Articles de revues : « Volatile in magmas »

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Degruyter, Wim, Andrea Parmigiani, Christian Huber et Olivier Bachmann. « How do volatiles escape their shallow magmatic hearth ? » Philosophical Transactions of the Royal Society A : Mathematical, Physical and Engineering Sciences 377, n^o 2139 (7 janvier 2019) : 20180017. http://dx.doi.org/10.1098/rsta.2018.0017.

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Only a small fraction (approx. 1–20%) of magmas generated in the mantle erupt at the surface. While volcanic eruptions are typically considered as the main exhaust pipes for volatile elements to escape into the atmosphere, the contribution of magma reservoirs crystallizing in the crust is likely to dominate the volatile transfer from depth to the surface. Here, we use multiscale physical modelling to identify and quantify the main mechanisms of gas escape from crystallizing magma bodies. We show that most of the outgassing occurs at intermediate to high crystal fraction, when the system has reached a mature mush state. It is particularly true for shallow volatile-rich systems that tend to exsolve volatiles through second boiling, leading to efficient construction of gas channels as soon as the crystallinity reaches approximately 40–50 vol.%. We, therefore, argue that estimates of volatile budgets based on volcanic activity may be misleading because they tend to significantly underestimate the magmatic volatile flux and can provide biased volatile compositions. Recognition of the compositional signature and volumetric dominance of intrusive outgassing is, therefore, necessary to build robust models of volatile recycling between the mantle and the surface. This article is part of the Theo Murphy meeting issue ‘Magma reservoir architecture and dynamics’.

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Perinelli, Cristina, Silvio Mollo, Mario Gaeta, Serena De Cristofaro, Danilo Palladino et Piergiorgio Scarlato. « Impulsive Supply of Volatile-Rich Magmas in the Shallow Plumbing System of Mt. Etna Volcano ». Minerals 8, n^o 11 (25 octobre 2018) : 482. http://dx.doi.org/10.3390/min8110482.

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Magma dynamics at Mt. Etna volcano are frequently recognized as the result of complex crystallization regimes that, at shallow crustal levels, unexpectedly change from H2O-undersaturated to H2O-saturated conditions, due to the impulsive and irregular arrival of volatile-rich magmas from mantle depths. On this basis, we have performed hydrous crystallization experiments for a quantitative understanding of the role of H2O in the differentiation of deep-seated trachybasaltic magmas at the key pressure of the Moho transition zone. For H2O = 2.1–3.2 wt %, the original trachybasaltic composition shifts towards phonotephritic magmas never erupted during the entire volcanic activity of Mt. Etna. Conversely, for H2O = 3.8–8.2 wt %, the obtained trachybasalts and basaltic trachyandesites reproduce most of the pre-historic and historic eruptions. The comparison with previous low pressure experimental data and natural compositions from Mt. Etna provides explanation for (1) the abundant release of H2O throughout the plumbing system of the volcano during impulsive ascent of deep-seated magmas; (2) the upward acceleration of magmas feeding gas-dominated, sustained explosive eruptions; (3) the physicochemical changes of gas-fluxed magmas ponding at shallow crustal levels; and (4) the huge gas emissions measured at the summit craters and flank vents which result in a persistent volcanic gas plume.

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Rasmussen, Daniel J., Terry A. Plank, Diana C. Roman et Mindy M. Zimmer. « Magmatic water content controls the pre-eruptive depth of arc magmas ». Science 375, n^o 6585 (11 mars 2022) : 1169–72. http://dx.doi.org/10.1126/science.abm5174.

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Vanguard efforts in forecasting volcanic eruptions are turning to physics-based models, which require quantitative estimates of magma conditions during pre-eruptive storage. Below active arc volcanoes, observed magma storage depths vary widely (~0 to 20 kilometers) and are commonly assumed to represent levels of neutral buoyancy. Here we show that geophysically observed magma depths (6 ± 3 kilometers) are greater than depths of neutral buoyancy, ruling out this commonly assumed control. Observed depths are instead consistent with predicted depths of water degassing. Intrinsically wetter magmas degas water and crystallize deeper than dry magmas, resulting in viscosity increases that lead to deeper stalling of ascending magma. The water–depth relationship provides a critical constraint for forecasting models by connecting depth of eruption initiation to its volatile fuel.

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Nizametdinov, I. R., D. V. Kuzmin, S. Z. Smirnov, A. V. Rybin et I. Yu Kulakov. « Water in parental basaltic magmasof the Menshiy Brat volcano (Iturup Island, Kurile islands) ». Доклады Академии наук 486, n^o 1 (10 mai 2019) : 93–97. http://dx.doi.org/10.31857/s0869-5652486193-97.

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The paper presents study of the liquidus assemblage of olivine and spinel in high-magnesian basalts (MgO up to 10 mas. %) of the Menshiy Brat volcano (Iturup Island). It was possible to reconstruct the water content and evolution of volatile components in the primary parental magmas that took part in the formation of the Medvezhya Caldera, Iturup Islands. It is shown that the initial water content in the primary melts could reach 5 mas. % with oxygen fugacity corresponding to oxygen buffer NNO + 0.4 log. units. The evolution of magmas involved continuous degassing while magma rises to the surface. The water-rich fluid, which is constantly separated by evolving magma, could play a significant role in the formation of large siliceous magma chambers, which participated in catastrophic caldera eruptions.

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Boudreau, Alan E. « The Stillwater Complex, Montana – Overview and the significance of volatiles ». Mineralogical Magazine 80, n^o 4 (juin 2016) : 585–637. http://dx.doi.org/10.1180/minmag.2016.080.063.

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AbstractThe geology of the 2.7 Ga Stillwater Complex in South-Central Montana is reviewed with a focus on the role of volatiles in locally modifying both the crystallization sequence of the evolving parent magma and the initially precipitated solid assemblages to favour olivine ± chromite. A secondary origin for these two minerals is particularly probable for the olivine-bearing rocks of the Banded series and, at a minimum, also increasing their modal abundance in the Peridotite zone of the Ultramafic series. Direct evidence for volatiles includes the presence of high-temperature fluid inclusions in pegmatoids and hydrous melt inclusions (now crystallized) in chromite and olivine from both the Ultramafic and the Banded series rocks. Indirect evidence includes the boninitic character of the parent magma, the presence of volatile-bearing minerals including high-temperature carbonates, rock textures, and Cl / F variations in apatite. Mechanisms which favour the formation of olivine (± chromite) over pyroxene include volatile phase boundary shifts induced by added H2O, incongruent melting of pyroxene by hydration of a partly-molten mush, and the near- to sub-solidus replacement of pyroxene by olivine and chromite by silica-undersaturated fluids. These mechanisms cast doubt that magmas with different liquid lines of descent were involved in the crystallization of the Stillwater Complex. A dry Stillwater magma would have been mineralogically and modally much less varied and lacking in high-grade platinum-group element and chromium deposits.

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Russell, J. Kelly, R. Stephen J. Sparks et Janine L. Kavanagh. « Kimberlite Volcanology : Transport, Ascent, and Eruption ». Elements 15, n^o 6 (1 décembre 2019) : 405–10. http://dx.doi.org/10.2138/gselements.15.6.405.

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Kimberlite rocks and deposits are the eruption products of volatile-rich, silica-poor ultrabasic magmas that originate as small-degree mantle melts at depths in excess of 200 km. Many kimberlites are emplaced as subsurface cylindrical-to-conical pipes and associated sills and dykes. Surficial volcanic deposits of kimberlite are rare. Although kimberlite magmas have distinctive chemical and physical properties, their eruption styles, intensities and durations are similar to conventional volcanoes. Rates of magma ascent and transport through the cratonic lithosphere are informed by mantle cargo entrained by kimberlite, by the geometries of kimberlite dykes exposed in diamond mines, and by laboratory-based studies of dyke mechanics. Outstanding questions concern the mechanisms that trigger and control the rates of kimberlite magmatism.

