Modeling clinopyroxene and plagioclase growth kinetics at Mt. Etna and Stromboli: a time-integrated, polybaric and polythermal perspective

Basaltic volcanoes (e.g., Mt. Etna, Stromboli, Hawaii, etc.) are characterized by a range of effusive to explosive activities with variable intensity, which can pose different type of threats to local populations. Challenges in modern volcanology and petrology involve the attempt to constrain pre-eruptive magmatic processes, which provide the basis for volcanic hazard assessment. Although the recent literature has reported constant advancements in this respect, several key questions remain unanswered. Understanding how magma is stored, migrates and feeds eruption is not a trivial task, requiring for renewed improvements over the years. In this context, both textural maturation and compositional variability of minerals crystallizing in basaltic systems represent valuable sources of information to quantify the physio-chemical conditions experienced by magmas upon the effect of changing and complex plumbing system dynamics. This study aims to provide new insights on the solidification behavior of mafic alkaline magmas erupted at Mt. Etna and Stromboli (Italy). Such open conduit volcanoes are characterized by the ubiquitous stability of clinopyroxene from mantle depths to shallow crustal levels. More evolved magmas are also saturated with plagioclase, especially at lower temperatures, melt-water contents, and pressures. Thus, clinopyroxene and plagioclase crystals represent powerful recorders of the intricate ascent dynamics explored by mafic alkaline magmas during their ascent paths towards the surface. By focusing on textural and chemical features of natural/synthetic clinopyroxene, plagioclase and coexisting glasses, I have provided new tools for interpreting polythermal-polybaric changes of magmas, together with several guidelines and a secure methodology to model pre- and syn-eruptive conditions. The temporal evolution of Etnean and Strombolian magmas has been also tracked via timescale modeling to better constrain the cooling-decompression paths of magmas rising and accelerating through the vertically extended, highly dynamic plumbing systems. In the first part of this PhD thesis, I have experimentally explored the role of supersaturation and relaxation phenomena on clinopyroxene nucleation and growth processes, which affect the final crystal cargo of variably undercooled magmas. A certain degree of undercooling is pivotal to promote the growth and textural maturation of crystals. With increasing crystallization time, however, the crystal growth rate decreases as the system approaches to near-equilibrium conditions that minimize the effect of melt supersaturation. By quantifying the textural features of synthetic and natural crystals it has been possible to parameterize clinopyroxene growth kinetics under a broad range of isothermal-isobaric, decompression, and cooling conditions representative of crystallization scenarios typically encountered in open-conduit volcanoes. This model parameterization has been combined with the textural analysis of natural clinopyroxene crystals erupted during lava fountain events at Mt. Etna allowing to unlock timescale of growth for microphenocryst and microlite populations. The retrieved temporal information has been used to develop a new conceptual model for the timescale of magma dynamics recorded by the (dis)equilibrium textural evolution of clinopyroxene and for the rapid acceleration of magma ascending within the volcanic conduit, immediately before eruption at the vent. A more comprehensive work, focusing on plagioclase textural and compositional features, characterized the second part of my PhD thesis with the aim to identify disparate aspects of plagioclase growth scenarios. Following the same approach discussed above, timescale of plagioclase crystallization from mafic alkaline magmas has been parameterized as a function of growth rate by integrating experimental (i.e., isothermal-isobaric, decompression, and cooling experiments) and natural textural data from literature. Timescales of eruptive processes at Mt. Etna and Stromboli volcanoes have been quantified by considering phenocryst/microphenocryst and microlite crystals growing during lava flow and explosive eruptions. Statistical methodologies have been employed to assess the correlation between plagioclase growth rate and other system parameters governing the crystallization process. Special attention has been paid to disambiguate the role of temperature and melt-H2O content on plagioclase chemical zoning patterns at Stromboli and Mt. Etna. By using plagioclase components and major cation substitution mechanisms, I have refined and readapted equilibrium, thermometric, and hygrometric models for future investigations

