An empirical H2O solubility model for peralkaline rhyolitic melts

Allabar, Anja; Marks, Patricia Louisa; Eul, Dennis; Nowak, Marcus

doi:10.1007/s00410-022-01915-8

Cited by 3 publications

(9 citation statements)

References 39 publications

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“…Based on the EMPA data, the hybrid melt evolved to a dacitic composition, comparable to that of the Unzen dacite (Chen et al, 1993). Sato et al (1999) showed experimentally that 5.7 wt % H 2 O is soluble at 98 MPa and 1303 K, which is quite similar to our final experimental conditions of 100 MPa and 1348 K. Therefore, at P f the solubility of H 2 O in dacitic melt is much higher than in rhyolitic or basaltic melt, with 3.8 wt % and 3.3 wt %, respectively (Allabar et al, 2022;Berndt et al, 2002). This may explain the increase of ∼ 1 wt % H 2 O in each hybrid c H 2 O t profile.…”

Section: The C H 2 O T Profilessupporting

confidence: 80%

“…1). Under hydration conditions of 200 MPa and 1523 K, calculated H 2 O solubilities are 5.7 wt % H 2 O in the mildly peralkaline rhyolitic melt (Allabar et al, 2022) and 4.8 wt % H 2 O in the basaltic melt (Berndt et al, 2002). Consequently, the excess water added to the capsules ensured complete H 2 O saturation for both melt compositions.…”

Section: Hydration Experimentsmentioning

confidence: 98%

“…The c H 2 O t of the hydrated rhyolite samples was determined by measuring the peak heights of the ab- Allabar et al (2022) and in basaltic melt according to Berndt et al (2002). Sample Experiment P Eq , P i 0.17 0.230 X P Eq is equilibration/hydration pressure; P i is initial pressure prior to decompression; P f is final pressure; T Eq is equilibration/hydration temperature; Tr is run temperature during experiment; t Eq is equilibration/experiment duration; dP / dt is decompression rate; NQ: 16 K s −1 ; MQ: 44 K s −1 ; H 2 O sol.…”

Section: Ftir Spectroscopymentioning

confidence: 99%

“…A linear-tangential baseline correction (Behrens et al, 1996) was applied by setting points at ∼ 5400 and ∼ 4700 cm −1 , ensuring that the straight line is tangential to the spectrum between the absorption bands at 5210 and 4470 cm −1 . For evaluation, the linear molar absorption coefficients (ε Allabar et al (2022) were used. Because near-infrared measurements in transmission mode were impossible for basaltic glasses, the total H 2 O concentration was determined gravimetrically (c H 2 O g ) (Table 2).…”

Section: Ftir Spectroscopymentioning

confidence: 99%

“…Straight lines were fitted through the minima on both sides of the NIR combination bands related to H 2 O m and OH to evaluate the peak heights. As this baseline correction systematically underestimates the OH absorption (Ohlhorst et al, 2001), standard samples of rhyolitic composition (AOQ2, from Allabar et al, 2022) were baseline-corrected according to Behrens et al (1996) and Ohlhorst et al (2001) for comparison. The differences in c H 2 O t , calculated with the ε H 2 O m and ε OH of both corrections, are within the error in the c H 2 O t of each sample, except for sample Std_2_7 with the highest H 2 O content of 6.9 wt % (Table S1).…”

Section: Bimodal Reference Experimentsmentioning

confidence: 99%

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H₂O degassing triggered by alkali depletion in bimodal magma injection processes – a new experimental approach

Marks¹,

Allabar²,

Nowak³

2023

Eur. J. Mineral.

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Abstract. The injection of mafic magma into a hydrous felsic magma chamber is a potential trigger mechanism for bimodal explosive volcanism. As H2O is the most abundant volatile component in magmas, the interaction and the degassing behavior of mildly peralkaline hydrous rhyolitic melt in contact with hydrous basaltic melt were investigated by decompression experiments. Preparatory hydration experiments and bimodal magma decompression experiments, as well as reference experiments, were carried out in an internally heated argon pressure vessel. Pre-hydrated rhyolite and basalt cylinders were perfectly contacted together in a precious-metal capsule, heated to 1348 K at 210 MPa, and thermally equilibrated for 10 min. The initial sample properties were determined by a bimodal reference experiment, quenched immediately after equilibration. To simulate the magma ascent, three bimodal samples and a decompression experiment with two contacted rhyolite cylinders for testing the experimental setup were decompressed with 0.17 or 1.7 MPa s−1 to the final pressure of 100 MPa and then quenched. All decompression experiments resulted in vesiculated samples. The H2O vesicles observed in the decompressed sample of the monomodal rhyolite–rhyolite reference experiment are homogeneously distributed throughout the sample. The former interface between the contacted glass cylinders is invisible after decompression and quench. This reference experiment proves that the two-cylinder design does not influence the degassing behavior of the hydrous melt, e.g., an increased formation of vesicles at possible nucleation sites at the contact plane of the cylinders. The undecompressed bimodal rhyolite–basalt sample shows crystal-free rhyolitic glass, whereas 3 µm sized idiomorphic magnetite crystals coexist with glass in the basaltic part of the sample. Within the 10 min run time, a ∼ 300 µm wide hybrid composition zone developed between the hydrous rhyolitic and basaltic endmembers, caused by diffusion-induced mixing processes. Decompression and quenching of the bimodal melts resulted in vesiculated glass samples. A ∼ 100 µm wide zone of alkali-depleted rhyolitic glass as part of the ∼ 300–560 µm wide hybrid zone is covered with an enhanced number of H2O vesicles compared to the pristine rhyolitic and basaltic glass volumes. We suggest that this enhanced vesiculated zone forms by a rapid diffusional loss of alkalis from the mildly peralkaline rhyolitic melt into the basaltic melt of the sample. The reduced alkali concentration significantly reduces the H2O solubility of the rhyolitic melt. This process enhances the H2O supersaturation necessary for vesicle formation during decompression. In summary, the new findings imply that convective magma ascent driven by the injection of hot basaltic magma into a hydrous peralkaline rhyolitic melt reservoir leads to enhanced H2O vesicle formation near the melt interface and thus to efficient degassing. This in turn can accelerate buoyancy-driven magma ascent and mingling and mixing processes that induce further degassing and potentially trigger explosive volcanic eruptions.