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Holloway, John R., et Sigurdur Jakobsson. « Volatile solubilities in magmas : Transport of volatiles from mantles to planet surfaces ». Journal of Geophysical Research : Solid Earth 91, B4 (30 mars 1986) : 505–8. http://dx.doi.org/10.1029/jb091ib04p0d505.

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Martin, Audrey M., Etienne Médard, Kevin Righter et Antonio Lanzirotti. « Intraplate mantle oxidation by volatile-rich silicic magmas ». Lithos 292-293 (novembre 2017) : 320–33. http://dx.doi.org/10.1016/j.lithos.2017.09.002.

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Macdonald, R., et B. Bagiński. « The central Kenya peralkaline province : a unique assemblage of magmatic systems ». Mineralogical Magazine 73, n^o 1 (février 2009) : 1–16. http://dx.doi.org/10.1180/minmag.2009.073.1.1.

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The review focuses on the evolution of five contiguous peralkaline salic complexes in the south-central Kenya Rift Valley, stressing new developments of general significance to peralkaline magmatism. The complexes have evolved dominantly by combinations of fractional crystallization and magma mixing; volatile-melt interactions, remobilization of plutonic rocks and crystal mushes, and carbonate-silicate liquid immiscibility have been additional petrogenetic processes. Geochemical and experimental studies have shown that pantelleritic magmas can be generated by fractional crystallization of trachyte and high-silica rhyolite. Melts of comenditic composition were also formed by fractionation of trachyte but also locally by partial meltingof syenites. Studies of apparent partition coefficients have provided some of the first data on element distribution between phenocrysts and peralkaline silicic melts. Compositional zonation has been ubiquitous in the complexes, probably a result of the very low viscosity of the magmas.

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Madon, Baptiste, Lucie Mathieu et Jeffrey H. Marsh. « Oxygen Fugacity and Volatile Content of Syntectonic Magmatism in the Neoarchean Abitibi Greenstone Belt, Superior Province, Canada ». Minerals 10, n^o 11 (28 octobre 2020) : 966. http://dx.doi.org/10.3390/min10110966.

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Neoarchean syntectonic intrusions from the Chibougamau area, northeastern Abitibi Subprovince (greenstone belt), may be genetically related to intrusion related gold mineralization. These magmatic-hydrothermal systems share common features with orogenic gold deposits, such as spatial and temporal association with syntectonic magmatism. Genetic association with magmatism, however, remains controversial for many greenstone belt hosted Au deposits. To precisely identify the link between syntectonic magmas and gold mineralization in the Abitibi Subprovince, major and trace-element compositions of whole rock, zircon, apatite, and amphibole grains were measured for five intrusions in the Chibougamau area; the Anville, Saussure, Chevrillon, Opémisca, and Lac Line Plutons. The selected intrusions are representative of the chemical diversity of synvolcanic (TTG suite) and syntectonic (e.g., sanukitoid, alkaline intrusion) magmatism. Chemical data enable calculation of oxygen fugacity and volatile content, and these parameters were interpreted using data collected by electron microprobe and laser ablation-inductively coupled plasma-mass spectrometry. The zircon and apatite data and associated oxygen fugacity values in magma indicate that the youngest magmas are the most oxidized. Moreover, similar oxygen fugacity and high volatile content for both the Saussure Pluton and the mineralized Lac Line intrusion may indicate a possible prospective mineralized system associated with the syntectonic Saussure intrusion.

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Treloar, Peter J., et Howard Colley. « Variations in F and Cl contents in apatites from magnetite—apatite ores in northern chile, and their ore-genetic implications ». Mineralogical Magazine 60, n^o 399 (avril 1996) : 285–301. http://dx.doi.org/10.1180/minmag.1996.060.399.04.

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AbstractMagnetite—apatite deposits associated with the Atacama Fault Zone of northern Chile are interpreted here, on field criteria, as being the products either of hydrothermal fluids with a strong magmatic signature, or of late-stage Fe-rich magmas mixed with an aqueous fluid. Even in the Chilean iron belt, apatite-rich magnetite deposits are a rarity. Variations in F- and Cl- contents in apatites, strongly zoned with respect to halogens, are indicative of primary variations in fHF and fHCI in the hydrothermal fluids. Small variations in halogen fugacities in the aqueous fluid are capable of buffering large variations in halogen content within apatite crystals in equilibrium with that fluid. The recorded halogen zonation profiles are inconsistent with crystallization of the apatites simply from a volatile-rich, late-stage fractionation Fe-rich magma, or its derived magmatic vapour. It is more likely that they are the result of mineral—fluid buffering with a fluid that represents the mixing of a magmatically-derived aqueous fluid with a meteoric fluid that has variably scavenged Ca and Cl from within the country rocks. The source magma of the former is probably an Fe-P enriched acidic magma, derived by fractionation of primary calc-alkaline basic magmas.

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WYLLIE, PETER J., et IGOR D. RYABCHIKOV. « Volatile Components, Magmas, and Critical Fluids in Upwelling Mantle ». Journal of Petrology 41, n^o 7 (juillet 2000) : 1195–206. http://dx.doi.org/10.1093/petrology/41.7.1195.

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Correale, Alessandra, Vittorio Scribano et Antonio Paonita. « A Volcanological Paradox in a Thin-Section : Large Explosive Eruptions of High-Mg Magmas Explained Through a Vein of Silicate Glass in a Serpentinized Peridotite Xenolith (Hyblean Area, Sicily) ». Geosciences 9, n^o 4 (29 mars 2019) : 150. http://dx.doi.org/10.3390/geosciences9040150.

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Ultramafic magmas (MgO ≥ 18 wt%) are generally thought to be primary mantle melts formed at temperatures in excess of 1600 °C. Volatile contents are expected to be low, and accordingly, high-Mg magmas generally do not yield large explosive eruptions. However, there are important exceptions to low explosivity that require an explanation. Here we show that hydrous (hence, potentially explosive) ultramafic magmas can also form at crustal depths at temperatures even lower than 1000 °C. Such a conclusion arose from the study of a silicate glass vein, ~1 mm in thickness, cross-cutting a mantle-derived harzburgite xenolith from the Valle Guffari nephelinite diatreme (Hyblean area, Sicily). The glass vein postdates a number of serpentine veins already existing in the host harzburgite, thus reasonably excluding that the melt infiltrated in the rock at mantle depths. The glass is highly porous at the sub-micron scale, it also bears vesicles filled by secondary minerals. The distribution of some major elements corresponds to a meimechite composition (MgO = 20.35 wt%; Na2O + K2O < 1 wt%; and TiO2 > 1 wt%). On the other hand, trace element distribution in the vein glass nearly matches the nephelinite juvenile clasts in the xenolith-bearing tuff-breccia. These data strongly support the hypothesis that an upwelling nephelinite melt (MgO = 7–9 wt%; 1100 ≤ T ≤ 1250 °C) intersected fractured serpentinites (T ≤ 500 °C) buried in the aged oceanic crust. The consequent dehydroxilization of the serpentine minerals gave rise to a supercritical aqueous fluid, bearing finely dispersed, hydrated cationic complexes such as [Mg2+(H2O)n]. The high-Mg, hydrothermal solution "flushed" into the nephelinite magma producing an ultramafic, hydrous (hence, potentially explosive), hybrid magma. This hypothesis explains the volcanological paradox of large explosive eruptions produced by ultramafic magmas.

