Mantle-Derived Corundum-Bearing Felsic Dykes May Survive Only within the Lower (Refractory/Inert) Crust: Evidence from Zircon Geochemistry and Geochronology (Ivrea–Verbano Zone, Southern Alps, Italy)

Corundum-rich (up to 55 vol.%) felsic dykes formed with albite, +/- K-feldspar, +/- hercynite and +/- biotite-siderophyllite cut the lower crustal rocks exposed in the Ivrea–Verbano Zone (NW Italy). Zircon is an abundant accessory mineral and its investigation through laser ablation-inductively coupled plasma (multi-collector)-mass spectrometer (LA-ICP-(MC)MS) has allowed results to directly constrain the timing of emplacement, as well as petrology and geochemistry of parental melts. Zircons are characterized by very large concentration in rare earth elements (REE), Th, U, Nb and Ta, and negative Eu anomaly. U–Pb analysis points to Norian emplacement ages (223 +/- 7 Ma and 224 +/- 6 Ma), whereas large positive EHf(t) values (+13 on average) indicate a derivation from depleted to mildly enriched mantle source. The mantle signature and the corundum oversaturation were preserved thanks to limited crustal contamination of the host, high-temperature refractory granulites and mafic intrusives. According to the geochemical data and to the evidence of the development of violent explosions into the conduits, it is proposed that dykes segregated from peraluminous melts produced by exsolution processes affecting volatile-rich differentiates during alkaline magmatism. This work provides robust evidence about the transition of the geochemical affinity of Southern Alps magmatism from orogenic-like to anorogenic during Norian time, linked to a regional uprising of the asthenosphere and change of tectonic regime

Corrigendum: Experimental dissolution of carbonaceous materials in water at 1 GPa and 550°C: assessing the role of carbon forms and redox state on COH fluid production and composition during forearc subduction of organic matter

Biogenic carbonaceous material (CM) is the main carrier of organic carbon in the subduction zone and contributes to COH fluid production and volcanic arc gaseous emissions. Here we investigated the effect of the structural, textural and chemical heterogeneity of CM on its reactivity and redox dissolution by conducting short-lived (1 h) experiments, where synthetic analogues of CM (ordered graphite, graphite oxide (GO), mesoporous carbon (MC), Vulcan® carbon (VC) and glass-like carbon (GC)), are reacted with water at P = 1 GPa and T = 550°C—conditions typical of a warm forearc subduction—and fO2 buffered from ΔFMQ ≈ +4 to −7. We show that the amount of dissolved CM (CMdissolved) and the proportion of volatile carbon species (Cvolatile) in the fluid is related both to the structure and the peculiar surficial properties of the carbon forms, such as carbon sp2-and sp3-hybridization, amount of oxygen heteroatoms, presence of oxygenated functional groups (OFGs) and of active sites. MC and graphite (C(sp2) > 93 at%, O dissolved 2 + CO and CO2 + CH4 Cvolatile mixtures at ΔFMQ ≈ +4 and −7, respectively), while the latter has a maximum of Cvolatile production (CO2 + CH4) at ΔFMQ ≈0, which is not observed in a 10-day long run; partly-ordered GO (C(sp3) > 98 at%, O ∼31 at%, OFGs ∼41 at%) is the most reactive material at all redox conditions (CMdissolved > 2.6 mol%) and produces CO2 as the dominant Cvolatile species; disordered GC and VC (C(sp3) dissolved ∼1 mol%) and ΔFMQ ≈ –7 (CMdissolved > 1 mol%), where Cvolatile is dominantly CO2 and CH4, respectively. Besides the significant deviations from thermodynamically predicted graphite-saturated COH fluid composition and speciation, our results suggests that: 1) immature CM (disordered, rich in C(sp3), O, OFGs) is preferentially dissolved under high fluid fluxes and may buffer fluids to rather oxidizing conditions; 2) a descending flux of oxygen (and hydrogen) bond to CM may exist.</p

Effect of oxygen functionalities on the hydrous hydrazine decomposition over carbonaceous materials

Metal-free heterogeneous catalysis is promising in the context of H2 generation. Therefore, establishing structure-activity relationships is a crucial issue to improve the development of more efficient catalysts. Herein, to evaluate the reactivity of the oxygen functionalities in carbonaceous materials, functionalized commercial pyrolytically stripped carbon nanofibers (CNFs) were used as catalysts in the liquid-phase hydrous hydrazine decomposition process and compared its activity to the one of pristine CNF material. Different oxygenated groups were inserted by treating CNFs with hydrogen peroxide for 1h (O1-H2O2) and HNO3 for 1h (O1-HNO3) and 6h (O6-HNO3). An increase in activity was observed as a function of the oxidizing agent strength (HNO3 > H2O2) and the functionalization time (6h > 1h). A thorough characterization of the catalysts demonstrated that the activity could be directly correlated with the oxygen content (O6-HNO3 > O1-HNO3 > O1-H2O2 > CNFs) and pointed out the active sites for the reaction at carbon-oxygen double bond groups (C=O and COOH). Systematic DFT calculations supported rationalizing the experimental kinetic trends with respect to each oxygen group (C=O, C-O-C, C-OH and COOH)

