K/La, Ba/Nb), along with Pb isotope data of Type-1 MIs provide evidence for a heterogeneous mantle source resulting from mixing of three end-members with geochemical and isotopic characteristics of EM2, DMM, and HIMU components. Type-1 MIs composition does not support involvement of subduction-related components. Type-2 (from 2001 Lower and 2002 South vents) and Type-3 (2004 eruption) MIs reveal “ghost plagioclase signatures,” namely lower concentrations in strongly incompatible elements, and positive Sr, Ba, and Eu anomalies. Both Type-1 and Type-2 MIs occur in 2006 olivines, which highlight the occurrence of mixing between Type-1 and Type-2 end-members. Type-2/Type-3 MIs testify to en route processes (plagioclase assimilation and volatile fluxing) peculiar for “deep dike fed” eruptions. The latter are strongly controlled by tectonics or flank instability that occasionally promote upraise of undegassed, more radiogenic primitive magma, which may interact with plagioclase-rich crystal mush/cumulates before erupting. Type-2/Type-3 MIs approach the less radiogenic Pb isotopic composition of plagioclase from prehistoric lavas, thus suggesting geochemical overprinting of present-day melts by older products released from distinct mantle sources. Our study emphasizes that MIs microanalysis offers new insights on both source characteristics and en route processes, allowing to a link between melt composition and magma dynamics.

During planetary accretion, hydrogen behaves as a lithophile element and is unlikely to be a major element in planetary cores., Hydrogen has been thought to be an important light element in Earth’s core due to possible siderophile behavior during core-mantle segregation. We reproduced planetary differentiation conditions using hydrogen contents of 450 to 1500 parts per million (ppm) in the silicate phase, pressures of 5 to 20 GPa, oxygen fugacity varying within IW-3.7 and IW-0.2 (0.2 to 3.7 log units lower than iron-wüstite buffer), and Fe alloys typical of planetary cores. We report hydrogen metal-silicate partition coefficients of ~2 × 10−1, up to two orders of magnitude lower than reported previously, and indicative of lithophile behavior. Our results imply H contents of ~60 ppm in the Earth and Martian cores. A simple water budget suggests that 90% of the water initially present in planetary building blocks was lost during planetary accretion. The retained water segregated preferentially into planetary mantles.

International audience; Understanding the interaction between volcanic gases and ash is important 8 to derive gas compositions from ash leachates and to constrain the environmental 9 impact of eruptions. Volcanic HCl could potentially damage the ozone layer, but it is 10 unclear what fraction of HCl actually reaches the stratosphere. The adsorption of HCl 11 on volcanic ash was therefore studied from-76 to + 150 ˚C to simulate the behavior 12 of HCl in the dilute parts of a volcanic plume. Finely ground synthetic glasses of 13 andesitic, dacitic, and rhyolitic composition as well as a natural obsidian from 14 Vulcano (Italy) served as proxies for fresh natural ash. HCl adsorption is an 15 irreversible process and appears to increase with the total alkali content of the glass. 16 Adsorption kinetics follow a first order law with rate constants of 2.13. 10-6 s-1 to 17 1.80. 10-4 s-1 in the temperature range investigated. For dacitic composition, the 18 temperature and pressure dependence of adsorption can be described by the equation 19 ln c = 1.26 + 0.27 ln p-715.3/T, where c is the surface concentration of adsorbed HCl 20 in mg/m 2 , T is temperature in Kelvin, and p is the partial pressure of HCl in mbar. A 21 comparison of this model with a large data set for the composition of volcanic ash 22 suggests that adsorption of HCl from the gas phase at relatively low temperatures can 23 quantitatively account for the majority of the observed Cl concentrations. The model 24 implies that adsorption of HCl on ash increases with temperature, probably because of 25 2 the increasing number of accessible adsorption sites. This temperature dependence is 26 opposite to that observed for SO 2 , so that HCl and SO 2 are fractionated by the 27 adsorption process and the fractionation factor changes by four orders of magnitude 28 over a temperature range of 250 K. The assumption of equal adsorption of different 29 species is therefore not appropriate for deriving volcanic gas compositions from 30 analyses of adsorbates on ash. However, with the experimental data provided here, the 31 gas compositions in equilibrium with the ash surfaces can be calculated. In particular, 32 for dacitic composition, the molar ratio of S/Cl adsorbed to the ash surface is related 33 to the molar S/Cl ratio in the gas phase according to the equation ln (S/Cl) adsorbed = 34 2855 T-1 + 0.28 ln (S/Cl) gas-11.14. Our data also show that adsorption on ash will 35 significantly reduce the fraction of HCl reaching the stratosphere, only if the initial 36 HCl content in the volcanic gas is low (< 1 mole %). For higher initial HCl 37 concentrations, adsorption on ash has only a minor effect. While HCl scavenging by 38 hydrometeors may remove a considerable fraction of HCl from the eruption column, 39 recent models suggest that this process is much less efficient than previously thought. 40 Our experimental data therefore support the idea that the HCl loading from major 41 explosive eruptions may indeed cause severe depletions of stratospheric ozone. 42 43

