Renat R. Almeev

The Fe oxidation state in tourmaline is a potentially valuable indicator of oxygen fugacity in both magmatic and hydrothermal systems. Here we present the compositions of eight tourmaline crystals as reference materials to calibrate the electron probe microanalysis (EPMA) flank method for in situ determination of Fe3+/ΣFe ratios in tourmaline. A field emission microprobe equipped with two TAPL analyzing crystals provides stable and high-intensity Fe L spectra. Two synthetic garnet end-members, ferric and ferrous, were used to constrain the measuring positions in the flanks of FeLα and FeLβ, which theoretically correspond to maximal Fe3+L3 absorption and minimal Fe2+L2 absorption, respectively. For the suite of tourmaline reference materials (mainly elbaite and schorl), spanning the ranges of FeOT content within 3–15 wt% and Fe3+/ΣFe within 0.0–0.3, the Fe2+ contents show a linear correlation with the Lβ/Lα intensity ratios measured at the flank positions of FeLα and FeLβ. Based on this EPMA flank-method calibration, the Fe3+/ΣFe ratios of the tourmaline reference materials agree well with wet-chemistry results. The analytical accuracy in Fe3+/ΣFe is better than ±0.1 for tourmaline with FeOT ≥ 3 wt% and better than ±0.05 for those with FeOT ≥ 12 wt%.

Crystallisation kinetics play a fundamental role in controlling conduit dynamics and eruptive style. The degree of superheating is critical in controlling crystallisation kinetics; however, its effect is still debated and has an unclear impact on eruption dynamics. Here, we investigate how superheating influences clinopyroxene nucleation in tephritic magmas from the 2021 Tajogaite eruption (La Palma, Spain) through both in situ and ex situ view experiments. Our findings show that superheating delays nucleation by dissolving pre-existing nuclei, thereby inhibiting crystallisation upon return to subliquidus conditions. Using a numerical model, we investigate how different nucleation delays resulting from different degrees of superheating affect magma ascent dynamics. Depending on the initial thermodynamic conditions and on the pre-eruptive history of magma, an increased nucleation delay can significantly reduce crystal content during ascent, lowering magma viscosity and affecting eruptive style. These findings highlight the critical role of pre-eruptive thermal histories in controlling eruptive style, and provide constraints for refining experimental protocols and numerical models, with direct implications for improving volcanic hazard assessment and eruption forecasting.

Hydrous equilibrium crystallization experiments were conducted on two boninitic melt compositions, low-Si boninite (LSB) and high-Si boninite (HSB), from the Izu-Bonin-Mariana (IBM) fore-arc to constrain the evolution of magma storage conditions and differentiation during subduction initiation. Experiments were performed at temperatures between 1020 and 1220 °C and at pressures of 200 and 400 MPa. Additionally, 14 natural boninite samples were analyzed for direct comparison. The experiments show that boninitic magmas crystallize orthopyroxene and/or olivine (± Cr-spinel) along the liquidus followed by clinopyroxene and finally plagioclase. Magmatic H ₂ O suppresses plagioclase formation and stabilizes olivine and clinopyroxene relative to orthopyroxene. Residual melts follow calc-alkaline trends, evolving to high-Mg andesitic and (high-Mg) dacitic compositions. Experimental results indicate that IBM boninitic magmas were strongly hydrous. Water contents of 1–3 wt% are estimated for LSB magmas and 3–5 wt% for HSB magmas. These different H ₂ O contents confirm the increasing importance of water fluxes during the evolution of boninitic magmas, from LSB in early stages to HSB in later stages. In contrast, systematic differences in magma storage conditions could not be detected. LSBs and HSBs both differentiated at temperatures between 1220 and 1040 °C and were stored at pressures between 100 and 300 MPa prior to eruption. Our study demonstrates that residual boninitic melt evolution was controlled by (near-)equilibrium crystallization processes. However, the contributions of additional magmatic processes including magma mixing and reworking of harzburgitic/dunitic cumulates during early stages of subduction initiation result in the large compositional variety observed for Izu-Bonin-Mariana fore-arc eruptive products.