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LENSKY, N. G., V. LYAKHOVSKY et O. NAVON. « Expansion dynamics of volatile-supersaturated liquids and bulk viscosity of bubbly magmas ». Journal of Fluid Mechanics 460 (10 juin 2002) : 39–56. http://dx.doi.org/10.1017/s0022112002008194.

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We derive expressions for the bulk viscosity of suspension of gas bubbles in an incompressible Newtonian liquid that exsolves volatiles. The suspension is modelled as close packed spherical cells and is represented by a single cell (‘cell model’). A cell, consisting of a gas bubble centred in a spherical shell of a volatile-bearing liquid, is subjected to decompression that is applied at the cell boundary, and the resulting dilatational boundary motion and driving pressure are obtained. The dilatational motion and the driving pressure are used to define the bulk viscosity of the cell, as if it were composed of a homogeneously compressible fluid. By definition, the bulk viscosity is the relation between changes of the driving pressure and changes in the resulting expansion strain rate. The bulk viscosity of the suspension is obtained in terms of two-phase parameters, i.e. bubble radius, gas pressure and the properties of the incompressible continuous liquid phase. The resulting bulk viscosity is highly nonlinear. At the beginning of the expansion process, when gas exsolution is efficient, the expansion rate grows exponentially while the driving pressure decreases slightly, which means that the bulk viscosity is formally negative. This negative value reflects the release of the energy stored in the supersaturated liquid and its transfer to mechanical work during exsolution. Later, when bubbles are large and the gas influx decreases significantly, the strain rate decelerates and the bulk viscosity becomes positive as expected in a dissipative system. We demonstrate that amplification of seismic waves travelling through a volcanic conduit filled with a volatile saturated magma may be attributed to the negative bulk viscosity of the compressible magma. Amplification of an expansion wave may, at some level in the conduit, damage the conduit walls and initiate the opening of a new pathway for magma eruption. We also consider the energy related to positive and negative bulk viscosities.

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Yao, Zhuosen, James E. Mungall et Kezhang Qin. « A Preliminary Model for the Migration of Sulfide Droplets in a Magmatic Conduit and the Significance of Volatiles ». Journal of Petrology 60, n^o 12 (1 décembre 2019) : 2281–316. http://dx.doi.org/10.1093/petrology/egaa005.

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Abstract A close relationship between Ni–Cu–(PGE) sulfide deposits and magmatic conduit systems has been widely accepted, but our present understanding still rests on empirical inductions that sulfide liquids are entrained during magma ascent and aggregated at hydrodynamic traps such as the opening of a conduit into a larger magma body. In this contribution, a preliminary quantitative model for the dynamics of mm-scale sulfide droplets in a vertical magmatic conduit is developed, examining such limiting parameters as the size, transport velocity and the magmas’ maximum carrying capacity for sulfide droplets. Addition of numerous dense sulfide droplets significantly reduces magma buoyancy and rapidly increases the bulk viscosity, and the resulting pressure gradient in the propagating conduit dyke restricts the maximum volume fraction of droplets that can be carried by ascending magma. For sulfide droplets alone, the maximum carrying capacity is low, but it will be improved dramatically by the addition of volatiles which reduces the density and viscosity of silicate melt. Potential volatile degassing during decompression further facilitates sulfide entrainment by reducing bulk magma density, and the formation of buoyant compound vapour-sulfide liquid bubble drops also greatly enhances the carrying capacity. The breakdown of compound drops by detachment of parts of the vapour bubble or sulfide droplet may occur at low pressure, which liberates sulfide liquids from rising compound drops, potentially to collect in traps in the conduit system. When sulfide-laden magma flows through a widening conduit, many droplets can be captured by the re-circulation flow just downstream of the expanding section, followed by sulfide liquid accumulation and enhanced chemical interaction via diffusive exchange with the recirculating magma, potentially resulting in an economic, high-tonnage ore body. We apply our models to the emplacement of sulfide-rich magmatic suspensions at Noril’sk and show that the disseminated mineralization in intrusions could have formed when magmas carrying re-suspended sulfide liquid entrained from pre-existing sulfide accumulations in the conduit system reached their limiting sulfide carrying capacity as dictated by buoyancy and were deflected into blind sills flanking the principal conduit for flood basalt volcanism.

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Wade, Jennifer A., Terry Plank, William G. Melson, Gerardo J. Soto et Erik H. Hauri. « The volatile content of magmas from Arenal volcano, Costa Rica ». Journal of Volcanology and Geothermal Research 157, n^o 1-3 (septembre 2006) : 94–120. http://dx.doi.org/10.1016/j.jvolgeores.2006.03.045.

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Lucic, Gregor, Anne-Sophie Berg et John Stix. « Water-rich and volatile-undersaturated magmas at Hekla volcano, Iceland ». Geochemistry, Geophysics, Geosystems 17, n^o 8 (août 2016) : 3111–30. http://dx.doi.org/10.1002/2016gc006336.

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Słaby, E., K. Gros, H. J. Förster, A. Wudarska, Ł. Birski, M. Hamada, J. Götze et al. « Mineral–fluid interactions in the late Archean Closepet granite batholith, Dharwar Craton, southern India ». Geological Society, London, Special Publications 489, n^o 1 (8 janvier 2019) : 293–314. http://dx.doi.org/10.1144/sp489-2019-287.

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AbstractThe chemical composition of different rocks as well as volatile-bearing and volatile-free minerals has been used to assess the presence of fluids in the Closepet batholith and to estimate the intensity of the fluid–rock interactions. The data were processed using polytopic vector analysis (PVA). Additional data include measurements of water content in the structure of volatile-free minerals and an examination of growth textures. The composition of mineral domains indicated formation/transformation processes with common fluid–mineral interactions. In general, the results suggested that the processes occurred in a ternary system. Two end-members were likely magmas and the third was enriched in fluids. In contrast, analysis of the apatite domains indicated that they likely formed/transformed in a more complex, four-component system. This system was fluid-rich and included hybrid magma with a large mafic component. PVA implies that the fluids do not appear to come from one source, given their close affinity and partial association with mantle-derived fluids. A dynamic tectonic setting promoting heat influx and redistribution, and interaction of fluids suggests that the formation/transformation processes of minerals and rocks occurred in a hot-spot like environment.

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Cicconi, Maria Rita, Charles Le Losq, Roberto Moretti et Daniel R. Neuville. « Magmas are the Largest Repositories and Carriers of Earth’s Redox Processes ». Elements 16, n^o 3 (1 juin 2020) : 173–78. http://dx.doi.org/10.2138/gselements.16.3.173.

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Magma is the most important chemical transport agent throughout our planet. This paper provides an overview of the interplay between magma redox, major element chemistry, and crystal and volatile content, and of the influence of redox on the factors that drive igneous system dynamics. Given the almost infinite combinations of temperature, pressure, and chemical compositions relevant to igneous petrology, we focus on the concepts and methods that redox geochemistry provides to understand magma formation, ascent, evolution and crystallization. Particular attention is paid to the strong and complex interplay between melt structure and chemistry, and to the influence that redox conditions have on melt properties, crystallization mechanisms and the solubility of volatile components.

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Belenitskaya, G. A. « On the participation of natural salts in alkaline magmatism. Article 3. Genetic aspects of the model of salt-alkaline interactions ». LITHOSPHERE (Russia) 21, n^o 2 (26 avril 2021) : 172–97. http://dx.doi.org/10.24930/1681-9004-2021-21-2-172-197.

