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1. Depths in a Day—a New Era of Rapid-Response Raman-Based Barometry Using Fluid Inclusions

   https://doi.org/10.1093/petrology/egae119  

   DeVitre, Charlotte L; Wieser, Penny E; Bearden, Alexander T; Richie, Araela; Rangel, Berenise; Gleeson, Matthew_L M; Grimsich, John; Lynn, Kendra J; Downs, Drew T; Deligne, Natalia I; et al (November 2024, Journal of Petrology)

   Abstract Rapid-response petrological monitoring is a major advance for volcano observatories, allowing them to build and validate models of plumbing systems that supply eruptions in near-real time. The depth of magma storage has recently been identified as high-priority information for volcanic observatories, yet this information is not currently obtainable via petrological monitoring methods on timescales relevant to eruption response. Fluid inclusion barometry (using micro-thermometry or Raman spectroscopy) is a well-established petrological method to estimate magma storage depths and has been proposed to have potential as a rapid-response monitoring tool, although this has not been formally demonstrated. To address this deficiency, we performed a near-real-time rapid-response simulation for the September 2023 eruption of Kīlauea, Hawaiʻi. We show that Raman-based fluid inclusion barometry can robustly determine reservoir depths within a day of receiving samples—a transformative timescale that has not previously been achieved by petrological methods. Fluid inclusion barometry using micro-thermometric techniques has typically been limited to systems with relatively deep magma storage (>0.4 g/cm3 i.e.  > 7 km) where measurements of CO2 density are easy and accurate because the CO2 fluid homogenizes into the liquid phase. Improvements of the accuracy of Raman spectroscopy measurements of fluids with low CO2 density over the past couple of decades has enabled measurements of fluid inclusions from shallower magmatic systems. However, one caveat of examining shallower systems is that the fraction of H2O in the fluid may be too high to reliably convert CO2 density to pressure. To test the global applicability of rapid response fluid inclusion barometry, we compiled a global melt inclusion dataset (>4000 samples) and calculate the fluid composition at the point of vapor saturation ($${\mathrm{X}}_{{\mathrm{H}}_2\mathrm{O}}$$). We show that fluid inclusions in crystal hosts from mafic compositions (<57 wt. % SiO2)—likely representative of magmas recharging many volcanic systems worldwide—trap fluids with $${\mathrm{X}}_{{\mathrm{H}}_2\mathrm{O}}$$ low enough to make fluid inclusion barometry useful at many of the world’s most active and hazardous mafic volcanic systems (e.g. Iceland, Hawaiʻi, Galápagos Islands, East African Rift, Réunion, Canary Islands, Azores, Cabo Verde). 

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2. Constraints on deep, CO2-rich degassing at arc volcanoes from solubility experiments on hydrous basaltic andesite of Pavlof Volcano, Alaska Peninsula, at 300 to 1200 MPa

   https://doi.org/10.2138/am-2021-7531  

   Mangan, Margaret T.; Sisson, Thomas W.; Hankins, W. Ben; Shimizu, Nobumichi; Vennemann, Torsten (May 2021, American Mineralogist)