Carrier and dilution effects of CO2 on thoron emissions from a zeolitized tuff exposed to subvolcanic temperatures

Radon (222Rn) and thoron (220Rn) are two isotopes belonging to the noble gas radon (sensu lato) that is frequently employed for the geochemical surveillance of active volcanoes. Temperature gradients operating at subvolcanic conditions may induce chemical and structural modifications in rock-forming minerals and their related 222Rn-220Rn emissions. Additionally, CO2 fluxes may also contribute enormously to the transport of radionuclides through the microcracks and pores of subvolcanic rocks. In view of these articulated phenomena, we have experimentally quantified the changes of 220Rn signal caused by dehydration of a zeolitized tuff exposed to variable CO2 fluxes. Results indicate that, at low CO2 fluxes, water molecules and hydroxyl groups adsorbed on the glassy surface of macro- and micropores are physically removed by an intermolecular proton transfer mechanism, leading to an increase of the 220Rn signal. By contrast, at high CO2 fluxes, 220Rn emissions dramatically decrease because of the strong dilution capacity of CO2 that overprints the advective effect of carrier fluids. We conclude that the sign and magnitude of radon (sensu lato) changes observed in volcanic settings depend on the flux rate of carrier fluids and the rival effects between advective transport and radionuclide dilution

Parameterization of clinopyroxene growth kinetics via crystal size distribution (CSD) analysis. Insights into the temporal scales of magma dynamics at Mt. Etna volcano

There is increasing recognition that both textural and compositional changes of clinopyroxene crystallizing from mafic alkaline magmas are the direct expression of complex dynamic processes extending over a broad range of spatial and temporal scales. Among others, supersaturation and relaxation phenomena play a key role in controlling the final crystal cargo of variably undercooled magmas erupted from active alkaline volcanoes. Following this line of reasoning, we have carried out isothermal-isobaric, decompression, and cooling rate experiments on a basalt interpreted as the parental magma of mafic alkaline eruptions at Mt. Etna volcano (Sicily, Italy). The main purpose is to reconstruct and quantify the textural changes (i.e., length of major and minor axes, surface area per unit volume, area fraction, and maximum growth rate) of clinopyroxene at variable pressures (30-300 MPa), temperatures (1,050-1,100 °C), volatile contents (0-5 wt.% H2O and 0-0.2 wt.% CO2), and equilibration times (0.25-72 h). Melt supersaturation, corresponding to a degree of undercooling variable from 14 to 125 °C, drives the formation of clinopyroxene crystals with different textures and sizes as a function of growth rate and relaxation time. By integrating experimental data and thermodynamic modeling, the transition between interface-controlled (euhedral morphologies) and diffusion-controlled (anhedral morphologies) growth regimes has been determined to occur at degrees of undercooling higher than 30 °C. The decrease of clinopyroxene growth rate with increasing the equilibration time is combined with the crystal size distribution (CSD) analysis of naturally undercooled clinopyroxene crystals erupted during 2011-2012 lava fountain episodes at Mt. Etna volcano. Results indicate that the crystallization of microlites and microphenocrysts is on the order of ~100-101 min (large undercooling, short equilibration time) and ~101-102 h (small undercooling, long equilibration time), respectively. This temporal information allows to better constrain the cooling-decompression paths of Etnean magmas rising and accelerating along a vertically extended, highly dynamic plumbing system. While clinopyroxene microlites develop during the fast ascent of magmas (~100-101 m s-1) within the uppermost part of the conduit or immediately before ejection from the vent, the onset of microphenocryst crystallization occurs at depth and continues within the plumbing system during the slow ascent of magmas (~10-2 m s-1) that migrate through interconnected storage regions

Modeling the crystallization conditions of clinopyroxene crystals erupted during February-April 2021 lava fountains at Mt. Etna. Implications for the dynamic transfer of magmas

