Improvement of Electron Probe Microanalysis of Boron Concentration in Silicate Glasses

The determination of low boron concentrations in silicate glasses by electron probe microanalysis (EPMA) remains a significant challenge. The internal interferences from the diffraction crystal, i.e. the Mo-B4C large d-spacing layered synthetic microstructure crystal, can be thoroughly diminished by using an optimized differential mode of pulse height analysis (PHA). Although potential high-order spectral interferences from Ca, Fe, and Mn on the BKα peak can be significantly reduced by using an optimized differential mode of PHA, a quantitative calibration of the interferences is required to obtain accurate boron concentrations in silicate glasses that contain these elements. Furthermore, the first-order spectral interference from ClL-lines is so strong that they hinder reliable EPMA of boron concentrations in Cl-bearing silicate glasses. Our tests also indicate that, due to the strongly curved background shape on the high-energy side of BKα, an exponential regression is better than linear regression for estimating the on-peak background intensity based on measured off-peak background intensities. We propose that an optimal analytical setting for low boron concentrations in silicate glasses (≥0.2 wt% B2O3) would best involve a proper boron-rich glass standard, a low accelerating voltage, a high beam current, a large beam size, and a differential mode of PHA

Chemical interdiffusion between Na-series tephritic and phonolitic melts with different H2O content, temperature, and oxygen fugacity values

The diffusive exchange of major elements in Na-series tephrite–phonolite diffusion couples with compositions relevant to the Canary Islands magmatism was determined at 300 MPa and variable H2O concentrations (0.3 wt % to 3.3 wt %), temperatures (1150 to 1300 °C), and fO2 levels (NNO−1.5 to NNO+1.7). Composition-dependent effective binary diffusion coefficients were determined from concentration–distance profiles. Results show a wide range of diffusivities for different cations, consistently following the sequence Na Al K ≥ Mg = Fe = Ca > Si > Ti, with a mild diffusivity contrast (0.2–0.8 log units) between tephritic and phonolitic melts. Na is the fastest component, with diffusivities falling ∼ 1.0 log units above those of Si for any given condition. An anomalously fast Al diffusion is observed, with DAl falling ∼ 0.4 log units above Si and ∼ 0.6 log units below Na, suggesting a prevalence of Al–alkali coupling across our range of run conditions. The relationships between log D and H2O content in melt for all cations in an intermediate composition are strongly nonlinear and can be fitted using an exponential function with a convergence in diffusion coefficients for different temperatures with increasing H2O content. Thus, Arrhenius analyses result in a decrease in activation energies from 222–293 kJ mol−1 at 1.7 wt % H2O to 48–112 kJ mol−1 at 3.0 wt % H2O. These results provide new data on chemical interdiffusion in highly alkaline Na-rich melts and suggest that H2O content plays a key role in increasing the chemical efficiency of magma mixing at low temperatures. The obtained dataset is used to test chemical controls of magma mixing in the El Abrigo ignimbrite, Tenerife, where banded pumices involving basanitic–tephritic to phonolitic magmas are common in several compositionally bimodal ignimbrite units

Automated Segmentation of Olivine Phenocrysts in a Volcanic Rock Thin Section Using a Fully Convolutional Neural Network

