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The geochemical signature of magmas generated at convergent margins greatly depends on the nature of fluids and melts released during subduction. While major- and trace-elements transport capacity of ultrahigh pressure (UH P ) hydrous-silicate melts has been investigated, little is known about solute enrichment and fractionation in UH P (>3.5–4 GPa) solute-rich aqueous fluids released along colder geothermal gradients. Here, we performed in situ LA-ICP-MS trace-element analyses on selected UH P prograde-to-peak fluid inclusions trapped in a kyanite-bearing quartzite from Sulu (China). The alkali-aluminosilicate-rich aqueous fluid released from the meta-sediments by dehydration reactions is enriched in LILE, U, Th, Sr, and REE. Inclusions trapped at increasing temperature (and pressure) preserve a gradual and selective trace-element enrichment resulting from the progressive dissolution of phengite and carbonate and the partial dissolution of allanite/monazite. We show that, at the investigated P-T conditions, aqueous fluids generated by dissolution of volatile-bearing minerals fractionate trace-element distinctly from hydrous-silicate melts, regardless of the source lithology. The orogenic/post-orogenic magmas generated in a mantle enriched by metasomatic processes involving either solute-rich aqueous fluids or hydrous-silicate melts released by the slab at UH P conditions can preserve evidence of the nature of these agents.

Graphite is a critical raw material due to its pivotal role in the green transition; hence, there is a renewed interest in its exploration across Europe. The Chisone and Germanasca Valleys (Piemonte, IT) were home to significant graphite exploitation until the 20th century, owing to the widespread presence of graphite ore bodies hosted in the metasedimentary succession of the Pinerolo Unit in the Dora-Maira Massif (Western Alps). This contribution presents a renewed study on the geology, mineralogy, petrography, and geochemistry of graphite ores and their host rocks, employing OM, SEM-EDS, and BSE, μRaman, and ICP-OES/MS and INAA analyses. Mineralization occurs in two metasedimentary successions: (i) the Bourcet-type succession (meta-conglomerates and meta-sandstones intercalated with meta-siltstones/metapelites) and (ii) the Pons-type succession (meta-siltstones/metapelites intercalated with minor meta-arenites). Graphite occurs as (i) high-purity, fine-grained crystals dispersed within or concentrated in layers along the regional schistosity, or (ii) low-purity, coarse-grained crystals within shear zones. Based on crystallinity, three types of graphite were distinguished: high (Type I), intermediate (Type II), and poor (Type III) crystalline graphite, likely formed under different genetic conditions. The comparison of these findings has implications for future exploration and provides new insights into the metallogeny and geological evolution of the area.

The long-term carbon budget has major implications for Earth’s climate and biosphere, but the balance between carbon sequestration during subduction, and outgassing by volcanism is still poorly known. Although carbon-rich fluid inclusions and minerals are described in exhumed mantle rocks and xenoliths, compelling geophysical evidence of large-scale carbon storage in the upper mantle is still lacking. Here, we use a geophysical surface-wave seismic tomography model of the mantle wedge above the subducted European slab to document a prominent shear-wave low-velocity anomaly at depths greater than 180 km. We propose that this anomaly is generated by extraction of carbonate-rich melts from the asthenosphere, favoured by the breakdown of slab carbonates and hydrous minerals after cold subduction. The resulting transient network of carbon-rich melts is frozen in the mantle wedge without producing volcanism. Our results provide the first in-situ observational evidence of ongoing carbon sequestration in the upper mantle at a plate-tectonic scale. We infer that carbon sequestered during cold subduction may partly counterbalance carbon outgassed from ridges and oceanic islands. However, subducted carbon may be rapidly released during continental rifting, with global effects on long-term climate trends and the habitability of planet Earth.

Granulites and associated dykes from the less well-studied southern Ivrea–Verbano Zone (around Ivrea town) are characterized by combining field, macro, micro and chemical (major and trace-element mineral composition) data to identify chemical and rheological variations in the lower crust that could be relevant for geodynamic implications. The Ivrea granulites are similar to those in the Lower Mafic Complex of the central Ivrea–Verbano Zone. The mafic lithologies experienced stealth metasomatism (pargasitic amphibole and An-rich plagioclase) that occurred, at suprasolidus conditions, by a pervasive reactive porous flow of mantle-derived orogenic (hydrous) basaltic melts infiltrated along, relatively few, deformation-assisted channels. The chemical composition of the metasomatic melts is similar to that of melts infiltrating the central and northern Ivrea–Verbano Zone. This widespread metasomatism, inducing a massive regional hydration of the lowermost Southalpine mafic crust, promoted a plastic behavior in the lowermost part of the crust during the Early Mesozoic and, ultimately, the Triassic extension of the Variscan crust and the beginning of the Alpine cycle.

