Carbonate-silicate melts that originate in Earth's interior are described as transitional melts which possess compositions intermediate between carbonatitic and basaltic end members. The covariation of key oxides between carbonatite and basalt (e.g., 10-35 wt% SiO2 and 40-10 wt% CO2, respectively) is expected to have a strong effect on liquid properties. However, due to their paucity both in the record of terrestrial rocks and as quenched glasses, their molecular structure has remained poorly explored to date. We investigated the atomic structure of a synthetic carbonate-silicate liquid with chemical composition within the CaO-MgO-Al2O3-SiO2FeO-Na2O-ClO -CO2 oxide system having 18.28 wt% SiO2 and 22.54 wt% CO2 using multi-angle energy dispersive X-ray diffraction at pressures (P) and temperatures (T) of 1.4 GPa/1815 degrees C, 2.6 GPa/1865 degrees C, 4.3 GPa/1990 degrees C, 4.4 GPa/1950 degrees C. The results show that the intermediate range ordering of the structure decreases with an increase of both P and T. Based on this study, the carbonate-silicate magmas at upper mantle P-T conditions are expected to increase their viscosities during their ascent through the mantle as a result of increasing intermediate range ordering upon cooling and decompression. Additionally, spectroscopic measurements were carried out on the quenched glasses at ambient pressure using micro-Raman as well as micro-FTIR in reflection and transmission modes in the mid infrared range. High pressure investigation using micro-FTIR was also conducted. The distribution of Q(n) species obtained by deconvolution of the Raman spectra within the aluminosilicate region confirms the depolymerized nature of the quenched glasses as inferred by the low viscosities of the corresponding liquids; peculiar characteristics of the C vibrations would suggest a distorted environment surrounding the network modifying CO32- anion. No evidence of molecular CO2 was detected. Notably, we find evidence of both dissolved molecular CO and CO linked to a metal cation forming carbonyl complexes in the quenched glasses at P-T-fo(2) conditions compatible with a hot Archean upper mantle. This suggests a role for carbonate-silicate magmas as carriers of reduced gaseous C-O-H species towards the early atmosphere along with the mobilization of PGE-elements.

In subduction zones, carbon from the surface can be transported into Earth's interior. Fluids released during subduction can mobilize carbon bound in solid minerals in the subducting plate, and transport it into the overlying mantle. While there is increasing attention on the composition and redox state of carbon-bearing, subduction-mobilized fluids, the fate of fluids as they migrate through and react with overlying mantle rocks is less well characterized. Using thermodynamic modeling, we quantify the evolution of carbon-bearing aqueous fluids mobilized from subducting sedimentary rocks, as they follow several possible pathways through the overlying mantle. The extent of carbon mineralization in the overlying mantle depends on the source (siliciclastic vs carbonate rock) and composition of the fluids as well as the fluid path (isothermal, cooling, heating). A heating path towards the hot core of the mantle wedge does not favor sequestration of carbon into solid phases. Along cooling trajectories, most of the dissolved carbon (61 to 98 %) forms in solid minerals when carbon-rich, siliciclastic-equilibrated fluids react with mantle rocks. In contrast to fluids from siliciclastic sediments, less carbon is sequestered in the mantle when relatively carbon-poor, limestone-equilibrated fluids react with mantle rocks. The formation of quartz-rich (birbirite), quartz-magnesite (listvenite), talc-magnesite (soapstone), and serpentine-rich (serpentinite) parageneses is predicted at decreasing water-rock ratios. Soapstones are less favored to form along fractional crystallization paths and/or lower temperatures. Listvenite assemblages form from migrating fluids with dissolved carbon concentration > 20,000 ppm. Graphite is favored to form along closed reaction, cooling paths. Moreover, several cooling paths form reduced fluids rich in hydrogen (H2), methane (CH4), and organic species. Overall, this work provides quantitative models that can comprehensively inform studies on the movement of carbon in subduction zones and into the deep Earth.