Near-liquidus crystallization experiments were performed on a high-Mg andesite from the Izu-Bonin-Mariana forearc to study the effect of H2O on the orthopyroxene crystallization temperature. Experiments were conducted at 200 and 500 MPa in an internally heated pressure vessel at temperatures ranging from 1260 to 1075 °C. Orthopyroxene was the liquidus phase at both investigated pressures. H2O contents in quenched glasses were quantified via Raman spectroscopy. The liquidus depression for orthopyroxene at 200 MPa can be described by the following equation: ΔTdry liq=26.02(±1.84)·CH2O 0.933(±0.050), where ΔTdry liq is the difference between the anhydrous liquidus temperature (in °C) and the liquidus at a specific melt water concentration CH2O (in wt%). The liquidus depression curve at 500 MPa can be described by a linear equation: ΔTdry liq=25.72(±0.20)·CH2O. The experimental data set demonstrates that the orthopyroxene liquidus depression shows no significant pressure dependence with respect to absolute melt H2O concentrations. Thermodynamic and petrological models predict a more pronounced nonlinear behavior and generally overestimate the role of H2O on orthopyroxene liquidus depression at H2O contents below 5 wt% while partly underestimating it at higher H2O contents. Comparing our data set with experimental results taken from literature obtained for a high-Mg basaltic andesite and different boninites confirms the validity of our orthopyroxene liquidus parameterization for a wider range of melt compositions. A comparison with previous experimental data obtained for olivine and plagioclase shows that the effect of H2O on the depression of the liquidus temperature of orthopyroxene is slightly lower than for olivine while the plagioclase liquidus temperature is depressed more significantly in the presence of H2O. Our experimental data can be used for predicting liquidus temperatures for hydrous melts in which orthopyroxene appears as the liquidus phase. Moreover, the empirical parameterization can be incorporated in petrological models to improve modeling of crystallization paths of hydrous magmas.

Experiments were performed at 500 MPa, 1080 °C and water activities ( a H 2 O) from 0.0 to 1.0, in fluid-present and fluid-absent conditions, with the aim of constraining the effect of volatiles on phase equilibrium assemblages of a shoshonite from Vulcanello (Aeolian Islands, Italy). Experiments were run both under reducing and oxidizing conditions and results show that proportions, shapes and size of crystals vary as a function of the volatile composition ( X H 2 O and X CO 2 ) and volatile content. Clinopyroxene (Cpx) is the main crystallising phase and is compositionally analogous to Cpx crystals found in the natural rock. Plagioclase (Pl) is stable only for water activity lower than 0.1, whereas Fe–Ti oxides are present in all experimental runs, except for those where log f O 2 was lower than - 9, ( Δ NNO - 0.11) irrespective of the presence of CO 2 . The addition of CO 2 (2.8 wt%) in nominally dry experimental charges substantially reduces the crystallinity by ca. 1/3 compared to volatile free experiments. This result has important consequences upon the physical properties of the magma because it influences its viscosity and, as a consequence, velocity during its travel to the Earth surface. Assuming that the widths of Vulcanello conduits vary from 0.5 to 1.5 m, estimates of the ascent velocity vary in the range 1.5 × 10 - 4 –3.5 × 10 - 2 m · s - 1 for CO 2 free systems and from 5.7 × 10 - 4 –1.3 × 10 - 1 m · s - 1 for CO 2 bearing systems. Since shoshonitic magmas are common not only in the Italian volcanic provinces (Aeolian Arc, Campi Flegrei, Ischia Island, Pontine Islands, Monti Cimini, Monte Amiata, Capraia Island, Radicofani, Roccamonfina) but also in different volcanoes worldwide (Yellowstone, Mariana Arc, Kurile Arc, Tonga Arc, Andean Arc, Kamchatka Arc), we suggest that the new data will be useful to better understand shoshonitic magma behaviour under relevant geological scenarios. As such, we also suggest that hazard evaluation should incorporate the probability of very rapid ascent of poorly-evolved melts from depth.

This article reports a protocol on the application of Amp-TB2 (single-amphibole thermobarometry) based on detailed electron-microprobe analyses performed on homogeneous natural standards and synthetic glasses, and amphibole crystals (mostly phenocrysts) of volcanic products erupted by the Bezymianny volcano during its activity through time. The application of this protocol is facilitated by a new version of the model (Amp-TB2.1.xlsx) including an equation to identify heterogeneous domains (disequilibrium; not suitable for thermobarometric constraints) and homogenous (equilibrium) zones within amphibole crystals, which can be used to quantify the physicochemical parameters (i.e., pressure, P; temperature, T; volatile content in the melt, H₂O_melt; oxygen fugacity, fO₂) of “steady-state” magmatic crystallization. Application examples of the protocol, showing detailed core–rim microprobe data and physicochemical parameter variations in representative amphibole phenocrysts of the Bezymianny are also reported. The depth (and P) estimated by Amp-TB2.1 for this volcano are compared to seismic tomography results. Amp-TB2.1 results mainly show (1) that the Bezymianny is characterized by a very dynamic feeding system where the magma is stored at shallow crustal levels before recent activity periods characterized by climatic events and (2) that the pre-eruptive depth of magma storage generally increases with the age of the investigated products.

