Search

Subducted sections of dry oceanic mantle can undergo serpentinization through interaction with metamorphic fluids, recording a sequence of serpentinization events from the subseafloor to serpentine-out reactions. The geochemical patterns of these different stages of fluid-rock interaction trace the context and mechanisms of serpentinization processes. In this study, we used petrographic observations, micro-Raman spectroscopy, as well as bulk and in situ trace element analyses across a serpentinization front adjacent to a subduction-related tectonic contact in the Monte Maggiore massif, Alpine Corsica, France. Using a high-density sampling approach throughout the entire massif, we identified multiple and consistently distributed serpentine generations, reflecting successive hydration events in a chemically open system. Fluid-mobile elements (FME) exhibit a slight increase from lizardite- to antigorite-dominated generations and a more substantial rise with increasing serpentinization degrees towards the tectonic contact. When compared to other serpentinization fronts originated at different geodynamic settings – e.g., Semail Ophiolite (Oman) as a subduction serpentinization front; Lanzo Massif (W. Alps) as a subducted oceanic serpentinization front –, the Monte Maggiore FME patterns indicate a subduction-related serpentinization front, rather than subducted oceanic serpentinites. Our results and the comparison with literature data suggest that decreasing in situ serpentine FME concentrations across serpentinization fronts may be characteristic of subduction-zone serpentinization, whereas opposite trends may be diagnostic of subducted oceanic fronts. Our study also highlights the importance of tracing serpentinization as a spatially evolving process (across fronts), with least serpentinized rocks recording the latest stage(s) of serpentinization instead of relicts of incipient serpentinization.

Geodynamic models implying subduction of continental crust either consider this process happening during collision, when the continental margin of the lower plate attempts subduction, or in pre-collisional stages, when tectonic erosion of the upper plate or subduction of continental extensional allochthons drag continental crust in the subduction channel. In the Zagros orogen (W Iran), high-pressure rocks are known only from the Sanandaj-Sirjan Zone, NE of the Main Zagros Thrust. Here, eclogites of the North Shahrekord Metamorphic Complex suggest subduction of continental crust slices derived from the upper plate (Central Iran) during the onset of the Neo-Tethys subduction along the southern margin of Iran. Eclogites record a clockwise pressure-temperature-time path, with pre-eclogitic epidote-amphibolites-facies phase assemblages preserved in garnet cores, a high-pressure stage, and a subsequent retrogression at amphibolite-facies conditions. By means of forward thermodynamic modelling and 40Ar/39Ar geochronology, the peak metamorphism has been constrained at 1.9-2.1 GPa and 550-600 °C, in the 191-194 Ma time span. The following retrogression during exhumation lasted at least until 144 Ma. Our data suggest that the onset of the Neo-Tethys subduction traces back prior to 190 Ma, involving together with the Neo-Tethys oceanic lithosphere also slices of the upper plate continental crust scraped off by means of tectonic erosion processes.

The water-mediated mineral carbonation represents a promising solution for the capture and the storage of atmospheric CO2. Even though this reaction might be spontaneous for a number of Mg- and Ca-rich mineral phases, it is characterized by considerable activation barriers. In order to make it effective, associated energy costs related to the achievement of adequate reaction conditions must be minimized. Microwave chemistry is known to provide for substantial increments of the reaction rate for several systems. We applied here microwave chemistry to the process of carbonation of aqueous slurries of brucite, a model system of Mg-rich mineral, subjected to partial pressures of CO2 as low as 6 bar and to no other additive. The temperature of the reactor was finely varied while the radiation power and the reactor pressure were monitored in real-time. The radiation power was used to estimate the radiation energy budget needed to complete the carbonation process, whereas the reactor pressure was used as a proxy of reaction progression. We show a detailed evolution of the carbonate products obtained in terms of mineral phases, morphological properties, and degree of crystallinity, both as precipitate and as solid residue in the exsiccated supernatant reaction liquid.