Mount Somma–Vesuvius is a stratovolcano that represents a geological hazard to the population of the city of Naples and surrounding towns in southern Italy. Historically, volcanic eruptions at Mt. Somma–Vesuvius (SV) include high-magnitude Plinian eruptions, such as the infamous 79 CE eruption that occurred after 295 years of quiescence and killed thousands of people in Pompeii and surrounding towns and villages. The last eruption at SV was in 1944 and showed a Volcanic Explosivity Index (VEI) of 3 (0.01 km3 of volcanic material erupted). Following the 1944 eruption, SV has been dormant for the past nearly 79 years, with only minor fumarolic and seismic activity. During its long history, centuries of dormancy at SV have ended with Plinian eruptions (VEI 6) that signal the beginning of a new cycle of eruptive activity. Thus, the current dormancy stage demands a need to better understand the mechanism involved in high-magnitude eruptions in order to better predict future eruption magnitude and style. Despite centuries of research on the SV volcanic system, many questions remain, including the evolution of magmatic volatiles from deep primitive magmas to shallower more evolved magmas. Developing a better understanding of the physical and chemical processes associated with volatile evolution at SV can provide insights into magma dynamics and the mechanisms that trigger highly explosive eruptions at SV. In this study, we present new data for the pre-eruptive volatile contents of magmas associated with four Plinian and two inter-Plinian eruptions at SV based on analyses of reheated melt inclusions (MIs) hosted in olivine. We correct the volatile contents of bubble-bearing MIs by taking into account the volatile contents of bubbles in the MIs. We recognize two groups of MIs: one group hosted in high-Fo olivine (Fo85–90) and relatively rich in volatiles and the other group hosted in low-Fo olivine (Fo70–69) and relatively depleted in volatiles. The correlation between volatile contents and compositions of host olivines suggests that magma fractionation took place under volatile-saturated conditions and that more differentiated magmas reside at shallower levels relative to less evolved/quasi-primitive magmas. Using the CO2 contents of corrected MIs hosted in Fo90 olivine from SV, we estimate that 347 to 686 t d−1 of magmatic CO2 exsolved from SV magmas during the last 3 centuries (38–75 Mt in total) of volcanic activity. Although this study is limited to only few SV magmas, we suggest that further study applying similar methods could shed light on the apparent lack of correlation between the volatile contents of MIs and the style and age of eruptions. Further, such studies could provide additional constraints on the origin of CO2 and the interaction between the carbonate platform and ascending magmas below SV.

Abstract Fluid-mediated calcium metasomatism is often associated with strong silica mobility and the presence of chlorides in solution. To help quantify mass transfer at lower crustal and upper mantle conditions, we measured quartz solubility in H2O-CaCl2 solutions at 0.6–1.4 GPa, 600–900 °C, and salt concentrations to 50 mol%. Solubility was determined by weight loss of single-crystals using hydrothermal piston-cylinder methods. All experiments were conducted at salinity lower than salt saturation. Quartz solubility declines exponentially with added CaCl2 at all conditions investigated, with no evidence for complexing between silica and Ca. The decline in solubility is similar to that in H2O-CO2 but substantially greater than that in H2O-NaCl at the same pressure and temperature. At each temperature, quartz solubility at low salinity (XCaCl2 < 0.1) depends strongly on pressure, whereas at higher XCaCl2 it is nearly pressure independent. This behavior is consistent with a transition from an aqueous solvent to a molten salt near XCaCl2 ~0.1. The solubility data were used to develop a thermodynamic model of H2O-CaCl2 fluids. Assuming ideal molten-salt behavior and utilizing previous models for polymerization of hydrous silica, we derived values for the activity of H2O (aH2O), and for the CaCl2 dissociation factor (α), which may vary from 0 (fully associated) to 2 (fully dissociated). The model accurately reproduces our data along with those of previous work and implies that, at conditions of this study, CaCl2 is largely associated (<0.2) at H2O density <0.85 g/cm3. Dissociation rises isothermally with increasing density, reaching ~1.4 at 600 °C, 1.4 GPa. The variation in silica molality with aH2O in H2O-CaCl2 is nearly identical to that in H2O-CO2 solutions at 800 °C and 1.0 GPa, consistent with the absence of Ca-silicate complexing. The results suggest that the ionization state of the salt solution is an important determinant of aH2O, and that H2O-CaCl2 fluids exhibit nearly ideal molecular mixing over a wider range of conditions than implied by previous modeling. The new data help interpret natural examples of large-scale Ca-metasomatism in a wide range of lower crustal and upper mantle settings.