An example of automated characterization and interpretation of the textural and compositional characteristics of solids phases in thin sections using machine learning (ML) is presented. In our study, we focus on the characterization of olivine in volcanic rocks, which is a phase that is often chemically zoned with variable Mg/(Mg + Fe) ratios, so-called magnesian number or mg#. As the olivine crystals represent only less than 10 vol% of the volcanic rock, a pre-processing step is necessary to automatically detect the phases of interest in the images on a pixel level, which is achieved using Deep Learning. A major contribution of the presented approach is to use backscattered electron (BSE) images to: 1) automatically segment all olivine crystals present in the thin section; 2) determine quantitatively their mg#; and 3) identify different populations depending on zoning type (e.g., normal vs reversal zoning) and textural characteristics (e.g., microlites vs phenocrysts). The segmentation of the olivine crystals is implemented with a pretrained fully convolutional neural network model with DeepLabV3 architecture. The model is trained to identify olivine crystals in backscattered electron images using automatically generated training data. The training data are generated automatically from images which can easily be created from X-Ray element maps. Once the olivines are identified in the BSE images, the relationship between BSE intensity value and mg# is determined using a simple regression based on a set of microprobe measurements. This learned functional relationship can then be applied to all olivine pixels of the thin section. If the highest possible map resolution (1 micron per 1 pixel) is selected for the data acquisition, the full processing time of an entire thin section of (Formula presented.) containing more than 1,500 phenocrysts and 20.000 microliths required 140 h of data acquisition (BSE + X-Ray element maps), 8 h of training and 16 h of segmentation and classification. Our further tests demonstrated that the 140 h of data acquisition can be reduced at least by a factor of 4 since only a part of the thin section area (25% or even less) needs to be used for training. The characterization of each additional thin section would only require the BSE data acquisition time (less than 48 h for a whole thin section), without an additional training step. The paper describes the training and processing in detail, shows analytical results and outlines the potential of this Deep Learning approach for petrological applications, resulting in the automatic characterization and interpretation of mineral textures and compositions with an unprecedented high resolution. Copyright © 2022 Leichter, Almeev, Wittich, Beckmann, Rottensteiner, Holtz and Sester

Confocal μ-XANES as a tool to analyze Fe oxidation state in heterogeneous samples: the case of melt inclusions in olivine from the Hekla volcano

Here we present a confocal Fe K-edge μ-XANES method (where XANES stands for X-ray absorption near-edge spectroscopy) for the analysis of Fe oxidation state in heterogeneous and one-side-polished samples. The new technique allows for an analysis of small volumes with high spatial 3D resolution of <100 μm3. The probed volume is restricted to that just beneath the surface of the exposed object. This protocol avoids contamination of the signal by the host material and minimizes self-absorption effects. This technique has been tested on a set of experimental glasses with a wide range of Fe3+ / ςFe ratios. The method was applied to the analysis of natural melt inclusions trapped in forsteritic to fayalitic olivine crystals of the Hekla volcano, Iceland. Our measurements reveal changes in Fe3+ / ςFe from 0.17 in basaltic up to 0.45 in dacitic melts, whereas the magnetite-ilmenite equilibrium shows redox conditions with Fe3+ / ςFe ≤0.20 (close to FMQ, fayalite-magnetite-quartz redox equilibrium) along the entire range of Hekla melt compositions. This discrepancy indicates that the oxidized nature of glasses in the melt inclusions could be related to the post-entrapment process of diffusive hydrogen loss from inclusions and associated oxidation of Fe in the melt. The Fe3+ / ςFe ratio in silicic melts is particularly susceptible to this process due to their low FeO content, and it should be critically evaluated before petrological interpretation

Influence of volatiles (H 2 O and CO 2 ) on shoshonite phase equilibria

t. Experiments were performed at 500 MPa, 1080 °C and water activities (aH2O) 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 (XH2O and XCO2) 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 O2 was lower than −9, (∆NNO −0.11) irrespective of the presence of CO2. The addition of CO2 (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

Wie schnell steigt Magma auf? : Maschinelles Lernen (ML) hilft bei der Antwort

Wissenschaftler*innen der Geoinformatik und der Geowissenschaften bündeln ihre Expertisen, um Georisiken zu minimieren. In diesem Beitrag zeigen sie, dass maschinelles Lernen (ML) für die Auswertung von großen analytischen Datensätzen unumgänglich ist, um Prozesse in Magma-Reservoiren nachzuvollziehen. Der Einsatz von ML ermöglicht ein schnelles Reagieren bei laufenden vulkanischen Eruptionen, wie zum Beispiel La Palma

Viscosity, Boson Peak and Elastic Moduli in the Na2O-SiO2 System

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

Formation mechanisms of macroscopic globules in andesitic glasses from the Izu–Bonin–Mariana forearc (IODP Expedition 352)