Antigorite dehydration is a process able to release, in comparison with other minerals, the highest amount of H2O from a subducting slab. The released fluid delivers critical elements (e.g., S, Cu, and REE) to the overlying subarc mantle, modifying the mantle source of arc magmas and related ore deposits. Whether antigorite breakdown produces oxidising or reducing fluids is debated. Whereas previous studies have investigated antigorite dehydration in serpentinites (i.e., in a (C)AMFS-H2O system), this contribution is devoted to the CMFS-COHS carbonate system, which is representative of the metacarbonate sediments (or carbonate-dominated ophicarbonate rocks) that sit atop the slab. Thermodynamic modelling is used to investigate the redox effect of the carbonate-buffered antigorite dehydration reactions (i.e., brucite breakdown and antigorite breakdown) on electrolytic fluid geochemistry as a function of P-T-fO2. The influence of P-T-fO2 conditions on the solubility of C and S, solute-bound H2 and O2, fluid pH, the average valence states of dissolved C and S, and the fluid redox budget indicates that, in metacarbonate sediments, the CaCO3+antigorite reaction tends to produce reducing fluids. However, the redox state of such fluids is buffered not only by the redox state of the system but also, most importantly, by concomitantly dissolving redox-sensitive minerals (i.e., carbonates, graphite, pyrite, and anhydrite). A qualitative correlation between the redox state of the system and the possible depth of fluid release into the mantle wedge is also derived.

Carbon dragged at sub-arc depths and sequestered in the asthenospheric upper mantle during cold subduction is potentially released after millions of years during the breakup of continental plates. However, it is unclear whether these deep-carbon reservoirs can be locally remobilized on shorter-term timescales. Here we reveal the fate of carbon released during cold subduction by analyzing an anomalously deep earthquake in December 2020 in the lithospheric mantle beneath Milan (Italy), above a deep-carbon reservoir previously imaged in the mantle wedge by geophysical methods. We show that the earthquake source moment tensor includes a major explosive component that we ascribe to carbon-rich melt/fluid migration along upper-mantle shear zones and rapid release of about 17,000 tons of carbon dioxide when ascending melts exit the carbonate stability field. Our results underline the importance of carbon-rich melts at active continental margins for emission budgets and suggest their potential episodic contributions to atmospheric carbon dioxide.

In deep and cold subduction such as that experienced by the UHP Units of the Western Alps, carbon dissolution is a relevant mechanism for carbon transfer from the slab into the mantle. The UHP impure Cal-Dol-marbles from the Dora-Maira Massif are studied to investigate the poorly known evolution of dolomite during deep subduction. Dolomite shows four stages of growth, from pre-Alpine to early-retrograde Alpine, coupled with chemical variations and distinct included mineral assemblages. To explain the evidence for growth and partial reabsorption of dolomite through HP prograde, UHP peak, and UHP early-retrograde Alpine metamorphism, a chemically simple marble (Cal, Dol, Di, Fo, and retrograde Atg, Tr, Mg-Chl) has been studied in detail. Microstructural relationships, coupled with mineral chemistry, indicate the growth of the assemblage dolomite+diopside+forsterite±aragonite during HP prograde, UHP peak, and UHP early-retrograde evolution. Mixed-volatile P-T projection modeled in the simple CaO-(FeO)-MgO-SiO2-H2O-CO2 system and T-P-XCO2 petrogenetic grids and pseudosections predict the prograde (1.7 GPa, 560°C) growth of dolomite in equilibrium with diopside and forsterite through the breakdown of antigorite+aragonite. In a H2O-CO2-saturated system, the subsequent HP-UHP evolution is predicted in the Di+Fo+Dol+Arg stability field in equilibrium with a dominantly aqueous COH fluid [0.000326.3 wt% of NaCleq) COH fluid, containing Ca, Mg, and Si as dissolved cations was present during the growth of the UHP assemblage Dol+Cpx+Ol+Arg. The complex zoning of dolomite is therefore interpreted as due to protracted episodes of dissolution and precipitation in saline aqueous fluids at HP/UHP conditions. Kinetics of dolomite dissolution in aqueous fluids is poorly known, and experimental and thermodynamic data under HP conditions are still lacking. Data on calcite indicate that dissolution at HP is enhanced by a prograde increase in both P and T, by high salinity in aqueous fluids, and/or low-pH conditions. In the studied marble, the P-T path and the occurrence of free high-saline fluids represent favorable conditions: (1) for the inferred dissolution-precipitation processes of the stable dolomite in a closed system, and (2) for possible migration of the dissolved carbonate, if the system would have been open during subduction.