Completely carbonated peridotites represent a window to study reactions of carbon‐rich fluids with mantle rocks. Here, we present details on the carbonation history of listvenites close to the basal thrust in the Samail ophiolite. We use samples from Oman Drilling Project Hole BT1B, which provides a continuous record of lithologic transitions, as well as outcrop samples from listvenites, metasediments, and metamafics below the basal thrust of the ophiolite. 87 Sr/ 86 Sr of listvenites and serpentinites, ranging from 0.7090 to 0.7145, are significantly more radiogenic than mantle values, Cretaceous seawater, and other peridotite hosted carbonates in Oman. The Hawasina sediments that underlie the ophiolite, on the other hand, show higher 87 Sr/ 86 Sr values of up to 0.7241. δ 13 C values of total carbon in the listvenites and serpentinites range from −10.6‰ to 1.92‰. We also identified a small organic carbon component with δ 13 C as low as −27‰. Based on these results, we propose that during subduction at temperatures above >400°C, carbon‐rich fluids derived from decarbonation of the underlying sediments migrated updip and generated the radiogenic 87 Sr/ 86 Sr signature and the fractionated δ 13 C values of the serpentinites and listvenites in core BT1B.

Earth and Space Science Open Archive Presented WorkOpen AccessYou are viewing the latest version by default [v1]Subducting Carbon: Heterogeneity RulesAuthorsTerryPlankiDAlbertoMalinvernoiDCraigManningiDSee all authors Terry PlankiDCorresponding Author• Submitting AuthorLamont-Doherty Earth Observatory of Columbia UniversityiDhttps://orcid.org/0000-0002-1665-0484view email addressThe email was not providedcopy email addressAlberto MalinvernoiDLamont -Doherty Earth Observatory of Columbia UniversityiDhttps://orcid.org/0000-0003-4442-9055view email addressThe email was not providedcopy email addressCraig ManningiDUniv California Los AngelesiDhttps://orcid.org/0000-0002-1463-3701view email addressThe email was not providedcopy email address

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The precise form of internalization of the provisions of the Council of Europe’s Framework Convention for the Protection of National Minorities in domestic law is crucial in ensuring its long-term effectiveness. Experiences in the Western Balkans raise important questions about the role of minority (or community) rights legislation in deeply divided societies. This article uses the case-studies of Bosnia and Herzegovina, Kosovo and the Republic of North Macedonia to highlight key themes and limitations that have emerged. Comparative analysis reveals a surprising divergence of approaches to internalization in the region. The article further demonstrates that the ‘nation-cum-state paradigm’ remains prevalent, despite the premise of universality. It argues that such legislation can play an important symbolic and practical role, but that legal internalization needs to be seen as an ongoing process. It concludes that attention needs to be given to ensuring the continued particularization and adaptation of such legislation in light of both the limitations and changing circumstances, providing a key lesson also for other divided societies.