Wetting angles of hydrous carbonatitic liquids and reversal in wettability for silicate and carbonatitic magmas in the mantle

Carbon-bearing solids, fluids, and melts in the Earth’s deep interior play an important role in the long-term carbon cycle. Carbonatite magmas have been suggested as important agents of mantle metasomatism and yet, their physical features are expected to control the mobility from the source region to shallow Earth. The mobility and infiltration rates of carbonatitic melts, together with their influence on the annealing of mantle peridotites are poorly constrained processes. Although natural carbonatitic melts are complex chemical systems with C-O-H species as a major component, previous work has been performed in anhydrous model systems. Here we present a quantitative laboratory simulation of variables and processes controlling the ascent, mobility and connectivity of carbonatites in a model mantle material investigating the dihedral angle of hydrous carbonatitic liquids. We aim at comparing the texturally equilibrated volume proportions of volatile-rich carbonatitic melts with silicate melts in a partially molten peridotite, and we examine whether carbonatitic liquids are always more wetting than silicate melts. The infiltration experiments were performed employing an end loaded piston-cylinder apparatus, at T= 1200°C and P = 2.5 GPa to investigate the percolation of carbonatitic liquids and interconnectivity of melt pockets in a peridotitic matrix. Hydrous carbonatitic melt pockets were found along olivine grain boundaries; image analysis on electron back scattered and X-ray maps allow us quantifying the apparent dihedral angles between the liquid and olivine and to calculate the grain boundary wetness. Experiments performed at 5 wt.% of water contents result in dihedral angles evolving from ~31° to ~41° with a volume of liquids from 2 to 10 vol.%, while experiments carried out at 30wt. % of water content show a dihedral angle values of almost 50° with a range of volume infiltrated melts between 4 to 9vol.%. These results indicate that dihedral angles progressively increase with increasing water dissolved from 25°-28° in anhydrous carbonatitic liquids up to 50° in water-rich carbonatitic liquids, and, as expected, the volume of interstitial liquid decreases with water increasing. The increase of wetting angles is representative of a sintering process of the solid matrix, which evolves with time in the development of channels of pores, as highlighted relating the grain boundary wetness with fraction of liquid infiltrated. We suggest that the low grain boundary wetness measured may be due to a relatively low melt-rock interfaces which develop with channelized liquid, and that channelization is promoted by chemical gradient, as established by a carbonatitic segregate in the silicate matrix. If H2O is available, we expect that H2O strongly partitions into carbonatitic liquids. As a result, their dihedral angle may evolve up to 50°, a value which is significantly higher than that characterizing silicate melts at similar mantle conditions

The Cogne magnetite deposit (Western Alps, Italy): A Late Jurassic seafloor ultramafic-hosted hydrothermal system?

The Cogne magnetite deposit (Western Alps, Italy) is the largest in a series of apatite and sulphide-free magnetite orebodies that are hosted in serpentinites belonging to western Alpine ophiolitic units. The nearly endmember composition of magnetite, which is unusual for an ultramafic setting, and the relatively high tonnage of the deposit (18*10^6 tonnes at 45\u201350 wt% Fe) make Cogne an intriguing case study to explore magnetite-forming processes in ophiolites. The Cogne magnetite shows variable textures, including nodular ores, veins and fine-grained disseminations in serpentinites after mantle peridotites and totally serpentinized melt-impregnated peridotites (troctolites). An increase in Co/Ni ratio from magnetite-poor serpentinized peridotites (0.05) to nodular magnetite ores (>1) is observed. Trace element analyses of magnetite from different sites and lithologies by laser-ablation inductively-coupled mass spectrometry indicate that magnetites have typical hydrothermal compositions, characterized by high Mg and Mn (median values up to 24,100 and 5000 ppm, respectively), and low Cr, Ti and V (median values up to 30, 570 and 60 ppm, respectively). Moreover, the variations in trace element compositions distinguish magnetite that is hydrothermal fluid-controlled [highest (Mg, Mn, Co, Zn)/Ni ratios] from magnetite whose composition is affected by host-rock chemistry (highest Ni \ub1 Ti \ub1 V). U-Th-Pb dating of magnetite-associated uraninite constrains the formation of the deposit to the Late Jurassic (ca. 150 Ma), during an advanced stage of the opening of the Alpine Tethys. Thermodynamic modelling of fluid-rock interactions indicates that fluids produced by seawater\u2013peridotite or seawater\u2013Fe-gabbro are not sufficiently Fe-rich to account for the formation of the Cogne deposit. This suggests that fractionation processes such as phase separation were critical to generate hydrothermal fluids capable to precipitate large amounts of magnetite in various types of ultramafic host-rocks. The oceanic setting and geochemical and mineralogical similarities with some modern ultramafic-hosted volcanogenic massive sulphide deposits on mid-ocean ridges suggest that the exposed mineralized section at Cogne may represent the deep segment of a seafloor, high-temperature (300\u2013400 C) hydrothermal system. The occurrence of similar magnetite enrichments in present-day oceanic settings could contribute to explain the presence of significant magnetic anomalies centred on active and inactive ultramafic-hosted hydrothermal fields