Mantle to surface degassing of alkalic magmas at Erebus volcano, Antarctica

International audienceContinental intraplate volcanoes, such as Erebus volcano, Antarctica, are associated with extensional tectonics, mantle upwelling and high heat flow. Typically, erupted magmas are alkaline and rich in volatiles (especially CO2), inherited from low degrees of partial melting of mantle sources. We examine the degassing of the magmatic system at Erebus volcano using melt inclusion data and high temporal resolution open-path Fourier transform infrared (FTIR) spectroscopic measurements of gas emissions from the active lava lake. Remarkably different gas signatures are associated with passive and explosive gas emissions, representative of volatile contents and redox conditions that reveal contrasting shallow and deep degassing sources. We show that this unexpected degassing signature provides a unique probe for magma differentiation and transfer of CO2-rich oxidised fluids from the mantle to the surface, and evaluate how these processes operate in time and space. Extensive crystallisation driven by CO2 fluxing is responsible for isobaric fractionation of parental basanite magmas close to their source depth. Magma deeper than 4 kbar equilibrates under vapour-buffered conditions. At shallower depths, CO2-rich fluids accumulate and are then released either via convection-driven, open-system gas loss or as closed-system slugs that ascend and result in Strombolian eruptions in the lava lake. The open-system gases have a reduced state (below the QFM buffer) whereas the closed-system gases preserve their deep oxidised signatures (close to the NNO buffer)

Pre- and syn-eruptive degassing and crystallisation processes of the 2010 and 2006 eruptions of Merapi volcano, Indonesia

The 2010 eruption of Merapi (VEI 4) was the volcano’s largest since 1872. In contrast to the prolonged and effusive dome-forming eruptions typical of Merapi’s recent activity, the 2010 eruption began explosively, before a new dome was rapidly emplaced. This new dome was subsequently destroyed by explosions, generating pyroclastic density currents (PDCs), predominantly consisting of dark coloured, dense blocks of basaltic andesite dome lava. A shift towards open-vent conditions in the later stages of the eruption culminated in multiple explosions and the generation of PDCs with conspicuous grey scoria and white pumice clasts resulting from sub-plinian convective column collapse. This paper presents geochemical data for melt inclusions and their clinopyroxene hosts extracted from dense dome lava, grey scoria and white pumice generated during the peak of the 2010 eruption. These are compared with clinopyroxene-hosted melt inclusions from scoriaceous dome fragments from the prolonged dome-forming 2006 eruption, to elucidate any relationship between pre-eruptive degassing and crystallisation processes and eruptive style. Secondary ion mass spectrometry analysis of volatiles (H2O, CO2) and light lithophile elements (Li, B, Be) is augmented by electron microprobe analysis of major elements and volatiles (Cl, S, F) in melt inclusions and groundmass glass. Geobarometric analysis shows that the clinopyroxene phenocrysts crystallised at depths of up to 20 km, with the greatest calculated depths associated with phenocrysts from the white pumice. Based on their volatile contents, melt inclusions have re-equilibrated during shallower storage and/or ascent, at depths of ~0.6–9.7 km, where the Merapi magma system is interpreted to be highly interconnected and not formed of discrete magma reservoirs. Melt inclusions enriched in Li show uniform “buffered” Cl concentrations, indicating the presence of an exsolved brine phase. Boron-enriched inclusions also support the presence of a brine phase, which helped to stabilise B in the melt. Calculations based on S concentrations in melt inclusions and groundmass glass require a degassing melt volume of 0.36 km3 in order to produce the mass of SO2 emitted during the 2010 eruption. This volume is approximately an order of magnitude higher than the erupted magma (DRE) volume. The transition between the contrasting eruptive styles in 2010 and 2006 is linked to changes in magmatic flux and changes in degassing style, with the explosive activity in 2010 driven by an influx of deep magma, which overwhelmed the shallower magma system and ascended rapidly, accompanied by closed-system degassing

Hydrogen partitioning between melt, clinopyroxene, and garnet at 3 GPa in a hydrous MORB with 6 wt.% H2O