This paper provides an overview of research on core from Oman Drilling Project Hole BT1B and the surrounding area, plus new data and calculations, constraining processes in the Tethyan subduction zone beneath the Samail ophiolite. The area is underlain by gently dipping, broadly folded layers of allochthonous Hawasina pelagic sediments, the metamorphic sole of the Samail ophiolite, and Banded Unit peridotites at the base of the Samail mantle section. Despite reactivation of some faults during uplift of the Jebel Akdar and Saih Hatat domes, the area preserves the tectonic “stratigraphy” of the Cretaceous subduction zone. Gently dipping listvenite bands, parallel to peridotite banding and to contacts between the peridotite and the metamorphic sole, replace peridotite at and near the basal thrust. Listvenites formed at less than 200°C and (poorly constrained) depths of 25–40 km by reaction with CO 2 ‐rich, aqueous fluids migrating from greater depths, derived from devolatilization of subducting sediments analogous to clastic sediments in the Hawasina Formation, at 400°–500°. Such processes could form important reservoirs for subducted CO 2 . Listvenite formation was accompanied by ductile deformation of serpentinites and listvenites—perhaps facilitated by fluid‐rock reaction—in a process that could lead to aseismic subduction in some regions. Addition of H 2 O and CO 2 to the mantle wedge, forming serpentinites and listvenites, caused large increases in the solid mass and volume of the rocks. This may have been accommodated by fractures formed as a result of volume changes, mainly at a serpentinization front.

Processes for formation, cooling, and altering Earth's ocean crust are not yet completely understood due to challenges in access and sampling. Here, we use contiguous micro‐imaging infrared spectroscopy to develop complete‐core maps of mineral occurrence and investigate spatial patterns in the hydrothermal alteration of 1.2 km of oceanic crust recovered from Oman Drilling Project Holes GT1A, GT2A, and GT3A drilled in the Samail Ophiolite, Oman. The imaging spectrometer shortwave infrared sensor measured reflectance of light at wavelengths 1.0–2.6 μm at 250–260 μm/pixel, resulting in >1 billion independent measurements. We map distributions of nine key primary and secondary minerals/mineral groups—clinopyroxene, amphibole, calcite, chlorite, epidote, gypsum, kaolinite/montmorillonite, prehnite, and zeolite—and find differences in their spatial occurrences and pervasiveness. Accuracy of spectral mapping of occurrence is 68%–100%, established using X‐ray diffraction measurements from the core description. The sheeted dikes and gabbros of upper oceanic crust Hole GT3A show more pervasive alteration and alteration dominated by chlorite, amphibole, and epidote. The foliated/layered gabbros of GT2A from intermediate crustal depths have similarly widespread chlorite but more zeolite and little amphibole and epidote. The layered gabbros of the lower oceanic crust (GT1A) have remnant pyroxene and 2X less chlorite, but alteration is extensive within and surrounding major fault zones with widespread occurrences of amphibole. The results indicate greater distribution of higher temperature alteration minerals in the upper oceanic crust relative to deeper gabbros and highlight the importance of fault zones in hydrothermal convection in the lower ocean crust.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

The solubility of calcite in salt-H2O fluids was determined at 700 degrees C and 8 kbar using hydrothermal piston-cylinder methods. The investigated salts included NaCl, KCl, LiCl, and CsCl. Results show that, at constant pressure and temperature, calcite solubility increases with increasing concentration of any individual salt and all investigated salt mixtures. Data regression indicates a simple dependence of solubility on the square of salt mole fraction. At a given salt concentration, solubility enhancement increases with decreasing salt cation size. Experiments on fluids with mixtures of multiple salts were used to derive simple relations that can be used to predict calcite solubility in a wide range of salt solutions at the studied conditions. The results provide a basis for extending this approach to different pressures, temperatures, and salt compositions.