Understanding the interactions between surface and deep Earth processes is important for research in many diverse scientific areas including climate, environment, energy, georesources and biosphere. The TOPO-EUROPE initiative of the International Lithosphere Program serves as a pan-European platform for integrated surface and deep Earth sciences, synergizing observational studies of the Earth structure and fluxes on all spatial and temporal scales with modelling of Earth processes. This review provides a survey of scientific developments in our quantitative understanding of coupled surface-deep Earth processes achieved through TOPO-EUROPE. The most notable innovations include (1) a process-based understanding of the connection of upper mantle dynamics and absolute plate motion frames; (2) integrated models for sediment source-to-sink dynamics, demonstrating the importance of mass transfer from mountains to basins and from basin to basin; (3) demonstration of the key role of polyphase evolution of sedimentary basins, the impact of pre-rift and pre-orogenic structures, and the evolution of subsequent lithosphere and landscape dynamics; (4) improved conceptual understanding of the temporal evolution from back-arc extension to tectonic inversion and onset of subduction; (5) models to explain the integrated strength of Europe's lithosphere; (6) concepts governing the interplay between thermal upper mantle processes and stress-induced intraplate deformation; (7) constraints on the record of vertical motions from high-resolution data sets obtained from geo-thermochronology for Europe's topographic evolution; (8) recognition and quantifications of the forcing by erosional and/or glacial-interglacial surface mass transfer on the regional magmatism, with major implications for our understanding of the carbon cycle on geological timescales and the emerging field of biogeodynamics; and (9) the transfer of insights obtained on the coupling of deep Earth and surface processes to

We disclose the crystallisation evolution of magnetite-bearing multiphase inclusions hosted in metamorphic olivine of harzburgites from the Cerro de Almirez (Betic Cordillera, Spain), which have been interpreted as final products of the trapping of the aqueous fluid produced by the subduction-zone dehydration of former serpentinites. The chemical exchange between inclusion fluid and olivine started soon after entrapment, at peak P-T conditions of 1.6–1.9 GPa and 650–700 °C, and continued during cooling along the retrograde path, with the coexistence of olivine and magnetite with orthopyroxene, chlorite, talc, antigorite and the destabilisation of olivine and antigorite into brucite and low-temperature chrysotile serpentine, as recognised by Raman analyses. Thermodynamic modelling and mass balance calculations demonstrate that the water component of fluid trapped in the inclusions of metamorphic olivine is expected to trigger the oxidation of the fayalite component in olivine, producing a mineral assemblage made of magnetite + orthopyroxene and molecular hydrogen, where the elemental redox processes are Fe2+ of olivine that oxidizes to Fe3+ and H+ of water that reduces to H2. Probable H2 trapped in the olivine host close to the inclusion wall has been detected by Raman spectroscopy. To corroborate its presence, we performed quantitative mass spectrometry analyses of the fluid phase trapped in the multiphase inclusions and of the olivine crystals hosting the inclusions, revealing that 1 kg of olivine matrix contains 6.2 ± 0.1 mmol of H2. We identify two synergistic driving forces of the whole process, which has the peculiarity to produce molecular hydrogen at apparently oxidising conditions: i) the building up of an epitaxial interface between olivine and magnetite, and ii) the olivine ability to trap H2 at high pressures. The olivine + H2O system of these natural microreactors simulates a process of oxidation of the mantle olivine by water, with production of H2 at p

Silicate nanoparticles occur in various astrophysical environments where they experience substantial processing due to events such as grain-grain collisions and irradiation. However, the structure and chemical evolution together with the origin of these grains are still poorly understood and intensively debated. For this purpose, we performed liquid-phase nanosecond pulsed laser ablation on olivine single crystals to (i) simulate space weathering in a water environment (e.g., hydrous or volatile-rich bodies) and (ii) study the chemical and structural evolution of both the target surface and the ablated material. In particular, optical spectroscopy analyses have been performed on the ablated material and correlated with high-resolution transmission electron microscopy and diffraction, whereas compositional variations of the ablated target surface were determined by X-ray photoelectron spectroscopy. Our results show that the target material is enriched in Fe and depleted in Mg after the ablation process, with the water environment triggering the oxidation of Fe2+ into Fe3+ in a region confined at the solid-liquid interface and thus promoting the formation of magnetite on the sample surface. On the other hand, in the ablated material we find olivine crystalline fragments with shock features together with Mg-rich crystalline nanoparticles. Notably, no metallic iron nanoparticles have been detected in the ablated material. Our simulation of space weathering in the water environment revealed structural and chemical changes which are expected to give rise to distinctive features in the reflectance spectra when compared to those from airless bodies of the inner solar system.