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Research subject. An analysis of regional and global geological material characterizing the spatio-temporal relationships between alkaline magmatic and saline complexes allowed the author to propose and justify a new geological-genetic model of alkaline magmatism. This model considers saline complexes, located along the paths of the upward movement of deep magmas, as additional sources of alkaline and volatile components.Materials and methods. Three articles are devoted to the discussion and justification of this model. Two articles were devoted to geological aspects of the problem. The prerequisites and signs of the participation of ancient saline complexes in alkaline magmatism were characterized. It was shown that the presence of saline rocks in the deep zones of the earth's crust along the paths of the upward movement of deep magma flows is a geologically natural and common phenomenon. Natural alkaline-salt associations (spatio-temporal combinations of alkaline and salt objects) were indicated; their tectonic types were distinguished. A global overview of their different-age analogues (neo- and paleogeodynamic) was given.Results and discussion. The collected data made it possible to evaluate older (than magmas) salt-bearing complexes (deeply buried in the substrate) as a possible important and active participant in the ontogenesis of alkaline complexes, to give a positive assessment of the geological aspects of the “magma halocontamination” model and salt-magmatic interactions; to formulate the main geological-genetic provisions of this model.Conclusion. This article focuses on the discussion of the genetic aspects of the proposed model with an assessment of the probable role and significance of various halophilic components in the formation of alkaline magmas and their features. For this purpose, the similarity features in the spatial and quantitative distribution of halophilic and foydaphilic components in salt and alkaline rocks are considered; the probable role of various halophilic components in the formation of alkaline specialization of magmas, in the emergence of a rich set of unusual features of alkaline rocks (material, structural, morphological, etc.) is discussed. The probability of participation of the complex of paragenic (non-salt) members of the halophilic community (dolomites, anhydrites, black shales and associated ore components) in the interaction with hot magma is estimated. A comparative analysis of some basic provisions of the model under consideration with other geological-genetic models of alkaline petrogenesis is performed. The advantages of this model and its predictive capabilities are evaluated. A number of ideas have been proposed by the author for the first time, thus requiring further elucidation.

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Edmonds, Marie, Emily Mason et Olivia Hogg. « Volcanic Outgassing of Volatile Trace Metals ». Annual Review of Earth and Planetary Sciences 50, n^o 1 (31 mai 2022) : 79–98. http://dx.doi.org/10.1146/annurev-earth-070921-062047.

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Volcanoes play a key role in the cycling of volatile metals (e.g., chalcophile elements such as Tl, Pb, and Cu and metalloids such as As, Te, and Se) on our planet. Volatile metals and metalloids are outgassed by active volcanoes, forming particulate volcanic plumes that deliver them in reactive form to the environment, where they may be nutrients (e.g., Cu and Zn) or pollutants (e.g., Hg, As, Pb). Volcanic outgassing rates of these elements compare to those associated with building ore deposits in the crust and to anthropogenic emission rates. There are distinct compositional differences between volcanic plumes in different tectonic settings, related to the enrichment of arc magmas in metals transported in slab fluids, metal speciation, and partitioning between silicate melt, vapor, and magmatic sulfide. Volcanic gases have compositions similar to those of quartz-hosted fluid inclusions found in mineralized granites, albeit with a lower density and salinity. Volatile volcanic metals are transported as soluble aerosols in volcanic plumes and may persist for hundreds of kilometers in the troposphere. Volcanic metal chloride aerosols in tropospheric volcanic plumes at high latitudes are recorded in ice cores. ▪ Volcanoes emit significant fluxes of volatile trace metals such as Cu, Tl, and Pb, as gases and particulates, to the surface environment. ▪ There is a distinct metal compositional fingerprint in volcanic and hydrothermal plumes at subduction and hotspot volcanoes and mid-ocean ridges, controlled by magma and fluid chemistry. ▪ Volcanic gases are the less saline equivalent of the fluids forming economic porphyry deposits of chalcophile metals (e.g., Cu) in the crust. ▪ The metals in tropospheric volcanic plumes may be rained out near the vent, but in dry environments they may persist for thousands of kilometers and be deposited in ice cores.

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Zajacz, Zoltán, Jung Hun Seo, Philip A. Candela, Philip M. Piccoli, Christoph A. Heinrich et Marcel Guillong. « Alkali metals control the release of gold from volatile-rich magmas ». Earth and Planetary Science Letters 297, n^o 1-2 (août 2010) : 50–56. http://dx.doi.org/10.1016/j.epsl.2010.06.002.

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Broska, Igor, et Michal Kubiš. « Accessory minerals and evolution of tin-bearing S-type granites in the western segment of the Gemeric Unit (Western Carpathians) ». Geologica Carpathica 69, n^o 5 (1 octobre 2018) : 483–97. http://dx.doi.org/10.1515/geoca-2018-0028.

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Abstract The S-type accessory mineral assemblage of zircon, monazite-(Ce), fluorapatite and tourmaline in the cupolas of Permian granites of the Gemeric Unit underwent compositional changes and increased variability and volume due to intensive volatile flux. The extended S-type accessory mineral assemblage in the apical parts of the granite resulted in the formation of rare-metal granites from in-situ differentiation and includes abundant tourmaline, zircon, fluorapatite, monazite-(Ce), Nb–Ta–W minerals (Nb–Ta rutile, ferrocolumbite, manganocolumbite, ixiolite, Nb–Ta ferberite, hübnerite), cassiterite, topaz, molybdenite, arsenopyrite and aluminophosphates. The rare-metal granites from cupolas in the western segment of the Gemeric Unit represent the topaz–zinnwaldite granites, albitites and greisens. Zircon in these evolved rare-metal Li–F granite cupolas shows a larger xenotime-(Y) component and heterogeneous morphology compared to zircons from deeper porphyritic biotite granites. The zircon Zr/Hfwt ratio in deeper rooted porphyritic granite varies from 29 to 45, where in the differentiated upper granites an increase in Hf content results in a Zr/Hfwt ratio of 5. The cheralite component in monazite from porphyritic granites usually does not exceed 12 mol. %, however, highly evolved upper rare-metal granites have monazites with 14 to 20 mol. % and sometimes > 40 mol. % of cheralite. In granite cupolas, pure secondary fluorapatite is generated by exsolution of P from P-rich alkali feldspar and high P and F contents may stabilize aluminophosphates. The biotite granites contain scattered schorlitic tourmaline, while textural late-magmatic tourmaline is more alkali deficient with lower Ca content. The differentiated granites contain also nodular and dendritic tourmaline aggregations. The product of crystallization of volatile-enriched granite cupolas are not only variable in their accessory mineral assemblage that captures high field strength elements, but also in numerous veins in country rocks that often contain cassiterite and tourmaline. Volatile flux is documented by the tetrad effect via patterns of chondrite normalized REEs (T1,3 value 1.46). In situ differentiation and tectonic activity caused multiple intrusive events of fluid-rich magmas rich in incompatible elements, resulting in the formation of rare-metal phases in granite roofs. The emplacement of volatile-enriched magmas into upper crustal conditions was followed by deeper rooted porphyritic magma portion undergoing second boiling and re-melting to form porphyritic granite or granite-porphyry during its ascent.

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Holtz, F., B. Scaillet, H. Behrens, F. Schulze et M. Pichavant. « Water contents of felsic melts : application to the rheological properties of granitic magmas ». Earth and Environmental Science Transactions of the Royal Society of Edinburgh 87, n^o 1-2 (1996) : 57–64. http://dx.doi.org/10.1017/s0263593300006477.