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Section: The C H 2 O T Profilessupporting

confidence: 80%

Section: Hydration Experimentsmentioning

confidence: 98%

Section: Ftir Spectroscopymentioning

confidence: 99%

Section: Ftir Spectroscopymentioning

confidence: 99%

Section: Bimodal Reference Experimentsmentioning

confidence: 99%

See 3 more Smart Citations

H₂O degassing triggered by alkali depletion in bimodal magma injection processes – a new experimental approach

Marks¹,

Allabar²,

Nowak³

2023

Eur. J. Mineral.

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Magma storage conditions beneath a peralkaline caldera in the Main Ethiopian Rift

Colby,

Pyle,

Fontijn

et al. 2024

Journal of Volcanology and Geothermal Research

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Extreme Differentiation along Multiple Liquid Lines of Descent in Strongly Peralkaline Magma Series at Pantelleria (Italy)

White

Macdonald

Bagiński

et al. 2023

Journal of Petrology

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The liquid line of descent (LLOD) from trachyte to pantellerite is controlled primarily by fractional crystallization of alkali feldspar, with whole-rock compositions following a fractionation path along the “thermal valley” in the peralkaline haplogranite system Qz-Ab-Or-Ac-Ns and terminating at a minimum on the feldspar-quartz cotectic. Although whole-rock compositions for different pantelleritic suites follow nearly identical paths in a Qz-Ab-Or projection that terminate near the experimental minimum (Qz40.5Or34.5Ab25 at 100 MPa, projected from Ac-Ns), matrix glass from samples with near-minimum compositions record extreme differentiation and form a “cotectic delta” beyond the terminus of the “thermal valley”. Although each glass trend shows a continuing increase in Zr to >3000 μg/g, the most evolved compositions in each suite differ in peralkalinity (mol [Na + K] / Al) and in the proportions of FeOT, Qz, Ab, Or, and other components, which are related to subtle variations in the mafic phases controlled mainly by differences in oxygen fugacity (fO2) and pressure (P). To determine the controls over mafic mineral crystallization in pantelleritic magmas and the various paths these suites take beyond the apparent (whole-rock) minimum, amphibole-phyric suites from the ~159 ka Cala dell’Altura and Cala Gadir volcanic centers and the ~8-10 ka Cuddia Mida volcanic center on Pantelleria have been analyzed and compared with each other and with the well-characterized and amphibole-free, compositionally zoned Green Tuff, the ~46 ka caldera-forming ignimbrite of the Cinque Denti caldera. Differences between the extended fractionation trends may be ultimately attributed to variations in oxygen fugacity, depth of emplacement, and water saturation. Shallower (lower pressure) magma reservoirs such as the one for the Green Tuff are water-saturated and undergo degassing, which leads to an increase in relative oxygen fugacity. Deeper (higher pressure) magma reservoirs remain water-undersaturated and retain water in the melt, which both maintains lower relative oxygen fugacities and enables the crystallization of amphibole. Amphibole formation appears to require melt water contents >4 wt%, low oxygen fugacity (<ΔNNO-1.5), and low temperatures (<700°C), although fluorine may stabilize it at higher temperatures in some rocks.

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An empirical H2O solubility model for peralkaline rhyolitic melts

Cited by 3 publications

References 39 publications

H₂O degassing triggered by alkali depletion in bimodal magma injection processes – a new experimental approach

H₂O degassing triggered by alkali depletion in bimodal magma injection processes – a new experimental approach

Magma storage conditions beneath a peralkaline caldera in the Main Ethiopian Rift

Extreme Differentiation along Multiple Liquid Lines of Descent in Strongly Peralkaline Magma Series at Pantelleria (Italy)

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An empirical H2O solubility model for peralkaline rhyolitic melts

Cited by 3 publications

References 39 publications

H2O degassing triggered by alkali depletion in bimodal magma injection processes – a new experimental approach

H2O degassing triggered by alkali depletion in bimodal magma injection processes – a new experimental approach

Magma storage conditions beneath a peralkaline caldera in the Main Ethiopian Rift

Extreme Differentiation along Multiple Liquid Lines of Descent in Strongly Peralkaline Magma Series at Pantelleria (Italy)

Contact Info

Product

Resources

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H₂O degassing triggered by alkali depletion in bimodal magma injection processes – a new experimental approach

H₂O degassing triggered by alkali depletion in bimodal magma injection processes – a new experimental approach