Granulites and associated dykes from the less well-studied southern Ivrea–Verbano Zone (around Ivrea town) are characterized by combining field, macro, micro and chemical (major and trace-element mineral composition) data to identify chemical and rheological variations in the lower crust that could be relevant for geodynamic implications. The Ivrea granulites are similar to those in the Lower Mafic Complex of the central Ivrea–Verbano Zone. The mafic lithologies experienced stealth metasomatism (pargasitic amphibole and An-rich plagioclase) that occurred, at suprasolidus conditions, by a pervasive reactive porous flow of mantle-derived orogenic (hydrous) basaltic melts infiltrated along, relatively few, deformation-assisted channels. The chemical composition of the metasomatic melts is similar to that of melts infiltrating the central and northern Ivrea–Verbano Zone. This widespread metasomatism, inducing a massive regional hydration of the lowermost Southalpine mafic crust, promoted a plastic behavior in the lowermost part of the crust during the Early Mesozoic and, ultimately, the Triassic extension of the Variscan crust and the beginning of the Alpine cycle.

Antigorite dehydration is a process able to release, in comparison with other minerals, the highest amount of H[sub.2] O from a subducting slab. The released fluid delivers critical elements (e.g., S, Cu, and REE) to the overlying subarc mantle, modifying the mantle source of arc magmas and related ore deposits. Whether antigorite breakdown produces oxidising or reducing fluids is debated. Whereas previous studies have investigated antigorite dehydration in serpentinites (i.e., in a (C)AMFS-H[sub.2] O system), this contribution is devoted to the CMFS-COHS carbonate system, which is representative of the metacarbonate sediments (or carbonate-dominated ophicarbonate rocks) that sit atop the slab. Thermodynamic modelling is used to investigate the redox effect of the carbonate-buffered antigorite dehydration reactions (i.e., brucite breakdown and antigorite breakdown) on electrolytic fluid geochemistry as a function of P-T-fO[sub.2] . The influence of P-T-fO[sub.2] conditions on the solubility of C and S, solute-bound H[sub.2] and O[sub.2] , fluid pH, the average valence states of dissolved C and S, and the fluid redox budget indicates that, in metacarbonate sediments, the CaCO[sub.3] +antigorite reaction tends to produce reducing fluids. However, the redox state of such fluids is buffered not only by the redox state of the system but also, most importantly, by concomitantly dissolving redox-sensitive minerals (i.e., carbonates, graphite, pyrite, and anhydrite). A qualitative correlation between the redox state of the system and the possible depth of fluid release into the mantle wedge is also derived.

Summary Petrographic, minerochemical, and geothermobarometric data are reported for a suite of spinel-lherzolite ± pargasite xenoliths hosted in a Quaternary basanitic lava flow from the North-Western Ethiopian Plateau (Injibara, Lake Tana Province). Protogranular to porphyroclastic ( deformed ) rocks show evidence of a modal metasomatism, represented by a Cl-rich pargasitic amphibole, coupled with cryptic enrichment in Fe and Al. Equigranular rocks ( granular ) record a further cryptic metasomatism, represented by enrichment in Fe, Al, Na, and depletion in Ni, Cr and Cl. Some xenoliths ( transitional ) show intermediate textural and compositional characters, indicating that the granular samples represent an evolution of the deformed ones. All xenoliths give the same P–T equilibration conditions for Opx-Cpx pairs (947–1015 °C and 1.3–2.0 GPa), but in granular samples, recrystallised olivine and spinel record T about 100 °C higher. Two distinct metasomatic processes, probably connected with the emplacement of the Afar plume, are proposed. The first one is a pervasive modal metasomatism produced by water-rich fluids. The latter is a non-pervasive cryptic metasomatism, probably connected to migration of melts. The comparison the mantle beneath the Ethiopian Volcanic Plateau, the southern Main Ethiopian Rift and the central Main Ethiopian Rift suggests spatial heterogeneity of the mantle and variable mantle processes during asthenospheric upwelling.