The circulation of carbon in Earth's interior occurs through the formation, migration, and ascent of CO2-bearing magmas throughout the convective mantle. Their chemical composition spans from carbonatitic to kimberlitic as a result of either temperature and pressure variations or local redox conditions at which partial melting of carbonated mantle mineral assemblages occurs. Previous experiments that focused on melting relations of synthetic CO2-bearing mantle assemblages revealed the stability of carbonate-silicate melts, or transitional melts, that have been generally described to mark the chemical evolution from kimberlitic to carbonatitic melts at mantle conditions. The migration of these melts upward will depend on their rheology as a function of pressure and temperature. In this study, we determined the viscosity of carbonate-silicate liquids (similar to 18 wt% SiO2 and 22.54 wt% CO2) using the falling-sphere technique combined with in situ synchrotron X-ray radiography. We performed six successful experiments at pressures between 2.4 and 5.3 GPa and temperature between 1565 degrees C and 2155 degrees C. At these conditions, the viscosity of transitional melts is between 0.02 and 0.08 Pa's; that is, about one order of magnitude higher than what was determined for synthetic carbonatitic melts at similar P-T conditions, likely due to the polymerizing effect of the SiO2 component in the melt.

Understanding the viscosity of mantle-derived magmas is needed to model their migration mechanisms and ascent rate from the source rock to the surface. High pressure–temperature experimental data are now available on the viscosity of synthetic melts, pure carbonatitic to carbonate–silicate compositions, anhydrous basalts, dacites and rhyolites. However, the viscosity of volatile-bearing melilititic melts, among the most plausible carriers of deep carbon, has not been investigated. In this study, we experimentally determined the viscosity of synthetic liquids with ~31 and ~39 wt% SiO2, 1.60 and 1.42 wt% CO2 and 5.7 and 1 wt% H2O, respectively, at pressures from 1 to 4.7 GPa and temperatures between 1265 and 1755 °C, using the falling-sphere technique combined with in situ X-ray radiography. Our results show viscosities between 0.1044 and 2.1221 Pa·s, with a clear dependence on temperature and SiO2 content. The atomic structure of both melt compositions was also determined at high pressure and temperature, using in situ multi-angle energy-dispersive X-ray diffraction supported by ex situ microFTIR and microRaman spectroscopic measurements. Our results yield evidence that the T–T and T–O (T = Si,Al) interatomic distances of ultrabasic melts are higher than those for basaltic melts known from similar recent studies. Based on our experimental data, melilititic melts are expected to migrate at a rate ~from 2 to 57 km·yr−1 in the present-day or the Archaean mantle, respectively.

Fluids are essential to the physical and chemical processes in subduction zones. Two types of subduction-zone fluids can be distinguished. First, shallow fluids, which are relatively dilute and water rich and that have properties that vary between subduction zones depending on the local thermal regime. Second, deep fluids, which possess higher proportions of dissolved silicate, salts and non-polar gases relative to water content, and have properties that are broadly similar in most subduction systems, regardless of the local thermal structure. We review key physical and chemical properties of fluids in two key subduction-zone contexts-along the slab top and beneath the volcanic front- to illustrate the distinct properties of shallow and deep subduction-zone fluids.

The interaction of fluids with rock-forming minerals plays an important role in the chemical evolution of mafic and ultramafic rocks in the lower crust and upper mantle. Recent work highlights the importance of salt-rich fluids in element transport in settings such as the mantle wedge above subduction zones and high-grade granulite facies metamorphism. Forsterite (Mg2SiO4), enstatite (MgSiO3) and diopside (CaMgSi2O6) are key rock-forming minerals in these settings in the system CaO-MgO-SiO2. We determined experimentally the solubilities of diopside, enstatite and forsterite in H2O-NaCl fluids at a range of pressures and temperatures. Forsterite solubility was determined at 1 GPa, 800 and 900 °C, in pure H2O and in H2O-NaCl solutions. Forsterite dissolved congruently at nearly all conditions. Its solubility in pure H2O is low, but increases greatly with rising NaCl concentration in the fluid. Enstatite solubility was investigated in H2O-NaCl solutions at 1 GPa, 800 and 900 °C. Enstatite dissolved incongruently to yield forsterite at all conditions. Addition of excess silica led to suppression of forsterite and showed that fluids in equilibrium with enstatite with or without forsterite are strongly enriched in Si relative to Mg, though Mg solubility is significant at high salinity. Diopside solubility was determined in pure H2O at 650–900 °C and 0.7–1.5 GPa, and in H2O-NaCl solutions at 800 °C and 1 GPa, with NaCl concentrations approaching halite saturation. Diopside dissolves incongruently yielding residual forsterite at all conditions investigated. The solubility of diopside in pure H2O increases with increasing pressure, temperature and salinity. Diopside dissolution in H2O-NaCl solutions displays a dependence on fluid salinity similar to that of forsterite and wollastonite. The results of forsterite solubility experiments in H2O-NaCl solutions were used to calculate the compositions of fluid coexisting with enstatite or diopside where forsterite was present. The concentration of solutes coexisting with enstatite decreases with rising NaCl, similar to quartz. In contrast, bulk solutes coexisting with diopside increase with NaCl, similar to wollastonite and forsterite. These patterns imply complexing among rock-forming components and fluid components, that Ca-chloride species are substantially more stable than Mg-chloride species, and that hydrous Na-silicate complexes are important components of deep H2O-NaCl fluids. The results show that salt-bearing brines have substantial metasomatic power and may exert significant control on the chemical evolution of lower crustal and upper mantle mafic and ultramafic rocks.