   Abstract The solubility of CO2 in hydrous basaltic andesite was examined in fO2-controlled experiments at a temperature of 1125 °C and pressures between 310–1200 MPa. Concentrations of dissolved H2O and CO2 in experimental glasses were determined by ion microprobe calibrated on a subset of run glasses analyzed by high-temperature vacuum manometry. Assuming that the solubility of H2O in mafic melt is relatively well known, estimates of XH2Ofluid and PH2Ofluid in the saturating fluid were modeled, and by difference, values for XCO2fluid and PCO2fluid were obtained (XCO2 ~0.5–0.9); fCO2 could be then calculated from the fluid composition, temperature, and pressure. Dissolved H2O over a range of 2.3–5.5 wt% had no unequivocal influence on the dissolution of CO2 at the pressures and fluid compositions examined. For these H2O concentrations, dissolved CO2 increases with fCO2 following an empirical power-law relation: dissolved CO2 (ppmw) = 14.9−3.5+4.5[fCO2 (MPa)]0.7±0.03. The highest-pressure results plot farthest from this equation but are within its 1 standard-error uncertainty envelope. We compare our experimental data with three recent CO2-H2O solubility models: Papale et al. (2006); Iacono-Marziano et al. (2012); and Ghiorso and Gualda (2015). The Papale et al. (2006) and Iacono-Marizano et al. (2012) models give similar results, both over-predicting the solubility of CO2 in a melt of the Pavlof basaltic andesite composition across the fCO2 range, whereas the Ghiorso and Gualda (2015) model under-predicts CO2 solubility. All three solubility models would indicate a strong enhancement of CO2 solubility with increasing dissolved H2O not apparent in our results. We also examine our results in the context of previous high-pressure CO2 solubility experiments on basaltic melts. Dissolved CO2 correlates positively with mole fraction (Na+K+Ca)/Al across a compositional spectrum of trachybasalt-alkali basalt-tholeiite-icelandite-basaltic andesite. Shortcomings of current solubility models for a widespread arc magma type indicate that our understanding of degassing in the deep crust and uppermost mantle remains semi-quantitative. Experimental studies systematically varying concentrations of melt components (Mg, Ca, Na, K, Al, Si) may be necessary to identify solubility reactions, quantify their equilibrium constants, and thereby build an accurate and generally applicable solubility model. 

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3. Effects of H2O–CO2 Fluids, Temperature, and Peridotite Fertility on Partial Melting in Mantle Wedges and Generation of Primary Arc Basalts

   https://doi.org/10.1093/petrology/egad047  

   Lara, Michael; Dasgupta, Rajdeep (July 2023, Journal of Petrology)

   Abstract Many lines of evidence from high P–T experiments, thermodynamic models, and natural observations suggest that slab-derived aqueous fluids, which flux mantle wedges contain variable amounts of dissolved carbon. However, constraints on the effects of H2O–CO2 fluids on mantle melting, particularly at mantle wedge P–T conditions, are limited. Here, we present new piston cylinder experiments on fertile and depleted peridotite compositions with 3.5 wt.% H2O and XCO2 [= molar CO2 / (CO2 + H2O)] of 0.04–0.17. Experiments were performed at 2–3 GPa and 1350°C to assess how temperature, peridotite fertility, and XCO2 of slab-derived fluid affects partial melting in mantle wedges. All experiments produce olivine + orthopyroxene +7 to 41 wt.% partial melt. Our new data, along with previous lower temperature data, show that as mantle wedge temperature increases, primary melts become richer in SiO2, FeO*, and MgO and poorer CaO, Al2O3, and alkalis when influenced by H2O–CO2 fluids. At constant P–T and bulk H2O content, the extent of melting in the mantle wedge is largely controlled by peridotite fertility and XCO2 of slab-fluid. High XCO2 depleted compositions generate ~7 wt.% melt, whereas, at identical P–T, low XCO2 fertile compositions generate ~30 to 40 wt.% melt. Additionally, peridotite fertility and XCO2 have significant effects on peridotite partial melt compositions. At a constant P–T–XCO2, fertile peridotites generate melts richer in CaO and Al2O3 and poorer in SiO2, MgO + FeO, and alkalis. Similar to previous experimental studies, at a constant P–T fertility condition, as XCO2 increases, SiO2 and CaO of melts systematically decrease and increase, respectively. Such distinctive effects of oxidized form of dissolved carbon on peridotite partial melt compositions are not observed if the carbon-bearing fluid is reduced, such as CH4-bearing. Considering the large effect of XCO2 on melt SiO2 and CaO concentrations and the relatively oxidized nature of arc magmas, we compare the SiO2/CaO of our experimental melts and melts from previous peridotite + H2O ± CO2 studies to the SiO2/CaO systematics of primitive arc basalts and ultra-calcic, silica-undersaturated arc melt inclusions. From this comparison, we demonstrate that across most P–T–fertility conditions predicted for mantle wedges, partial melts from bulk compositions with XCO2 ≥ 0.11 have lower SiO2/CaO than all primitive arc melts found globally, even when correcting for olivine fractionation, whereas partial melts from bulk compositions with XCO2 = 0.04 overlap the lower end of the SiO2/CaO field defined by natural data. These results suggest that the upper XCO2 limit of slab-fluids influencing primary arc magma formation is 0.04 < XCO2 < 0.11, and this upper limit is likely to apply globally. Lastly, we show that the anomalous SiO2/CaO and CaO/Al2O3 signatures observed in ultra-calcic arc melt inclusions can be reproduced by partial melting of either CO2-bearing hydrous fertile and depleted peridotites with 0 < XCO2 < 0.11 at 2–3 GPa, or from nominally CO2-free hydrous fertile peridotites at P > 3 GPa. 