International audience; During low–high temperature (T) cycles imposed on olivine-hosted melt inclusions (MIs) we observe a systematic increase in homogenization temperature (Th) with time, regardless of their initial major-element and H2O contents. Bubble persistence at high T suggests that inclusion internal pressure (Pint) is lower than its original, trapping pressure. We explore how reversible and irreversible processes modify the composition (X), volume (V) and Pint of heated MIs, and compare the results of theoretical modeling with experimental observations of MIs from FAMOUS Zone (FZ, Mid-Atlantic Ridge) and La Sommata (SOM, Vulcano, Aeolian Islands) basaltic samples. Due to olivine dissolution at inclusion walls and thermoelastic deformation, Pint–V–X conditions change significantly upon heating. Olivine dissolution induces changes in major-element composition (i.e., enrichment in Fe and Mg), morphology and volume (up to +25% at 1500 °C). We provide equations for the thermoelastic deformation of olivine bearing a two-phase, liquid–gas inclusion for the end-member cases of chemical equilibrium and no exchange between gas and liquid. These equations allow Pint–V evolution to be related to variations in bubble volume fraction. Upon heating, both Pint and V variations are smaller in the presence of a gas bubble than for a homogeneous liquid inclusion, at the same T. Dissolution–reprecipitation and thermoelastic deformation of the olivine host are reversible processes, so initial Pint–V–X conditions are restored upon cooling. On the contrary, water loss from MIs and plastic deformation of the olivine host are processes that irreversibly lower Pint, and account for the systematic increase of Th with time. Our theoretical and experimental investigations suggest that the increase of Th in volatile-rich SOM MIs is mainly related to progressive release of water. Compared to larger MIs located at a similar distance from the olivine rim, smaller MIs show a faster increase in Th with time, consistent with the effects of diffusive water loss. Nonetheless, we cannot exclude the combined effect of incipient plastic deformation, which would enhance water loss by diffusion along dislocations. The increase in Th in volatile-poor FZ MIs is driven mainly by elasto-plastic deformation of the olivine host, which becomes more marked with increasing T and decreased distance from MI wall to olivine rim. Occurrence of plastic deformation in FZ olivines is testified by dislocation patterns observed around inclusions. In general, conducting homogenization experiments at 1 atm prevents MI homogenization happening at a Th equal to entrapment T. This is due to a drop in Pint caused by the elastic deformation that affects olivine phenocrysts bearing pressurized MIs during magma ascent. Predicted increase in Th ranges from a few degrees to tens of degrees depending on entrapment conditions, melt composition and volatile contents.

