Volatiles are a critical component of magmas, driving volcanic eruptions, creating and modifying planetary atmospheres, and generating ore deposits. There has been an immense amount of work quantifying the solubility of individual volatile species in a wide variety of magma compositions and the creation of various modelling capabilities to look at magmatic degassing. This workshop aimed to bring together experimentalists, numerical modellers, and observational researchers with an interest in volatile solubility in magmas. Through group discussions, model demonstrations, and keynote talks, we explored the current landscape and looked for future directions. We started with a demonstration session of existing models and codes. There is currently no benchmarking for such codes, so we discussed options for a benchmarking exercise and test datasets. This was followed by discussions on future directions, covering: 1. What are the experimental gaps (e.g., melt compositions, volatile species, pressure-temperature (PT) ranges, fugacity coefficients, non-ideal mixing, etc.)?2. What do observational researchers need these codes to be able to do? 3. Can we link monitoring needs to solubility outputs?
Volcanic Zone, Aotearoa New Zealand. Although best known for its high rates of explosive rhyolitic volcanism, there are several examples of basaltic to basaltic-andesite contributions to OVC eruptions. These range from minor involvement of basalt in rhyolitic eruptions to the exclusively basaltic 1886 C.E. plinian eruption of Tarawera. To explore the basaltic component supplying this dominantly rhyolitic area, we analyse the textures and compositions (minerals and melt inclusions) of four basaltic eruptions from within and around the OVC that have similar whole rock chemistry, namely: Terrace Rd, Rotomakariri, Rotokawau, and Tarawera. Data from these basaltic deposits provide constraints on the conditions of magma evolution and ascent in the crust prior to eruption, revealing that eruptions sample multiple distinct reservoirs during ascent to the surface. The most abundant basaltic component is generated by cooling-induced crystallisation of a common, oxidised, volatile-rich basaltic melt at various depths within the crust that mixes upon ascent. Despite similar bulk compositions, these four eruptions are texturally distinct from each other as a result of their wide variation in eruption style.
The cataclysmic basaltic eruption of Mt. Tarawera in 1886 represents a significant cultural and scientific event for New Zealand. This review utilises published and new observations, to reinterpret eruptive parameters encompassing the entirety of the eruption. The ∼17 km eruptive fissure, active for 4+ hours, extends across Mt. Tarawera to the hydrothermally active Waimangu region. Correlating published observations of bed thickness, componentry and microtextures from Mt. Tarawera to new bed descriptions and granulometry for the Rotomahana-Waimangu rift segment allows for a re-assessment of eruption variations along the length of the fissure. Variably thick pyroclastic fall sequences at Mt. Tarawera contrast with the pyroclastic surges and an eruption plume that together deposited the 'Rotomahana Mud' erupted along the Rotomahana-Waimangu segment. Providing insight into pre-eruptive conditions, new mineral chemistry from Mt. Tarawera provides the first constraints on crystallisation pressures (<2 kbar), temperatures (<1100°C), and magmatic water content (<2.8 wt%). Recalculated volumes indicate a bulk eruptive volume of 1.1–1.3 km3, and a juvenile basalt volume of up to 0.67 km3, which then lead to calculated discharge rates of 3.7 × 107–7.8 × 107 kg s−1 for the northern Mt. Tarawera segment of the fissure and 1.4–5.7 × 106 kg s−1 for the Rotomahana segment.
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 μrXANES 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.
Abstract Geobarometers are commonly used to determine the pressure (and hence depth) of magmatic bodies. For instance, at equilibrium, the concentration of dissolved volatiles in a vapor-saturated melt can be used as a barometer: this is the pressure of vapor-saturation (Psatv). Most determinations of Psatv assume that melt and vapor contain only oxidized C-O-H species. However, sulfur is the third most abundant volatile element in magmas, and oxygen fugacity (fO2) exerts a strong influence on the speciation of the melt and vapor. To explore how S and fO2 affect calculations of Psatv, we model a Hawaiian tholeiite that contains both reduced and oxidized C-O-H-S species in the melt and vapor. We find that excluding reduced C-O-H species in the system can result in significant underestimations of Psatv under reducing conditions (ΔFMQ < 0). The effect of S on Psatv is small except in the vicinity of the “sulfur solubility minimum” (SSmin; 0 < ΔFMQ < +2), where excluding S-bearing species can result in underestimates of Psatv. The implications of these results depend on the volatile concentration of the system being investigated, its fO2, and the melt composition and temperature. Our results suggest there will be little impact on Psatv calculated for mid-ocean ridge basalts because their fO2 is above where reduced C-O-H species become important in the melt and vapor and yet below the SSmin. However, the fO2 of ocean island and arc basalts are close enough to the SSmin and their S concentrations high enough to influence Psatv. However, high-CO2 and high-H2O concentrations are predicted to reduce the effect of the SSmin. Hence, Psatv calculated for shallowly trapped melt inclusions and matrix glass are more affected by the SSmin than deeply trapped melt inclusions. Lunar and martian magmas are typically more reduced than terrestrial magmas, and therefore accurate Psatv calculations for them require the inclusion of reduced C-O-H species.
We introduce three new reference materials and a new high-precision set-up for stable carbon isotope analysis in basaltic glasses using large-geometry secondary ion mass spectrometry (SIMS) instrument. The new hydrous basaltic reference materials, characterised for carbon concentration and isotope composition by step-heating gas extraction and manometry followed by isotope ratio mass spectrometry, show homogeneity for in situ analysis. Additionally, their hydrogen concentration and hydrogen isotope ratios are reported. Our SIMS protocol uses multi-collection, cycling between concurrent measurements of (_ ^12)C and (_ ^13)C on electron multipliers, and either (_ ^30)Si or (_ ^18)O, as a reference mass, on a 10^11 Ω resistor Faraday cup. The analysis involves rastering over an area of 20 〖μm〗^2 for 100 cycles, resulting in a 40 μm-wide analytical pit. This set-up achieves high internal precision for δ_ ^13 C down to ± 0.35 ‰ 1RSE at 1706_(-88)^(+89) μg g^(-1) CO_2, with precision of ± 1.00 ‰ 1RSE or better between 163_(-5.2)^(+5.1) and 267_(-8.9)^(+8.9) μg g^(-1) CO_2, depending on set-up sensitivity. Precision reported here is improved by a factor of three at comparable concentrations to that previously reported elsewhere. Carbon blanks were characterised by measuring carbon-free olivines, allowing for accurate blank corrections on δ_ ^13 C measurements. After correcting for blank signals and instrument mass fractionation, we measure δ_ ^13 C in glasses with low CO_2 concentrations down to 〖26.16〗_(-0.86)^(+0.85) μg g^(-1) CO_2 with a final measurement standard sample deviation of ± 2.97 ‰ 1s. We report in situ measurements on an ocean floor basaltic glass from the East Pacific Rise and a set of synthetic basaltic glasses are presented to demonstrate our approach. The reference materials and SIMS set-up can be used to significantly improve the accuracy and precision of del13C measurements in natural basaltic glasses and are applicable across a wide range of geologically relevant carbon contents.
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