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4. CO2 and H2O in Plagioclase-Hosted Melt Inclusions from Ocean-Ridge Lavas: An Indicator of Crystallization in the Lower Oceanic Crust

   • Kotash, A.; • Tepley, F. J.; • Nielsen, R.; • Bodnar, R. J. (December 2019, American Geophysical Union, Fall Meeting 2019, abstract #V13C-0172)

   Interpretation of erupted products we observe on the seafloor requires that we understand the petrogenesis of melts in the oceanic crust and where crystallization initially takes place. Our work focuses on estimating depth of crystallization of the plagioclase megacrysts using CO2 and H2O concentrations from plagioclase ultraphyric basalts (PUBs). Samples were analyzed from the Lucky Strike segment on the Mid-Atlantic Ridge and from three locations on the Juan de Fuca Ridge (West Valley, Endeavor Segment, and Axial Segment). Melt inclusions were re-homogenized to remove the effects of post-entrapment crystallization. The CO2 in the vapor bubbles present in the melt inclusions were analyzed at Virginia Tech using Raman spectroscopy, and associated glassy melt inclusions were analyzed at WHOI using the ion microprobe for CO2 and H2O. Vapor-saturation pressures calculated from these volatiles stored in melt inclusions and vapor bubbles range from 359-3994 bars, corresponding to depths of 1.0-11.4 km below the sea floor. The proportion of CO2 partitioned in the bubbles range from 11-98%. In summary, about 14% of the melt inclusions from Lucky Strike record crystallization depths of 3-4 km, consistent with the depth of the seismically imaged melt lens, whereas ~55% of melt inclusions crystallized at depths >4 km with a maximum at 9.8 km. These data are similar to depths of formation determined through olivine-hosted melt inclusions from the same segment (Wanless et al., 2015), although a greater portion of plagioclase-hosted melt inclusions record crystallization below the melt lens. At the Juan de Fuca ridge, ~24% of the melt inclusions record crystallization depths of 2-3 km, consistent with a seismically imaged mid-crustal magma chamber at the Endeavor Segment, while an additional ~62% crystallize at depths >3 km with a maximum at 11.4 km. This suggests that while crystallization can be focused within the melt lenses and magma chambers at these ridge localities, a significant and greater proportion of the megacrysts were sampled from the lower crust or upper mantle. 

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5. DensityX: A program for calculating the densities of magmatic liquids up to 1,627°C and 30 kbar

   https://doi.org/10.30909/vol.02.01.0110  

   Iacovino, Kayla; Till, Christy B (February 2019, Volcanica)

   Here we present a standalone program, DensityX, to calculate the densities of hydrous silicate melts (1000s of samples in a single model run) given pressures, temperatures, and major oxide compositions in wt.% in the 10-component system SiO2-TiO2-Al2O3-Fe2O3-FeO-MgO-CaO-Na2O-K2O-H2O. We use DensityX to analyze over 3,000 melt inclusions over a wide compositional range to visualize the distribution of natural silicate liquid densities in the Earth’s crust. The program is open-source, written in Python, and can be accessed and run via an online interface through a web browser at https://densityx.herokuapp.com or by downloading and running the code from a github repository. A companion Excel spreadsheet can also be used to run density calculations identical to those in the Python script but only for one sample at a time. In another example application, we demonstrate how DensityX can be used to constrain density-driven convective cycling in the phonolitic lava lake of Erebus volcano, Antarctica. 

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