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ABSTRACT:New experimental determinations of water solubility in haplogranitic melts (anhydrous compositions in the system Qz-Ab-Or and binary joins) and of the viscosity of hydrous Qz28Ab38Or34 melts (normative proportions) and natural peraluminous leucogranitic melt (Gangotri, High Himalaya) are used to constrain the evolution of viscosity of ascending magmas, depending on their P-T paths.At constant pressure, in the case of fluid-absent melting conditions, with water as the main volatile dissolved in the melts, the viscosity of melts generated from quartzo-feldspathic protoliths is lower at low temperature than at, high temperature (difference of 1-2 log units between 700 and 900°C). This is due to the higher water contents of the melts at low temperature than at high temperature and to the fact that decreasing temperature does not counterbalance the effect of increasing melt water content. In ascending magmas generated from crustal material the magma viscosity does not change significantly whatever the P-T path followed (i.e. path with cooling and crystallisation; adiabatic path with decompression melting) as long as the crystal fraction is low enough to assume a Newtonian behaviour (30-50% crystals, depending on size and shape). Comparison of the properties of natural and synthetic systems suggests that both water solubility and the viscosity of multicomponent natural felsic melts (with less than 30-35% normative Qz) can be extrapolated from those of the equivalent synthetic feldspar melts.

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Metrich, N., et P. J. Wallace. « Volatile Abundances in Basaltic Magmas and Their Degassing Paths Tracked by Melt Inclusions ». Reviews in Mineralogy and Geochemistry 69, n^o 1 (1 janvier 2008) : 363–402. http://dx.doi.org/10.2138/rmg.2008.69.10.

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Sibik, Svetlana, Marie Edmonds, John Maclennan et Henrik Svensen. « Magmas Erupted during the Main Pulse of Siberian Traps Volcanism were Volatile-poor ». Journal of Petrology 56, n^o 11 (novembre 2015) : 2089–116. http://dx.doi.org/10.1093/petrology/egv064.

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Plechov, Pavel, Jon Blundy, Nikolay Nekrylov, Elena Melekhova, Vasily Shcherbakov et Margarita S. Tikhonova. « Petrology and volatile content of magmas erupted from Tolbachik Volcano, Kamchatka, 2012–13 ». Journal of Volcanology and Geothermal Research 307 (décembre 2015) : 182–99. http://dx.doi.org/10.1016/j.jvolgeores.2015.08.011.

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Robidoux, P., S. G. Rotolo, A. Aiuppa, G. Lanzo et E. H. Hauri. « Geochemistry and volatile content of magmas feeding explosive eruptions at Telica volcano (Nicaragua) ». Journal of Volcanology and Geothermal Research 341 (juillet 2017) : 131–48. http://dx.doi.org/10.1016/j.jvolgeores.2017.05.007.

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Robidoux, P., A. Aiuppa, S. G. Rotolo, A. L. Rizzo, E. H. Hauri et M. L. Frezzotti. « Volatile contents of mafic-to-intermediate magmas at San Cristóbal volcano in Nicaragua ». Lithos 272-273 (février 2017) : 147–63. http://dx.doi.org/10.1016/j.lithos.2016.12.002.

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Kamenetsky, Vadim S., Massimo Pompilio, Nicole Métrich, Alexander V. Sobolev, Dmitry V. Kuzmin et Rainer Thomas. « Arrival of extremely volatile-rich high-Mg magmas changes explosivity of Mount Etna ». Geology 35, n^o 3 (2007) : 255. http://dx.doi.org/10.1130/g23163a.1.

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Brady, A. E., et K. R. Moore. « A mantle-derived dolomite silicocarbonatite from the southwest coast of Ireland ». Mineralogical Magazine 76, n^o 2 (avril 2012) : 357–76. http://dx.doi.org/10.1180/minmag.2012.076.2.06.

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AbstractThe magma source and evolution of a zoned breccia pipe on the southern Beara Peninsula in southwest Ireland are investigated using the geochemistry of the host mineral assemblages. The clast-poor inner zone of the pipe has a magnesium-rich silicocarbonatite whole-rock composition (14.30 wt.% MgO; 31.80 wt.% SiO2). The silicocarbonatite has retained an ultimate mantle source 13C isotopic composition after metamorphism, consistent with the presence of mantle debris. The silicocarbonatite is Cr-, Ni- and Co-rich (847 ppm, 611 ppm and 60 ppm, respectively) but REE depleted compared with volcanic dolomite carbonatites worldwide. The mineral assemblage consists of Sr-rich (0.55 wt.% SrO) ferroan dolomite, magnesite and pseudomorphs of chlorite after phlogopite, consistent with derivation from a carbonated and hydrated mantle. However, chrome spinel crystals (≤4 40.14 wt.% Cr2O3) are compositionally indistinguishable from unmetasomatized spinel macrocrysts in kimberlites. The silicocarbonatite is inferred to represent a magma produced by partial melting of metasomatized mantle at physical conditions between those in which primary dolomite carbonatite and ultramafic magmas of high-pressure origin form. The primary silicocarbonatite magma ascended and sampled mantle material in a manner similar to kimberlite, and subsequently lost volatile components due to release of metasomatic fluids and later metamorphism.

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Henderson, C. M. B., F. R. Richardson et J. M. Charnock. « The Highwood Mountains potassic igneous province, Montana : mineral fractionation trends and magmatic processes revisited ». Mineralogical Magazine 76, n^o 4 (août 2012) : 1005–51. http://dx.doi.org/10.1180/minmag.2012.076.4.16.

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AbstractPotassium-rich mafic dykes and lavas from the Highwood Mountains Igneous Province, USA were studied by electron-microprobe and bulk-rock analysis. For the mafic phonolites, compositional trends for olivine and augite phenocrysts and groundmass biotite, alkali feldspar and titanomagnetites are presented and substitution mechanisms discussed. Phenocrysts of biotite and augite in the minettes are also characterized, together with groundmass alkali feldspar and titanomagnetite. The alkali feldspars and biotites are commonly enriched in Ba. Olivine, clinopyroxene and biotite phenocrysts are generally quite magnesium-rich, which is consistent with the primitive natures of the least evolved rocks.Bulk-rock major-element compositions are combined with modal and microprobe data for the principal phenocrysts to calculate model residual liquid compositions for mafic phonolites, minettes and a syenitic rock. On the basis of phase-equilibria, it is suggested that the main controls of differentiation are polybaric involving crystallization during transport of primary magmas from the mantle for the minettes, and low-pressure differentiation for the mafic phonolites. Whereas magma mixing might have contributed to petrogenesis, many of the disequilibrium features exhibited by clinopyroxene and biotite phenocrysts can also be attributed to pre-existing phenocrysts undergoing decompression melting during magma uprise from its mantle source, followed by rapid crystal growth and episodic volatile loss in sub-volcanic magma chambers.

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Sutcliffe, R. H., J. M. Sweeny et A. D. Edgar. « The Lac des Iles Complex, Ontario : petrology and platinum-group-elements mineralization in an Archean mafic intrusion ». Canadian Journal of Earth Sciences 26, n^o 7 (1 juillet 1989) : 1408–27. http://dx.doi.org/10.1139/e89-120.