We studied the elemental and isotopic (Pb, B and Li isotopes) composition of melt inclusions hosted in highly forsteritic (Fo(83-91)) olivines that were collected from San Bartolo lava and pumice (ST79p, ST82p and ST531p) samples erupted by Stromboli in historical times. The studied melt inclusions have primitive calcalkaline to shoshonitic basaltic compositions. They cover a compositional range far wider than that exhibited by the whole-rocks and differ in key trace element ratios. San Bartolo melt inclusions are characterized by lower incompatible trace element abundances, higher ratios between fluid-mobile (B, Pb, U and LILE) and less fluid-mobile (REE, Th, HFSE) elements and lower La/Yb ratios relative to the pumice-hosted melt inclusions and pumiceous melts erupted during paroxysmal events. Trace elements, along with different Pb, B and Li isotopic signatures, attest to source heterogeneity on the small scale and provide new insights into subducted components beneath Stromboli. Results of a mixing model suggest that metasomatism of the mantle source of pumice-hosted melt inclusions was driven by solute-rich high-pressure fluids (< 20%) expelled from the deep portion of the slab. Heterogeneous Pb isotopic composition together with light delta B-11 (-8.6 to -13.7aEuro degrees) and delta Li-7 (+2.3 to -1.7aEuro degrees) indicates that high-pressure liquids were released in variable proportions from highly dehydrated metabasalts and metasediments. On the other hand, the elemental and isotopic (delta B-11 similar to -1.9 to -5.9aEuro degrees) composition of San Bartolo melt inclusions is better explained by the addition of a prevalent aqueous component (similar to 2 to 4%) escaped at shallower depths from sediments and altered basaltic crust in almost equivalent proportions, with a smaller contribution by high-pressure fluids. Owing to the high-angle dip of the subducted cold Ionian slab, aqueous fluids and high-pressure fluids would rise through the mantle wedge and locally superimpose on each other, thus giving origin to variously metasomatized mantle domains.

International audience; To understand the solidification processes of natural magma and the texture evolution of igneous rocks, we have carried out in situ observation of the crystallization of a high-K basaltic melt cooling from ~1,240 °C in a moissanite cell. In a series of experiments with different thermal history, olivine or clinopyroxene (cpx) appeared as the liquidus phase before the formation of plagioclase. During cooling at 100 °C/h, the morphology of olivine and cpx transited from tabular to hopper habit. To first order approximation, crystal grow rate (2 × 10−9 to 7 × 10−9 m/s for olivine and 6 × 10−9 to 17 × 10−9 m/s for cpx), probably limited by chemical diffusion, is proportional to crystal size. In one experiment dominated by olivine crystallization, the good image quality allows the analysis of texture evolution over an extended period. Nucleation of olivine occurred only in a narrow temperature and time interval below the liquidus. Two-dimensional length- and area-based crystal size distributions (CSDs) show counterclockwise rotation around axes of 8 μm and 100 μm2, which is consistent with the proportionate crystal growth. Both CSDs and direct observation show the dissolution of small crystals and Ostwald ripening. These data suggest that conventional analyses of crystal size distributions of igneous rocks may be in error—the slope of the CSD cannot be interpreted in terms of a uniform growth rate, and the intercept with the vertical axis does not correspond to a nucleation density.

An experimental apparatus is described that allows for optical in situ observation of samples at 1 bar and temperatures up to 1250 °C for durations up to several days. The apparatus resembles a Bassett-type externally heated diamond cell, where the diamond anvils are replaced by cylinders of synthetic moissanite (SiC). The sample is placed inside a gasket of glassy carbon between the two moissanite windows. The moissanite cell allows for the direct and continuous observation of the sample with an optical resolution in the range of a few micrometers. Partially molten systems can be investigated during cooling-heating cycles within a single experimental run. Texture evolution can be observed continuously through time. The technique also allows for direct measurements of crystal growth and dissolution rates at defined cooling and heating rates. The method was tested by observing the crystallization of KNO 3 -LiNO 3 and NaCl-KCl salt melts as well as of a basaltic-trachyandesitic melt. Dendrite formation, static grain growth, and dissolution were observed in the salt melts. During textural coarsening (static grain growth) of rock salt in the presence of melt, grain coalescence was observed. Both nitrate and rock salt grains growing in contact with melt generally showed curved boundaries and roundish shapes, rather than facets. In one experiment, the final texture did not contain any traces of the previous textural evolution anymore. In a basaltic trachyandesite at temperatures near the liquidus several phenomena, in particular the interactions between crystals, magmatic flow and bubbles during nucleation, growth and dissolution of crystals, and crystal-clusters were observed simultaneously. Crystal growth rates and dissolution rates were also measured.