The Izu–Bonin–Mariana volcanic arc is situated at a convergent plate margin where subduction initiation triggered the formation of MORB-like forearc basalts as a result of decompression melting and near-trench spreading. International Ocean Discovery Program (IODP) Expedition 352 recovered samples within the forearc basalt stratigraphy that contained unusual macroscopic globular textures hosted in andesitic glass (Unit 6, Hole 1440B). It is unclear how these andesites, which are unique in a stratigraphic sequence dominated by forearc basalts, and the globular textures therein may have formed. Here, we present detailed textural evidence, major and trace element analysis, as well as B and Sr isotope compositions, to investigate the genesis of these globular andesites. Samples consist of K2O-rich basaltic globules set in a glassy groundmass of andesitic composition. Between these two textural domains a likely hydrated interface of devitrified glass occurs, which, based on textural evidence, seems to be genetically linked to the formation of the globules. The andesitic groundmass is Cl rich (ca. 3000μg/g), whereas globules and the interface are Cl poor (ca. 300μg/g). Concentrations of fluid-mobile trace elements also appear to be fractionated in that globules and show enrichments in B, K, Rb, Cs, and Tl, but not in Ba and W relative to the andesitic groundmass, whereas the interface shows depletions in the latter, but is enriched in the former. Interestingly, globules and andesitic groundmass have identical Sr isotopic composition within analytical uncertainty (87Sr/86Sr of 0.70580±10), indicating that they likely formed from the same source. However, globules show high δ11B (ca. + 7‰), whereas their host andesites are isotopically lighter (ca. – 1 ‰), potentially indicating that whatever process led to their formation either introduced heavier B isotopes to the globules, or induced stable isotope fractionation of B between globules and their groundmass. Based on the bulk of the textural information and geochemical data obtained from these samples, we conclude that these andesites likely formed as a result of the assimilation of shallowly altered oceanic crust (AOC) during forearc basaltic magmatism. Assimilation likely introduced radiogenic Sr, as well as heavier B isotopes to comparatively unradiogenic and low δ11B forearc basalt parental magmas (average 87Sr/86Sr of 0.703284). Moreover, the globular textures are consistent with their formation being the result of fluid-melt immiscibility that was potentially induced by the rapid release of water from assimilated AOC whose escape likely formed the interface. If the globular textures present in these samples are indeed the result of fluid-melt immiscibility, then this process led to significant trace element and stable isotope fractionation. The textures and chemical compositions of the globules highlight the need for future experimental studies aimed at investigating the exsolution process with respect to potential trace element and isotopic fractionation in arc magmas that have perhaps not been previously considered

High spatial resolution analysis of the iron oxidation state in silicate glasses using the electron probe

The iron oxidation state in silicate melts is important for understanding their physical properties, although it is most often used to estimate the oxygen fugacity of magmatic systems. Often high spatial resolution analyses are required, yet the available techniques, such as μXANES and μMössbauer, require synchrotron access. The flank method is an electron probe technique with the potential to measure Fe oxidation state at high spatial resolution but requires careful method development to reduce errors related to sample damage, especially for hydrous glasses. The intensity ratios derived from measurements on the flanks of FeLα and FeLβ X-rays (FeLβf/FeLαf) over a time interval (time-dependent ratio flank method) can be extrapolated to their initial values at the onset of analysis. We have developed and calibrated this new method using silicate glasses with a wide range of compositions (43–78 wt% SiO2, 0–10 wt% H2O, and 2–18 wt% FeOT, which is all Fe reported as FeO), including 68 glasses with known Fe oxidation state. The Fe oxidation state (Fe2+/FeT) of hydrous (0–4 wt% H2O) basaltic (43–56 wt% SiO2) and peralkaline (70–76 wt% SiO2) glasses with FeOT > 5 wt% can be quantified with a precision of ±0.03 (10 wt% FeOT and 0.5 Fe2+/FeT) and accuracy of ±0.1. We find basaltic and peralkaline glasses each require a different calibration curve and analysis at different spatial resolutions (∼20 and ∼60 μm diameter regions, respectively). A further 49 synthetic glasses were used to investigate the compositional controls on redox changes during electron beam irradiation, where we found that the direction of redox change is sensitive to glass composition. Anhydrous alkali-poor glasses become reduced during analysis, while hydrous and/or alkali-rich glasses become oxidized by the formation of magnetite nanolites identified using Raman spectroscopy. The rate of reduction is controlled by the initial oxidation state, whereas the rate of oxidation is controlled by SiO2, Fe, and H2O content