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The Lac des Iles Complex is a late Archean, mafic to ultramafic complex that is host to 20.4 × 106 t Pt, Pd, and Au mineralization with an average grade of 6.34 g/t platinum-group elements (PGE). The 30 km2 complex, located in the Wabigoon Subprovince, consists of several coalescing mafic to ultramafic intrusive centers. The complex is emplaced into early gneissic tonalite and is contemporaneous with late granitoids.The magmatic evolution of the complex reflects the emplacement of multiple pulses of previously fractionated magma, some of which underwent subsequent in situ fractionation. Distinct magma sequences are recognized on the basis of intrusive relationships, differences in the cumulus minerals, crystallization order, and trace-element chemistry. These sequences are (1) hornblende gabbro, (2) gabbro and gabbronorite, and (3) ultramafic. Where there is evidence of intrusive relations, the most primitive sequences are emplaced late and occur toward the north end of the complex. Microprobe analyses indicate the following compositional ranges of mineral phases within the complex: olivine–Fo84–76, orthopyroxene—En82–67Fs15–30Wo2–3, and clinopyroxene—En53–46Fs3–12Wo41–51. Cyclic layering is developed only in the northern ultramafic part of the complex where several cycles of mesocumulate- to adcumulate-textured wehrlite, clinopyroxenite, and websterite are identified.Although the parental magma compositions are not well constrained, model calculations using major-element and rare-earth-element (REE) contents of cumulus phases and adcumulate rocks indicate that the parental magmas had mol MgO/(MgO + FeO) ranging from at least 0.61 to 0.54 and had fractionated REE abundances with CeN/YbN of approximately 5.0. Mineral chemistry and crystallization sequences indicate that the ultramafic and gabbroic parts of the complex had tholeiitic basalt parental magmas of picritic and high-alumina affinities, respectively.The major PGE mineralization at Lac des Iles occurs in the Roby Zone along the interface between the gabbro and gabbronorite in the southern part of the complex. This interface has been intruded by a sheet of pyroxene cumulate, which is probably part of the ultramafic sequence. The PGE sulphide mineralization in the Roby Zone is associated with disseminated chalcopyrite, pyrrhotite, pentlandite, pyrite, and altered silicates in (1) the pyroxene cumulate sheet, (2) the mixed gabbro–gabbronorite, and (3) the gabbroic pegmatite dikes and breccia zones. Cl-rich apatite and monazite are accessory phases associated with PGE mineralization PGE occurrences are also present within sulphide-bearing orthopyroxene–clinopyroxene cumulates in the northern ultramafic part of the complex. The Roby Zone mineralization is localized by mixing the PGE and sulphide-rich gabbronorite and pyroxene cumulates with the volatile-rich gabbro. The PGE are redistributed by late magmatic volatile activity, which generated mineralized gabbroic pegmatites and breccia zones.

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Coulson, Ian M., James K. Russell et Gregory M. Dipple. « Origins of the Zippa Mountain pluton : a Late Triassic, arc-derived, ultrapotassic magma from the Canadian Cordillera ». Canadian Journal of Earth Sciences 36, n^o 9 (1 septembre 1999) : 1415–34. http://dx.doi.org/10.1139/e99-045.

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The Zippa Mountain intrusion is of Late Triassic age and is situated in the Iskut River area of northwest British Columbia. The pluton is elliptical in shape and 3.5 by 5 km in diameter. The pluton intrudes Palaeozoic and Triassic rocks within Stikinia and is compositionally zoned from clinopyroxenite at the pluton margins to a core of syenite. The Zippa Mountain pluton comprises aegirine-augite, potassium feldspar, and minor biotite, hornblende, nepheline, vishnevite, titanian andradite, titanite, and apatite. Based on new field, petrographic, and chemical data this intrusion is shown to be silica-undersaturated, strongly alkaline, and ultrapotassic. We interpret the pluton as a single pulse of magma, which entered a shallow-level crustal magma chamber. The potassic nature is a characteristic of the parental magma, but is enhanced by fractional crystallization and crystal sorting processes. The parental magma has affinities with arc-type magmas related to subduction (shoshonitic magma series), as is evidenced by high LILE/LREE ratio, and select depletion of HFSE. Upon emplacement, crystallization of clinopyroxene and then K-feldspar, and efficient physical sorting within the magma chamber, resulted in sidewall, marginal pyroxenite and roof-zone syenites, respectively. Continued fractionation in the core of the intrusion increased volatile contents and led to the crystallization of feldspathoids. Potentially, a mass of residual melt, and crystals of K-feldspar and feldspathoid, was buoyant relative to the surrounding pyroxenite, which allowed it it to rise and partly intrude the syenites.

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McCubbin, Francis M., et Jessica J. Barnes. « The chlorine-isotopic composition of lunar KREEP from magnesian-suite troctolite 76535 ». American Mineralogist 105, n^o 8 (1 août 2020) : 1270–74. http://dx.doi.org/10.2138/am-2020-7467.

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Abstract We conducted in situ Cl isotopic measurements of apatite within intercumulus regions and within a holocrystalline olivine-hosted melt inclusion in magnesian-suite troctolite 76535 from Apollo 17. These data were collected to place constraints on the Cl-isotopic composition of the last liquid to crystallize from the lunar magma ocean (i.e., urKREEP, named after its enrichments in incompatible lithophile trace elements like potassium, rare earth elements, and phosphorus). The apatite in the olivine-hosted melt inclusion and within the intercumulus regions of the sample yielded Cl-isotopic compositions of 28.3 ± 0.9‰ (2σ) and 30.3 ± 1.1‰ (2σ), respectively. The concordance of these values from both textural regimes we analyzed indicates that the Cl-isotopic composition of apatites in 76535 likely represents the Cl-isotopic composition of the KREEP-rich magnesian-suite magmas. Based on the age of 76535, these results imply that the KREEP reservoir attained a Cl-isotopic composition of 28–30‰ by at least 4.31 Ga, consistent with the onset of Cl-isotopic fractionation at the time of lunar magma ocean crystallization or shortly thereafter. Moreover, lunar samples that yield Cl-isotopic compositions higher than the value for KREEP are likely affected by secondary processes such as impacts and/or magmatic degassing. The presence of KREEP-rich olivine-hosted melt inclusions within one of the most pristine and ancient KREEP-rich rocks from the Moon provides a new opportunity to characterize the geochemistry of KREEP. In particular, a broader analysis of stable isotopic compositions of highly and moderately volatile elements could provide an unprecedented advancement in our characterization of the geochemical composition of the KREEP reservoir and of volatile-depletion processes during magma ocean crystallization, more broadly.

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Sokół, Krzysztof, Adrian A. Finch, William Hutchison, Jonathan Cloutier, Anouk M. Borst et Madeleine C. S. Humphreys. « Quantifying metasomatic high-field-strength and rare-earth element transport from alkaline magmas ». Geology 50, n^o 3 (3 décembre 2021) : 305–10. http://dx.doi.org/10.1130/g49471.1.

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Abstract Alkaline igneous rocks host many global high-field-strength element (HFSE) and rare-earth element (REE) deposits. While HFSEs are commonly assumed to be immobile in hydrothermal systems, transport by late-stage hydrothermal fluids associated with alkaline magmas is reported. However, the magnitude of the flux and the conditions are poorly constrained and yet essential to understanding the formation of REE-HFSE ores. We examined the alteration of country rocks (“fenitization”) accompanying the emplacement of a syenite magma at Illerfissalik in Greenland, through analysis of changes in rock chemistry, mineralogy, and texture. Our novel geochemical maps show a 400-m-wide intrusion aureole, within which we observed typically tenfold increases in the concentrations of many elements, including HFSEs. Textures suggest both pervasive and structurally hosted fluid flow, with initial reaction occurring with the protolith's quartz cement, leading to increased permeability and enhancing chemical interaction with a mixed Ca-K-Na fenitizing fluid. We estimated the HFSE masses transferred from the syenite to the fenite by this fluid and found ~43 Mt of REEs were mobilized (~12% of the syenite-fenite system total rare-earth-oxide [TREO] budget), a mass comparable to the tonnages of some of the world's largest HFSE resources. We argue that fenite can yield crucial information about the tipping points in magma evolution because retention and/or loss of volatile-bonded alkali and HFSEs are key factors in the development of magmatic zirconosilicate-hosted HFSE ores (e.g., Kringlerne, at Ilímaussaq), or the formation of the syenite-hosted Nb-Ta-REE (Motzfeldt-type) roof-zone deposits.

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Podladchikov, Yuri Y., et Stephen M. Wickham. « Crystallization of Hydrous Magmas : Calculation of Associated Thermal Effects, Volatile Fluxes, and Isotopic Alteration ». Journal of Geology 102, n^o 1 (janvier 1994) : 25–45. http://dx.doi.org/10.1086/629646.