During its 1800-year-long persistent activity the Stromboli volcano has erupted a highly porphyritic (HP) volatile-poor scoriaceous magma and a low porphyritic (LP) volatile-rich pumiceous magma. The HP magma is erupted during normal Strombolian explosions and lava effusions, while the LP one is related to more energetic paroxysms. During the March–April 2003 explosive activity, Stromboli ejected two typologies of juvenile glassy ashes, namely highly vesicular LP shards and volatile-poor HP shards. Their textural and in situ chemical characteristics are used to unravel mutual relationships between HP and LP magmas, as well as magma dynamics within the shallow plumbing system. The mantle-normalized trace element patterns of both ash types show the typical arc-lava pattern; however, HP glasses possess incompatible element concentrations higher than LP glasses, along with Sr and Eu negative anomalies. HP shards are generally characterized by higher Li contents (to ~20 ppm) and lower δ7Li values (+1.2 to −3.8‰) with respect to LP shards (Li contents of 7–14 ppm and δ7Li ranging between +4.6 and +0.9‰). Fractional crystallization models based on major and trace element compositions, combined with a degassing model based on open-system Rayleigh distillation and on the assumption that melt/fluidDLi > 1, show that abundant (~30%) plagioclase precipitation and variable degrees of degassing can lead the more primitive LP magma to evolve toward a differentiated (isotopically lighter) HP magma ponding in the upper conduit and undergoing slow continuous degassing-induced crystallization. This study also evidences that in March 2003 Stromboli volcano poured out a small early volume of LP magma that traveled slower within the conduit with respect to later and larger volumes of fast ascending LP magma erupted during the April 5 paroxysm. The different ascent rates and cooling rates of the two LP magma batches (i.e., pre- and post-paroxysm) resulted in small, but detectable, differences in their chemical signatures. Finally, this study highlights the high potential of in situ investigations of juvenile glassy ashes in petrologic and geochemical monitoring the volcanic activity and of Li isotopes as tracers of degassing processes within the shallow plumbing system.

The evolution of texture and crystal size distribution of igneous rocks is mainly determined by processes such as nucleation, grain growth, and Ostwald ripening. However, the role played by these processes on the final crystal size distribution is difficult to quantify with conventional methods. Here we present the first direct in situ observation of a crystallizing supercooled basaltic-andesitic liquid in order to directly assess the role of grain-scale processes occurring during magma solidification. The experiments were conducted at 900 °C and 1 bar with a new moissanite cell specifically designed for long-term high-temperature experiments. The method allowed the direct continuous observation of the crystallizing melt until the crystal fraction approached 0.7. Investigation of the evolving texture revealed that individual crystals followed distinct growth histories characterized by intervals of relatively uniform free growth, abrupt size increase due to grain coalescence, and growth reduction due to impingement; the latter played a dominant role at higher crystal fractions. Despite this growth dispersion, average growth rates calculated for single grains showed only little variation (4.9 × 10−6 to 1.9 × 10−6 mm/s). No clear relation between growth rate and crystal size was observed. Crystal coarsening through a combination of overgrowth and coalescence can also account for the counterclockwise rotation and the drop-off at smaller size fractions of the recovered crystal size distributions in our experiments. These results suggest that crystal size distributions of natural rocks do not record the original nucleation rates.

[1] Glass fragments in tephra erupted at Mt. Etna from May to December 1995 have been analyzed by laser ablation ICPMS. The trace element compositional variability of ashes deposited during this interval reveals the presence of discrete magma batches with different crystallization degrees in the shallow plumbing system. From May to October a highly crystalline magma is predominant within the conduit with only minor sporadic input of fresh and more primitive magma batches. After October new and less evolved magma batches become more prevalent and become progressively homogenized within more evolved resident magma. In December ashes closely match the chemistry of the volcanics subsequently erupted till February 1996. This study demonstrates that the trace element characterization of ashes has important implications for volcanic monitoring and is a useful tool for the forecasting of paroxysmal events at Mt. Etna.