Organic matter, showing variable degrees of crystallinity and thus of graphitization, is an important source of carbon in subducted sediments, as demonstrated by the isotopic signatures of deep and ultra-deep diamonds and volcanic emissions in arc settings. In this experimental study, we investigated the dissolution of sp(2) hybridized carbon in aqueous fluids at 1 and 3GPa, and 800 degrees C, taking as end-members (i) crystalline synthetic graphite and (ii) X-ray amorphous glass-like carbon. We chose glass-like carbon as an analogue of natural ``disordered" graphitic carbon derived from organic matter, because unlike other forms of poorly ordered carbon, it does not undergo any structural modification at the investigated experimental conditions, allowing approach to thermodynamic equilibrium. Textural observations, Raman spectroscopy, synchrotron Xray diffraction and dissolution susceptibility of char produced by thermal decomposition of glucose (representative of nontransformed organic matter) at the same experimental conditions support this assumption. The redox state of the experiments was buffered at Delta FMQ approximate to -0.5 using double capsules and either fayalite-magnetite-quartz (FMQ) or nickel-nickel oxide (NNO) buffers. At the investigated P-T-fO(2) conditions, the dominant aqueous dissolution product is carbon dioxide, formed by oxidation of solid carbon. At 1GPa and 800 degrees C, oxidative dissolution of glass-like carbon produces 16-19 mol% more carbon dioxide than crystalline graphite. In contrast, fluids interacting with glass-like carbon at the higher pressure of 3GPa show only a limited increase in CO2 (fH(2)NNO) or even a lower CO2 content (fH2FMQ) with respect to fluids interacting with crystalline graphite. The measured fluid compositions allowed retrieval of the difference in Gibbs free energy (DG) between glass-like carbon and graphite, which is +1.7(1) kJ/mol at 1GPa-800 degrees C and +0.51(1) kJ/mol (fH(2)NNO) at 3GPa-800 degrees C. Thermodynamic modeling suggests that the decline in dissolution susceptibility at high pressure is related to the higher compressibility of glass-like carbon with respect to crystalline graphite, resulting in G-P curves crossing at about 3.4GPa at 800 degrees(C), close to the graphite-diamond transition. The new experimental data suggest that, in the presence of aqueous fluids that flush subducted sediments, the removal of poorly crystalline "disordered" graphitic carbon is more efficient than that of crystalline graphite. This occurs especially at shallow levels of subduction zones, where the difference in free energy is higher and the availability of poorly organized metastable carbonaceous matter and of aqueous fluids produced by devolatilization of the downgoing slab is maximized. At depths greater than 110 km, the small differences in Delta G imply that there is minimal energetic drive for transforming "disordered" graphitic carbon to ordered graphite; "disordered" graphitic carbon could even be energetically slightly favored in a narrow P interval. (C) 2020 Elsevier Ltd. All rights reserved.