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Burnard, Pete. « Correction for volatile fractionation in ascending magmas : noble gas abundances in primary mantle melts ». Geochimica et Cosmochimica Acta 65, n^o 15 (août 2001) : 2605–14. http://dx.doi.org/10.1016/s0016-7037(01)00605-6.

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Gioncada, A., et P. Landi. « The pre-eruptive volatile contents of recent basaltic and pantelleritic magmas at Pantelleria (Italy) ». Journal of Volcanology and Geothermal Research 189, n^o 1-2 (janvier 2010) : 191–201. http://dx.doi.org/10.1016/j.jvolgeores.2009.11.006.

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Scaillet, B., et M. Pichavant. « Experimental constraints on volatile abundances in arc magmas and their implications for degassing processes ». Geological Society, London, Special Publications 213, n^o 1 (2003) : 23–52. http://dx.doi.org/10.1144/gsl.sp.2003.213.01.03.

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Metrich, N., et R. Clocchiatti. « Melt inclusion investigation of the volatile behaviour in historic alkali basaltic magmas of Etna ». Bulletin of Volcanology 51, n^o 3 (mai 1989) : 185–98. http://dx.doi.org/10.1007/bf01067955.

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Förster, Michael W., Yannick Bussweiler, Dejan Prelević, Nathan R. Daczko, Stephan Buhre, Regina Mertz-Kraus et Stephen F. Foley. « Sediment-Peridotite Reaction Controls Fore-Arc Metasomatism and Arc Magma Geochemical Signatures ». Geosciences 11, n^o 9 (3 septembre 2021) : 372. http://dx.doi.org/10.3390/geosciences11090372.

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Subduction of oceanic crust buries an average thickness of 300–500 m of sediment that eventually dehydrates or partially melts. Progressive release of fluid/melt metasomatizes the fore-arc mantle, forming serpentinite at low temperatures and phlogopite-bearing pyroxenite where slab surface reaches 700–900 °C. This is sufficiently high to partially melt subducted sediments before they approach the depths where arc magmas are formed. Here, we present experiments on reactions between melts of subducted sediments and peridotite at 2–6 GPa/750–1100 °C, which correspond to the surface of a subducting slab. The reaction of volatile-bearing partial melts derived from sediments with depleted peridotite leads to separation of elements and a layered arrangement of metasomatic phases, with layers consisting of orthopyroxene, mica-pyroxenite, and clinopyroxenite. The selective incorporation of elements in these metasomatic layers closely resembles chemical patterns found in K-rich magmas. Trace elements were imaged using LA-ICP-TOFMS, which is applied here to investigate the distribution of trace elements within the metasomatic layers. Experiments of different duration enabled estimates of the growth of the metasomatic front, which ranges from 1–5 m/ky. These experiments explain the low contents of high-field strength elements in arc magmas as being due to their loss during melting of sedimentary materials in the fore-arc.

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Coulson, I. M., K. M. Goodenough, N. J. G. Pearce et M. J. Leng. « Carbonatites and lamprophyres of the Gardar Province – a ‘window’ to the sub-Gardar mantle ? » Mineralogical Magazine 67, n^o 5 (octobre 2003) : 855–72. http://dx.doi.org/10.1180/0026461036750148.

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AbstractCarbonatite magmas are considered to be ultimately derived from mantle sources, which may include lithospheric and asthenospheric reservoirs. Isotopic studies of carbonatite magmatism around the globe have typically suggested that more than one source needs to be invoked for generation of the parental melts to carbonatites, often involving the interaction of asthenosphere and lithosphere.In the rift-related, Proterozoic Gardar Igneous Province of SW Greenland, carbonatite occurs as dykes within the Igaliko Nepheline Syenite Complex, as eruptive rocks and diatremes at Qassiarsuk, as a late plug associated with nepheline syenite at Grønnedal-Íka, and as small bodies associated with ultramafic lamprophyre dykes. The well-known cryolite deposit at Ivittuut was also rich in magmatic carbonate. The carbonatites are derived from the mantle with relatively little crustal contamination, and therefore should provide important information about the mantle sources of Gardar magmas. In particular, they are found intruded both into Archaean and Proterozoic crust, and hence provide a test for the involvement of lithospheric mantle.A synthesis of new and previously published major and trace element, Sr, Nd, C and O isotope data for carbonatites and associated lamprophyres from the Gardar Province is presented. The majority of Gardar carbonatites and lamprophyres have consistent geochemical and isotopic signatures that are similar to those typically found in ocean island basalts. The geochemical characteristics of the two suites of magmas are similar enough to suggest that they were derived from the same mantle source. C and O isotope data are also consistent with a mantle derivation for the carbonatite magmas, and support the theory of a cogenetic origin for the carbonatites and the lamprophyres. The differences between the carbonatites and lamprophyres are considered to represent differing degrees of partial melting of a similar source.We suggest that the ultimate source of these magmas is the asthenospheric mantle, since there is no geochemical or isotopic evidence for their having been derived directly from ancient, enriched sub-continental lithospheric mantle. However, it is likely that the magmas actually formed through a two-stage process, with small-degree volatile-rich partial melts rising from the asthenospheric mantle and being ‘frozen in’ as metasomites, which were then rapidly remobilized during Gardar rifting.

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Wang, Zaicong, Huai Cheng, Keqing Zong, Xianlei Geng, Yongsheng Liu, Jinhui Yang, Fuyuan Wu, Harry Becker, Stephen Foley et Christina Yan Wang. « Metasomatized lithospheric mantle for Mesozoic giant gold deposits in the North China craton ». Geology 48, n^o 2 (22 novembre 2019) : 169–73. http://dx.doi.org/10.1130/g46662.1.

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Abstract The origin of giant lode gold deposits of Mesozoic age in the North China craton (NCC) is enigmatic because high-grade metamorphic ancient crust would be highly depleted in gold. Instead, lithospheric mantle beneath the crust is the likely source of the gold, which may have been anomalously enriched by metasomatic processes. However, the role of gold enrichment and metasomatism in the lithospheric mantle remains unclear. Here, we present comprehensive data on gold and platinum group element contents of mantle xenoliths (n = 28) and basalts (n = 47) representing the temporal evolution of the eastern NCC. The results indicate that extensive mantle metasomatism and hydration introduced some gold (<1–2 ppb) but did not lead to a gold-enriched mantle. However, volatile-rich basalts formed mainly from the metasomatized lithospheric mantle display noticeably elevated gold contents as compared to those from the asthenosphere. Combined with the significant inheritance of mantle-derived volatiles in auriferous fluids of ore bodies, the new data reveal that the mechanism for the formation of the lode gold deposits was related to the volatile-rich components that accumulated during metasomatism and facilitated the release of gold during extensional craton destruction and mantle melting. Gold-bearing, hydrous magmas ascended rapidly along translithospheric fault zones and evolved auriferous fluids to form the giant deposits in the crust.

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Moretti, Roberto, Ilenia Arienzo, Valeria Di Renzo, Giovanni Orsi, Fabio Arzilli, Francesco Brun, Massimo D'Antonio, Lucia Mancini et Etienne Deloule. « Volatile segregation and generation of highly vesiculated explosive magmas by volatile-melt fining processes : The case of the Campanian Ignimbrite eruption ». Chemical Geology 503 (janvier 2019) : 1–14. http://dx.doi.org/10.1016/j.chemgeo.2018.10.001.

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Raia, Federica, James D. Webster et Benedetto De Vivo. « Pre-eruptive volatile contents of Vesuvius magmas : constraints on eruptive history and behavior. I - The medieval and modern interplinian activities ». European Journal of Mineralogy 12, n^o 1 (7 février 2000) : 179–93. http://dx.doi.org/10.1127/0935-1221/2000/0012-0179.