International audienceAbstract To understand partitioning of hydrogen between hydrous basaltic and andesitic liquids and coexisting clinopyroxene and garnet, experiments using a midocean ridge basalt (MORB) + 6 wt.% H2O were conducted at 3 GPa and 1,150–1,325°C. These included both isothermal and controlled cooling rate crystallization experiments, as crystals from the former were too small for ion microprobe (SIMS) analyses. Three runs at lower bulk water content are also reported. H2O was measured in minerals by SIMS and in glasses by SIMS, Fourier Transform infrared spectroscopy (FTIR), and from oxide totals of electron microprobe (EMP) analyses. At 3 GPa, the liquidus for MORB with 6 wt.% H2O is between 1,300 and 1,325C. In the temperature interval investigated, the melt proportion varies from 100 to 45% and the modes of garnet and clinopyroxene are nearly equal. Liquid composition varies from basaltic to andesitic. The crystallization experiments starting from above the liquidus failed to nucleate garnets, but those starting from below the liquidus crystallized both garnet and clinopyroxene. SIMS analyses of glasses with [7 wt.% H2O yield spuriously low concentrations, perhaps owing to hydrogen degassing in the ultra-high vacuum of the ion microprobe sample chamber. FTIR and EMP analyses show that the glasses have 3.4 to 11.9 wt.% water, whilst SIMS analyses indicate that clinopyroxenes have 1,340–2,330 ppm and garnets have 98–209 ppm H2O. DH cpx-gt is 11 ± 3, DH cpx-melt is 0.023 ± 0.005 and DH gt-melt is 0.0018 ± 0.0006. Most garnet/melt pairs have low values of DH gt-melt, but DH gt-melt increases with TiO2 in the garnet. As also found by previous studies, values of DH cpx-melt increase with Al2O3 of the crystal. For garnet pyroxenite, estimated values of DH pyroxenite-melt decrease from 0.015 at 2.5 GPa to 0.0089 at 5 GPa. Hydration will increase the depth interval between pyroxenite and peridotite solidi for mantle upwelling beneath ridges or oceanic islands. This is partly because the greater pyroxene/olivine ratio in pyroxenite will tend to enhance the H2O concentration of pyroxenite, assuming that neighboring pyroxenite and peridotite bodies have similar H2O in their pyroxenes

Hydrogen Isotope Composition of a Large Silicic Magma Reservoir Preserved in Quartz‐Hosted Glass Inclusions of the Bishop Tuff Plinian Eruption

Abstract Water controls magmatic crystallization and drives volcanic eruptions, but little is known about its primary source in silicic systems. The hydrogen isotope composition of volcanic products provides a metric that can track and identify magmatic source, fractionation, or degassing processes. Despite such promise, hydrogen isotope measurements have never previously been acquired for undegassed silicic melt. To explore whether hydrogen isotopes can identify the source and modification of water in a silicic magma reservoir, we analyzed D/H ratios and dissolved H2O content of quartz‐hosted, rhyolitic glass inclusions from the early Bishop Tuff, a time‐honored testing ground for innovative petrologic studies. The rhyolitic inclusions indicate the early Bishop reservoir had δD values ranging from −40‰ to −60‰ (Vienna Standard Mean Ocean Water). The measured hydrogen isotope ratios do not follow systematic trends that would be predicted for open‐system degassing, rehydration, or diffusive loss. Observed isotopic variability in the microanalyses is instead attributed to analytical artifacts. The large silicic reservoir degassed as a closed system, resulting in limited fractionation obscured by the uncertainty of the measurements. Significant modification of melt D/H ratios by assimilation and fractional crystallization are unlikely, as their projected contributions are not observed. Dynamic geologic processes are thus not recorded by the hydrogen isotope composition of the inclusions. Instead, the rhyolitic melt represents a distinct, largely homogenous isotopic reservoir. When compared to the global record of basaltic glass inclusions, the rhyolitic inclusions preserve an isotopic signature that is most similar to subduction‐related mafic melts

Tracking Radionuclide Fractionation in the First Atomic Explosion Using Stable Elements

Compositional analysis of postdetonation fallout is a tool for forensic identification of nuclear devices. However, the relationship between device composition and fallout composition is difficult to interpret because of the complex combination of physical mixing, nuclear reactions, and chemical fractionations that occur in the chaotic nuclear fireball. Using a combination of in situ microanalytical techniques (electron microprobe analysis and secondary ion mass spectrometry), we show that some heavy stable elements (Rb, Sr, Zr, Ba, Cs, Ba, La, Ce, Nd, Sm, Dy, Lu, U, Th) in glassy fallout from the first nuclear test, Trinity, are reliable chemical proxies for radionuclides generated during the explosion. Stable-element proxies show that radionuclides from the Trinity device were chemically, but not isotopically, fractionated by condensation. Furthermore, stable-element proxies delineate chemical fractionation trends that can be used to connect present-day fallout composition to past fireball composition. Stable-element proxies therefore offer a novel approach for elucidating the phenomenology of the nuclear fireball as it relates to the formation of debris and the fixation of device materials within debris

Table_1_Stability of Zircon and Its Isotopic Ratios in High-Temperature Fluids: Long-Term (4 months) Isotope Exchange Experiment at 850°C and 50 MPa.XLS