The formation of stable and insoluble Mg- and Ca-carbonates and/or hydroxy-carbonate hydrates, through water-mediated reaction of Mg(OH)2 (brucite) and Ca(OH)2 (portlandite) with carbon dioxide, is referred to as mineral carbonation (MC). MC is among the most promising potential solutions for long-term carbon capture and storage (CCS), since it is spontaneous under a wide range of conditions. However, kinetic barriers pose severe limitations to the practical exploitation of MC, implying energy requirements to reach sufficiently high reaction rates. Trying to overcome these hindrances, we show here the application of microwave (MW)-assisted processes for the carbonation of brucite, used as a model system for the carbonation occurring in a number of widely diffused minerals, such as serpentine. The mechanism, kinetics, and energy costs of the reaction, together with the chemical characteristics of the products obtained, are inferred by a combined study, carried out by both a MW synthesizer on bulk brucite water slurries and a purposedly developed microscopy platform, working on nano-scale regions of the surface of brucite single crystals.

Mineral Carbonation (MC) is included among the most promising potential solutions for long-term carbon capture and storage (CCS). This mineral treatment consists in the water-mediated reaction of the CaO and MgO components of minerals with carbon dioxide, with formation of stable and insoluble Ca- and Mg-carbonates and/or hydroxy-carbonate hydrates. Noteworthy, these latter compounds constitute also valuable secondary raw materials employed, e.g., in the construction sector. Despite the carbonation reaction is spontaneous under a wide range of conditions, kinetic barriers pose sever limitations for its practical exploitation. High fractions of MgO are available in serpentine minerals, characterized by alternated layers of silica [SiO2] and brucite [Mg(OH)2], quarried in a number of sites in Europe, with a relevant role played by Italy. This industrial activity is characterized by the production of a fraction as large as 50% of waste in the form of serpentine pebble and powder, representing potential reservoirs for fixing a quantity as large as 20 ktons of CO2 per site/per year in the form of re-utilizable Mg-carbonates. With the scope to contribute in making this possibility a reality, we show here the application of microwave (MW)-assisted processes for the in-depth study of the carbonation of brucite, representing a model systems for the reactive component of serpentine. MW reactors allow for a fine tuning of the temperature, pressure and irradiation energy, enabling us to infer the mechanism, kinetics, and energy costs of the reaction and to identify the chemical characteristics of the products.

The Borgo outcrop of the Monte Duria Area (Adula-Cima Lunga unit, Central Alps, Italy) is an excellent example of melt-peridotite interaction which occurred under a deformation regime at high pressure, that enabled the combination of porous and focused flow of eclogite-derived melts into garnet peridotites. Migmatised eclogites are in direct contact with retrogressed garnet peridotites and the contact is marked by a tremolitite layer, also occurring as boudins parallel to the garnet layering in the peridotites, derived from a garnet websterite precursor produced by the interaction between eclogite-derived melts with the peridotite at high pressure. LREE concentrations of tremolitite along a 120 m length profile, starting from the eclogite-peridotite contact to the inner part of the peridotite, show a progressive enrichment coupled with a peculiar fractionation. Numerical modelling assuming the eclogitic leucosome as the starting percolating melt reproduces the REE enrichment and LREE/HREE fractionation observed in tremolitite bulk rocks within the first 30 m. The comparison between the REE composition of the retrogressed garnet websterites along the profile and the result of our model suggests that reactive melt infiltration at high pressure is a plausible mechanism to modify the REE budget of mantle peridotites that lie on top of the subducting crustal slab.

Magnetite microinclusions in metamorphic harzburgites, derived from the deserpentinization of the subducted hydrated oceanic lithospheric mantle, were examined by synchrotron Mössbauer spectroscopy to investigate the chemical and magnetic environments of the Fe nuclei. The data reveal a critical susceptibility of the octahedral sites of the cubic structure of magnetite to chemical variations, which, in turn, influences their magnetic properties in terms of hyperfine magnetic field intensity and direction. Micromagnetites display substantial remanent magnetization; however, the magnetic moment direction can be significantly different among inclusions, even for those in close spatial proximity. This evidence points to a kinetic control of the composition of microcavities at mantle depths, implying that the use of the remanent magnetic field of included magnetic phases to infer large-scale implications on the Earth's magnetic field requires the development of complex geochemical and geodynamical models.