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Rasmussen, Daniel J., Terry A. Plank, Paul J. Wallace, Megan E. Newcombe et Jacob B. Lowenstern. « Vapor-bubble growth in olivine-hosted melt inclusions ». American Mineralogist 105, n^o 12 (1 décembre 2020) : 1898–919. http://dx.doi.org/10.2138/am-2020-7377.

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Abstract Melt inclusions record the depth of magmatic processes, magma degassing paths, and volatile budgets of magmas. Extracting this information is a major challenge. It requires determining melt volatile contents at the time of entrapment when working with melt inclusions that have suffered post-entrapment modifications. Several processes decrease internal melt inclusion pressure, resulting in nucleation and growth of a vapor bubble and, time permitting, diffusion of volatiles (especially CO2) into the vapor bubble. Previous studies have shown how this process may lead to most of the CO2 in the bulk melt inclusion being lost to the bubble. Without reconstruction, most of the melt inclusion data in the literature vastly underestimate the CO2 concentrations of magmas by measuring the glass phase only. Methods exist that attempt to reconstruct the entrapped CO2 contents, but they can be difficult to apply and do not always yield consistent results. Here, we explore bubble growth, evaluate CO2 reconstruction approaches, and develop improved experimental and computational approaches. Piston-cylinder experiments were conducted on olivine-hosted melt inclusions from Seguam (Alaska, U.S.A.) and Fuego (Guatemala) volcanoes at the following conditions: 500–800 MPa, 1140–1200 °C for Seguam and 1110–1140 °C for Fuego, 4–8 wt% H2O in the KBr brine filling the experimental capsules, and run durations of 10–120 min. Heated melt inclusions form well-defined S-CO2 trends consistent with degassing models. CO2 contents are enriched by a factor of ~2.5, on average, relative to those of the glasses in unheated melt inclusions, whereas S contents of heated and unheated melt inclusion glasses overlap, indicating that insignificant amounts of S partition into the vapor bubble. For naturally quenched melt inclusions, relatively low closure temperatures for CO2 diffusion enables some CO2 to enter vapor bubbles during quench, whereas higher closure temperatures for S diffusion limits its loss to vapor bubbles. We evaluate the timescales of post-entrapment processes and use the results to develop a new computational model to restore entrapped CO2 contents: melt inclusion modification corrections (MIMiC). Heated melt inclusion data are used as a benchmark to evaluate the results from MIMiC and other published methods of CO2 reconstruction. The methods perform variably well. Key advantages to our experimental technique are accurate measurements of CO2 contents and efficient rehomogenization of large quantities of melt inclusions. Our new computational model produces more accurate results than other computational methods, has similar accuracy to the Raman method of CO2 reconstruction in cases where Raman can be applied (i.e., no C-bearing phases in the bubble), and can be applied to the vast body of published melt inclusion data. To obtain the most robust data on bubble-bearing melt inclusions, we recommend taking both experimental- and MIMiC-based approaches.

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Thorpe, R. S., J. W. Gaskarth et P. J. Henney. « Composite Ordovician lamprophyre (spessartite) intrusions around the Midlands Microcraton in central Britain ». Geological Magazine 130, n^o 5 (septembre 1993) : 657–63. http://dx.doi.org/10.1017/s0016756800020963.

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AbstractLamprophyre sills and dykes of Ordovician age were emplaced within Cambrian–Lower Ordovician sedimentary rocks around the northern margins of the Midlands Microcraton. The intrusions show internal mineralogical and chemical variations indicating emplacement as multiple intrusions of co-magmatic pulses. The chemical characteristics of the lamprophyre magmas indicate formation by small-degree volatile-rich partial melting of lithospheric mantle enriched and modified by Lower Palaeozoic subduction (Th/Ta 5.3–11.6, La/Ta 29–82.3), together with a contribution from within-plate mantle source (Zr/Yc. 6) and/or mineralogically heterogeneous lithosphere, followed by varying degrees of fractional crystallization during uprise.

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Piquer, José, Pablo Sanchez-Alfaro et Pamela Pérez-Flores. « A new model for the optimal structural context for giant porphyry copper deposit formation ». Geology 49, n^o 5 (26 janvier 2021) : 597–601. http://dx.doi.org/10.1130/g48287.1.

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Abstract Porphyry-type deposits are the main global source of copper and molybdenum. An improved understanding of the most favorable structural settings for the emplacement of these deposits is necessary for successful exploration, particularly considering that most future discoveries will be made under cover based on conceptual target generation. A common view is that porphyry deposits are preferentially emplaced in pull-apart basins within strike-slip fault systems that favor local extension within a regional compressive to transpressive tectonic regime. However, the role of such a structural context in magma storage and evolution in the upper crust remains unclear. In this work, we propose a new model based on the integration of structural data and the geometry of magmatic-hydrothermal systems from the main Andean porphyry Cu-Mo metallogenic belts and from the active volcanic arc of southern Chile. We suggest that the magma differentiation and volatile accumulation required for the formation of a porphyry deposit is best achieved when the fault system controlling magma ascent is strongly misoriented for reactivation with respect to the prevailing stress field. When magmas and fluids are channeled by faults favorably oriented for extension (approximately normal to σ3), they form sets of parallel, subvertical dikes and veins, which are common both during the late stages of the evolution of porphyry systems and in the epithermal environment. This new model has direct implications for conceptual mineral exploration.

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Richards, Jeremy P. « Porphyry copper deposit formation in arcs : What are the odds ? » Geosphere 18, n^o 1 (16 novembre 2021) : 130–55. http://dx.doi.org/10.1130/ges02086.1.

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Abstract Arc magmas globally are H2O-Cl-S–rich and moderately oxidized (ΔFMQ = +1 to +2) relative to most other mantle-derived magmas (ΔFMQ ≤ 0). Their relatively high oxidation state limits the extent to which sulfide phases separate from the magma, which would otherwise tend to deplete the melt in chalcophile elements such as Cu (highly siderophile elements such as Au and especially platinum-group elements are depleted by even small amounts of sulfide segregation). As these magmas rise into the crust and begin to crystallize, they will reach volatile saturation, and a hydrous, saline, S-rich, moderately oxidized fluid is released, into which chalcophile and any remaining siderophile metals (as well as many other water-soluble elements) will strongly partition. This magmatic-hydrothermal fluid phase has the potential to form ore deposits (most commonly porphyry Cu ± Mo ± Au deposits) if its metal load is precipitated in economic concentrations, but there are many steps along the way that must be successfully negotiated before this can occur. This paper seeks to identify the main steps along the path from magma genesis to hydrothermal mineral precipitation that affect the chances of forming an ore deposit (defined as an economically mineable resource) and attempts to estimate the probability of achieving each step. The cumulative probability of forming a large porphyry Cu deposit at any given time in an arc magmatic system (i.e., a single batholith-linked volcanoplutonic complex) is estimated to be ~0.001%, and less than 1/10 of these deposits will be uplifted and exposed at shallow enough depths to mine economically (0.0001%). Continued uplift and erosion in active convergent tectonic regimes rapidly remove these upper-crustal deposits from the geological record, such that the probability of finding them in older arc systems decreases further with age, to the point that porphyry Cu deposits are almost nonexistent in Precambrian rocks. A key finding of this paper is that most volcanoplutonic arcs above subduction zones are prospective for porphyry ore formation, with probabilities only falling to low values at late stages of magmatic-hydrothermal fluid exsolution, focusing, and metal deposition. This is in part because of the high threshold required in terms of grade and tonnage for a deposit to be considered economic. Thus, the probability of forming a porphyry-type system in any given arc segment is relatively high, but the probability that it will be a large economic deposit is low, dictated to a large extent by mineral economics and metal prices.

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