In the period February–April 2021, seventeen energetic hours-long episodes of intense lava fountaining occurred at Mt. Etna, producing lava flows and ash plumes followed by heavy fallout. Clinopyroxene mesocrysts from these paroxysms show complex sector and concentric zoning patterns, with juxtaposition of Si-Mg-rich (Al-Tipoor) and Si-Mg-poor (Al-Ti-rich) crystal layers. Clinopyroxene-based equilibrium thermobarometry and hygrometry define an overall crystallization path in the range of ~170–480 MPa, ~1060–1110 ◦C, and ~ 1.2–2.7 wt% H2O, with a main magma storage region estimated at depths of ~11–15 km. From this perspective, we observe that 2021 lava fountains were fed by hotter magmas of deeper origin with respect to those feeding 2011–2012 paroxysms. Zoning patterns of 2021 clinopyroxene mesocrysts formed in a vertically-extended plumbing system upon the effect of mixing phenomena and crystal recycling caused by recurrent inputs of fresh magmas into interconnected mushy reservoirs. Kinetic growth modeling constrains the formation of 2021 clinopyroxene mesocrysts over timescales of ~30–90 h and small degrees of undercooling ≤28 ◦C. Fe–Mg diffusion chronometry confirms that the time elapsed between the formation of clinopyroxene rim and magma eruption is utterly related to growth kinetics caused by pre-eruptive dynamic transfer of magma at crustal depths. Kinetic effects are exacerbated for clinopyroxene microlites/microcrysts forming at the syn-eruptive stage, when magma decompression, degassing, and cooling become more effective in the last 1.5 km below the vent of Mt. Etna. Kinetic growth modeling reveals that eruption dynamics within the conduit promote an exceptionally rapid disequilibrium growth of clinopyroxene microlites/microcrysts in only ~0.4–3.3 min upon large degrees of undercooling >60 ◦C. The resulting ascent velocity of 2021 magmas within the conduit is ~8–63 m/s, a factor of ~3 higher than the less energetic 2011–2012 paroxysms

Micro-Raman water calibration in ultrapotassic silicate glasses:Application to phono-tephrites and K-foidites of Colli Albani Volcanic District (Central Italy)

This study investigates the potential use of micro-Raman spectroscopy for the quantification of water in ultrapotassic silicate glasses. A calibration was developed using experimental phono-tephritic glasses with water content ranging from ~1 to ~3 wt%. The calibration curve showed a typical direct proportionality between water content and the ratio of high- (3100–3750 cm⁻¹) and low-wavenumber (100–1500 cm⁻¹) spectral regions, with a linear fit coefficient m = 1.74. The comparison with the m coefficients available in literature for other silicate compositions showed a deviation of our composition as a function of some major oxides such as FeO, TiO2 and K2O, highlighting the possible influence of the polymerization degree (NBO/T: non-bridging oxygens per tetrahedron) on m coefficient. In this respect, we observed a linear relationship between m coefficient and NBO/T and a positive correlation between the area underneath the silicate region (100–1500 cm⁻¹) and NBO/T for the phono-tephrite of this study and for other compositions spanning from basalts to phonolite and rhyolites available in literature. For ultrapotassic natural and experimental glasses characterized by the presence of CO2, documented by the carbonate peak at 1062–1092 cm⁻¹, it has been possible to extrapolate the CO2 content by using the model of Morizet et al. (2013) obtaining values of ~1.1 ± 0.3 and ~ 1.7 ± 0.2 wt%, respectively. The obtained m coefficient was applied to estimate water content of natural phono-tephritic glasses belonging to the Colli Albani Volcanic District. Moreover, we estimated water content also for some natural K-foiditic glasses from the same volcanic District. Since the m coefficient results to be strongly dependent on the chemical composition of the sample of interest, the coefficient estimated for the phono-tephrites of this study could result in significant overestimation or underestimation of the water content of the CAVD K-foiditic natural samples. Thus, we extrapolated the m coefficient for the K-foiditic samples by means of an equation obtained in this study as function of the polymerization degree (NBO/T)