<p>Stability of zircon in hydrothermal fluids and vanishingly slow rates of diffusion identify zircon as a reliable recorder of its formation conditions in recent and ancient rocks. Debate, however, persists on how rapidly oxygen and key trace elements (e.g., Li, B, Pb) diffuse when zircon is exposed to silicate melt or hot aqueous fluids. Here, we report results of a nano- to micrometer-scale investigation of isotopic exchange using natural zircon from Mesa Falls Tuff (Yellowstone) treated with quartz-saturated, isotopically (¹⁸O, D, ⁷Li, and ¹¹B) labeled water with a nominal δ¹⁸O value of +450%0 over 4 months at 850°C and 50 MPa. Frontside (crystal rim inwards) δ¹⁸O depth profiling of zircon by magnetic sector SIMS shows initially high but decreasing ¹⁸O/¹⁶O over a ~130 nm non-Fickian profile, with a decay length comparable to the signal from surficial Au coating deposited onto zircon. In contrast, backside (crystal interior outwards) depth profiling on a 2-3 μm thick wafer cut and thinned from treated zircon by focused ion beam (FIB) milling lacks any significant increase in ¹⁸O/¹⁶O during penetration of the original surface layer. Near-surface time-of-flight (TOF-SIMS) frontside profiles of uncoated zircon from 4-month and 1-day-long experiments as well as untreated zircons display similar enrichments of ¹⁸O over a distance of ~20 nm. All frontside ¹⁸O profiles are here interpreted as transient surface signals from nm-thick surface enrichment or contamination unrelated to diffusion. Likewise, frontside depth profiling of H, Li, and B isotopes are similar for long- and short-duration experiments. Additionally, surface U-Pb dating of zircon from the 4-month experiment returned U-Pb ages by depth profiling with ~1 μm penetration that were identical to untreated samples. Frontside and backside depth-profiling thus demonstrate that diffusive ¹⁸O enrichment in the presence of H₂O is much slower than predicted from experiments in Watson and Cherniak (1997). Instead, intracrystalline exchange of oxygen between fluid and zircon in wet experimental conditions with excess silica occurred over length-scales equivalent to those predicted for dry diffusion. Oxygen diffusion coefficients even under wet conditions and elevated temperatures (850°C) are ≤ 1–3 × 10⁻²³ m²/s, underscoring a virtual lack of oxygen diffusion and an outstanding survivability of zircons and its isotopic inventory under most metamorphic and hydrothermal conditions.</p

Presentation_1_Stability of Zircon and Its Isotopic Ratios in High-Temperature Fluids: Long-Term (4 months) Isotope Exchange Experiment at 850°C and 50 MPa.pdf

<p>Stability of zircon in hydrothermal fluids and vanishingly slow rates of diffusion identify zircon as a reliable recorder of its formation conditions in recent and ancient rocks. Debate, however, persists on how rapidly oxygen and key trace elements (e.g., Li, B, Pb) diffuse when zircon is exposed to silicate melt or hot aqueous fluids. Here, we report results of a nano- to micrometer-scale investigation of isotopic exchange using natural zircon from Mesa Falls Tuff (Yellowstone) treated with quartz-saturated, isotopically (¹⁸O, D, ⁷Li, and ¹¹B) labeled water with a nominal δ¹⁸O value of +450%0 over 4 months at 850°C and 50 MPa. Frontside (crystal rim inwards) δ¹⁸O depth profiling of zircon by magnetic sector SIMS shows initially high but decreasing ¹⁸O/¹⁶O over a ~130 nm non-Fickian profile, with a decay length comparable to the signal from surficial Au coating deposited onto zircon. In contrast, backside (crystal interior outwards) depth profiling on a 2-3 μm thick wafer cut and thinned from treated zircon by focused ion beam (FIB) milling lacks any significant increase in ¹⁸O/¹⁶O during penetration of the original surface layer. Near-surface time-of-flight (TOF-SIMS) frontside profiles of uncoated zircon from 4-month and 1-day-long experiments as well as untreated zircons display similar enrichments of ¹⁸O over a distance of ~20 nm. All frontside ¹⁸O profiles are here interpreted as transient surface signals from nm-thick surface enrichment or contamination unrelated to diffusion. Likewise, frontside depth profiling of H, Li, and B isotopes are similar for long- and short-duration experiments. Additionally, surface U-Pb dating of zircon from the 4-month experiment returned U-Pb ages by depth profiling with ~1 μm penetration that were identical to untreated samples. Frontside and backside depth-profiling thus demonstrate that diffusive ¹⁸O enrichment in the presence of H₂O is much slower than predicted from experiments in Watson and Cherniak (1997). Instead, intracrystalline exchange of oxygen between fluid and zircon in wet experimental conditions with excess silica occurred over length-scales equivalent to those predicted for dry diffusion. Oxygen diffusion coefficients even under wet conditions and elevated temperatures (850°C) are ≤ 1–3 × 10⁻²³ m²/s, underscoring a virtual lack of oxygen diffusion and an outstanding survivability of zircons and its isotopic inventory under most metamorphic and hydrothermal conditions.</p

Unravelling the Consequences of SO₂-Basalt Reactions for Geochemical Fractionation and Mineral Formation

Palm is supported by a Commonwealth Supported Place for the Masters of Earth Science (Advanced) degree and the John and Kerry Lovering Scholarship (RSES, ANU). Palm, King, Renggli, Troitzsch and Mernagh are supported by Australian Research Council grants to King (DP150104604 and FT130101524)

Analytical techniques for probing small-scale layers that preserve information on gas-solid interactions

This work was supported by Australian Research Council funding to King (DP150104604 and FT130101524) that provided for the contributions from King, Renggli, Troitzsch, and Palm; Australian Synchrotron funding to King, Troitzsch, Mernagh, Yue and Palm; plus an Australian Commonwealth Supported Place and the Kerry and John Lovering scholarship to Pal