This article reports a protocol on the application of Amp-TB2 (single-amphibole ther-mobarometry; [1]) based on detailed electron microprobe analyses performed on homogeneous natural standards and synthetic glasses, and amphibole crystals (mostly phenocrysts) of volcanic products erupted by Bezymianny volcano during its activity through time. The application of this protocol is facilitated by a new version of the model (Amp-TB2.1.xlsx) including an equation to identify heterogeneous domains (disequilibrium; not suitable for thermobarometric constraints) and homogenous (equilibrium) zones within amphibole crystals, which can be used to quantify the physico-chemical parameters (i.e., pressure, P; temperature, T; volatile content in the melt, H2Omelt; oxygen fugacity, fO2) of “steady-state” magmatic crystallization. Application examples of the protocol, showing detailed core-rim microprobe data and physico-chemical parameter variations in representative amphibole phenocrysts of Bezymianny are also reported. The depth (and P) estimated by Amp-TB2.1 for this volcano are compared to seismic tomography results. Amp-TB2.1 results mainly shows (1) that Bezymianny is characterized by a very dynamic feeding system where the magma is stored at shallow crustal levels before recent activity periods characterized by climatic events and (2) that the pre-eruptive depth of magma storage generally increases with the age of the investigated products.

The temperature and chemical dependence of the melt viscosity are ubiquitous in the model development of the volcanic dynamics, as well as in the glass production and design. We focussed on the yet-explored relationship between the bulk and shear moduli ratio and boson peak with the melt fragility of their parental glasses. Here, we explored the extension of the observed trend by testing the conventional binary system Na₂O-SiO₂, thus providing new evidence supporting the link between the flow of melts and supercooled liquids and the vibrational dynamics of their parental glasses. This was accomplished by integrating new low-frequency Raman measurements and integrating data from the literature on Brillouin light scattering and viscometry. This approach allows us to feed the MYEGA equation with reliable input parameters to quantitatively predict the viscosity of the Na₂O-SiO₂ system from the liquid up to the glass transition.

Subduction‐related lavas have higher Fe³⁺/∑Fe than midocean ridge basalts (MORB). Hypotheses for this offset include imprint from subducting slabs and differentiation in thickened crust. These ideas are readily tested through examination of the time‐dependent evolution of slab‐derived signatures, thickening crust of the overriding plate, and evolving redox during subduction initiation. Here, we present Fe³⁺/ΣFe and volatile element abundances of volcanic glasses recovered from International Ocean Discovery Program (IODP) Expedition 352 to the Izu‐Bonin‐Mariana (IBM) forearc. The samples include forearc basalts (FAB) that are stratigraphically overlain by low‐ and high‐silica boninite lavas. The FAB glasses have 0.18–0.85 wt% H₂O, 75–233 ppm CO₂, S contents controlled by saturation with a sulfide phase (602–1,386 ppm), Ba/La from 3.9‐10, and Fe³⁺/ΣFe ratios from 0.136 to 0.177. These compositions are similar to MORB and suggest that decompression melting of dry and reduced mantle dominates the earliest stages of subduction initiation. Low‐ and high‐silica boninite glasses have 1.51–3.19 wt% H₂O, CO₂ below detection, S contents below those required for sulfide saturation (5–235 ppm), Ba/La from 11 to 29, and Fe³⁺/∑Fe from 0.181 to 0.225. The compositions are broadly similar to modern arc lavas in the IBM arc. These data demonstrate that the establishment of fluid‐fluxed melting of the mantle, which occurs in just 0.6–1.2 my after subduction initiation, is synchronous with the production of oxidized, mantle‐derived magmas. The coherence of high Fe³⁺/∑Fe and Ba/La ratios with high H₂O contents in Expedition 352 glasses and the modern IBM arc rocks strongly links the production of oxidized arc magmas to signatures of slab dehydration.