A review of plagioclase growth rate and compositional evolution in mafic alkaline magmas. Guidelines for thermometry, hygrometry, and timescales of magma dynamics at Stromboli and Mt. Etna

Mafic alkaline magmas, such as those feeding the persistent eruptive activity of Stromboli and Mt. Etna volcanoes in Italy, are dominated by the crystallization of plagioclase via cooling and degassing phenomena related to the dynamics of shallow crustal reservoirs and eruptive conduits. Because plagioclase textures and compositions are extremely sensitive to the changes of intensive variables in subvolcanic plumbing systems, the phenomenological variability of erupted crystals preserves detailed evidence of complex growth histories. From this point of view, we reappraise the textural maturation and compositional complexity of plagioclase by allying thermodynamic and kinetic principles to natural and experimental observations, with the purpose of drawing up guidelines for reconstructing magma dynamics in mafic alkaline volcanic settings. A multifaceted statistical method is adopted to parameterize the decay of crystal growth rate with increasing crystallization time, as relaxation kinetics prevails over melt supersaturation effects. This model parameterization is combined with the textural analysis of natural plagioclase crystals to quantify the residence time of phenocrysts in equilibrium with magmas at Stromboli and Mt. Etna and/or the timescale of rapid microlite growth during disequilibrium ascent of magmas within the conduit. The role played by temperature and melt-water content on plagioclase components and major cation substitution mechanisms is also evaluated under both isobaric-isothermal and decompression conditions. The emerging paradigm is that the influence of dissolved water on anorthite-albite exchange between plagioclase and melt is overwhelmingly mitigated by changes in temperature at conditions of P = 30-300 MPa, T = 1050-1150 °C, fO2 = NNO+1.9-NNO+2.3, and melt-H2O = 0.6-4.4 wt.%. As a corollary, anorthite and albite melt activities are almost fully encapsulated in the variation of anhydrous melt components as the crystallization of plagioclase proceeds during magma cooling. Following this line of reasoning, we propose an integrated modeling approach to decipher complex zoning patterns in natural plagioclase phenocrysts from mafic alkaline eruptions. Key findings from our re-assessment of equilibrium, thermometric, and hygrometric models indicate that temperature and dissolved water can be iteratively estimated for different plagioclase textural patterns if crystals are sufficiently strongly zoned and probability-based criteria are applied to determine the maximum probability distribution from kernel density analysis

Deep versus shallow sources of CO2 and Rn from a multi-parametric approach: the case of the Nisyros caldera (Aegean Arc, Greece)

Estimating the quantity of CO2 diffusively emitted from the Earth’s surface has important implications for volcanic surveillance and global atmospheric CO2 budgets. However, the identification and quantification of non-hydrothermal contributions to CO2 release can be ambiguous. Here, we describe a multi-parametric approach employed at the Nisyros caldera, Aegean Arc, Greece, to assess the relative influence of deep and shallow gases released from the soil. In April 2019, we measured diffuse soil surface CO2 fluxes, together with their carbon isotope compositions, and at a depth of 80 cm, the CO2 concentration, soil temperature, and the activities of radon and thoron. The contributions of deep CO2 and biogenic CO2 fluxes were distinguished on the basis of their carbon isotope compositions. A Principal Component Analysis (PCA), performed on the measured parameters, effectively discriminates between a deep- and a shallow degassing component. The total CO2 output estimated from a relatively small testing area was two times higher with respect to that observed in a previous survey (October 2018). The difference is ascribed to variation in the soil biogenic CO2 production, that was high in April 2019 (a wet period) and low or absent in October 2018 (a dry period). Accounting for seasonal biogenic activity is therefore critical in monitoring and quantifying CO2 emissions in volcanic areas, because they can partially- or completely overwhelm the volcanic-hydrothermal signal.ISSN:2045-232