Geology of the southern Monviso metaophiolite complex (W-Alps, Italy)

The Monviso metaophiolite complex (W. Alps) is an almost intact fragment of Tethyan oceanic lithosphere metamorphosed to eclogite-facies peak metamorphic conditions during Alpine subduction. This 1:20.000 scale geological map encompasses, in an area of ∼35 km2, the Monviso Unit (MU) and the Lago Superiore Unit (LSU). Major focus was given to the Lower Shear Zone sub-unit (LSZ), where in the strongly deformed serpentinite-rich matrix are embedded blocks of variably brecciated metagabbros. Here, the occurrence of eclogite-facies mylonitic foliation (paragenesis: omphacite + rutile + garnet ± ex-lawsonite ± quartz) cut by breccia planes (cemented by omphacite + garnet ± ex-lawsonite) indicates brecciation at pristine eclogitic conditions. This map (i) provides new lithological, structural and morphological insights regarding the stratigraphy of the Monviso metaophiolite complex and (ii) supplies an unprecedented detail on the distribution of eclogite-facies breccia blocks inside the Lower Shear Zone that crosscuts the Lago Superiore Unit

DFT Simulation of the X-Ray Emission and Absorption Spectra for Different Lithium Compounds

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Calculation of emission spectra of lithium compounds

International audienceAbstract We studied the electronic structure of lithium metal, lithium fluoride, and spodumene using density functional theory. We performed calculations of the total, local and partial density of states, and x‐ray emission spectra. We observed changes in the Li K spectra shape and energy position due to the chemical structure and composition modification. We also outline possible interferences between emission bands

Massive formation of lawsonite in subducted sediments from the Schistes Lustrés (W. Alps): Implications for mass transfer and decarbonation in cold subduction zones

International audienceThis study investigates the reactions allowing crystallization of large amounts of lawsonite (CaAl 2 Si 2 O 7 (OH) 2 • [H 2 O]) found in calcschists metamorphosed in a subduction zone setting. Previous studies of lawsonite-forming reactions in metasediments have highlighted its importance for the large-scale budget of CO 2 , as the calcium required to form lawsonite is thought to originate from decarbonation reactions. Yet, thermodynamic modelling as well as isotopic measurements have indicated that 80 to 90% of the carbon is retained in sediments, and there is no evidence of major decarbonation in the field. As lawsonite contains abundant H 2 O and has a large stability field, understanding its crystallization is also important to assess fluid migration and mass transfer in a critical part of the subduction system where slow earthquakes are nucleating (such as low frequency earthquakes or episodic tremor and slip). The upper units of the blueschist facies metasediments of the Schistes Lustrés complex (Western Alps), buried to depths of~30-40 km, have been selected as an ideal case study as they host up to 40 vol% of lawsonite. Lawsonite is found crystallized over several generations, in veins and in reactions fronts as well as in the rock matrix. Three types of lawsonite were identified. The most abundant type of lawsonite is associated with quartz and ankerite. This assemblage formed from phyllosilicates and calcite in a continuous reaction: chlorite + calcite + kaolinite = lawsonite + ankerite + quartz + H 2 O. According to thermodynamic modelling, this reaction is restricted to a narrow pressure-temperature domain and initiates around 180°C and 0.4 GPa. Lawsonite is also observed and predicted to grow from Fe-Mg-carpholite at higher metamorphic conditions. None of these reactions allow efficient net export of carbon, as one carbonate replaces another, and most observations are consistent with closed-system behaviour at outcrop-scale, in agreement with geochemical studies. At sample-scale and below, crystallization of lawsonite is linked to homogenization of carbonate and pelitic domains in geologically fast reactions. Dissolution of calcite produces reactive fluids prone to react with pelitic domains and crystallize lawsonite and another carbonate. Although metamorphic veins are ubiquitous to the upper units of the Schistes Lustrés Complex, most of them result from local reactions and do no indicate large-scale mass transfer

Chloritization of granites in shear zones: an open window on fluid pathways, equilibrium length-scales and porosity formation down to nanoscale

International audienceStrain localisation in the upper crust is strongly influenced by the presence of phyllosilicates (e.g. white mica, biotite, chlorite), systematically observed in shear zones in granites. Identifying reactions involving phyllosilicates at low-grade metamorphic conditions is crucial to understand crust mechanics and fluid-granite interactions during deformation. In the 305 Ma old basement of the Bielsa massif (Axial Zone, Pyrenees), extensive pre-orogenic (i.e. pre-Alpine) alteration related to feldspar sericitization and chloritization of biotite and amphibole occurred at temperatures of 270-350°C at 230-300 Ma. This event was followed by mylonitization and fracturing at 40-70 Ma, and fluid-rock interaction at 200-280°C marked by replacement and new crystallization of chlorite and white mica. In undeformed parts of the granite, compositional maps reveal in situ reaction, high local heterogeneities and low element mobility (migration over few µm) for most elements. Transmission electron microscopy (TEM) shows disconnected reaction-induced nanoporosity in chloritized amphiboles and ripplocations in chloritized biotite. Chloritization reaction varies over tens of nanometres, indicating high variability of element availability. Equilibrium is reached locally due to isolation of fluid in pockets. In samples with fractures, both elemental maps and TEM images show two chlorite groups: alpine chlorites in fractures have homogeneous composition while pre-alpine chlorites in the matrix show patchy compositions. Channelization of fluids in fractures and sealing by chlorite prevented replacement of the matrix chlorite. High element mobility was therefore limited to fractures. In mylonites, compositional maps show secondary chlorites up to 1 mm around cracks and only partial replacement of chlorite within the matrix. This suggests fluids could percolate from cracks to the matrix along chlorite grain boundaries. TEM images show nanocracks at the boundary of chlorite crystallites where replacement is localised. Crystallites were individually replaced by dissolution-reprecipitation reactions and not by intra-crystallite mineral replacement, explaining the patchy compositional variations. While fracturing did not allow chlorite sheets to be progressively re-oriented, a continuous, brittle-ductile deformation in mylonites did, making preferential fluid pathways progressively change. Despite high strain, chlorite replacement was not complete even in mylonites. Replacement appears to be controlled by matrix-fracture porosity contrasts and the location and connection of nanoporosity between crystallites, criteria that may be only transiently met in space during deformation. These mechanisms need to be taken into account when attempting to reconstruct the metamorphic history of shear zones as well as the evolution of their mechanical behaviour since they affect the scale of the thermodynamic equilibrium and the preservation of hydrothermal metamorphism in granites

Transient and periodic brittle deformation of eclogites during intermediate-depth subduction

International audienceBrecciated eclogite clasts from the Monviso ophiolite in the Western Alps fossil subduction zone preserve healed fractures within garnet crystals. These fractures are divided into two distinct populations based on chemical and textural criteria. Diffusion modeling on profiles across fractures identifies a characteristic diffusive timescale for each population, with a mean calculated difference between population timescales of ∼2 Myr. We interpret this relative difference in diffusional timescales to represent the maximum period between individual fracturing events during prograde subduction at eclogite facies P-T conditions. The period between the two distinct fracturing events is likely controlled by transient fluid pressure and/or strain rate increases during prograde subduction. An increase in pore fluid pressure may be related to the dehydration of adjacent portions of the down-going plate during prograde subduction, while an increase in strain rate may be related to seismicity at shallower depths in the subducting slab. The absolute duration for diffusion, as constrained by diffusion modeling, implies extremely short timescales near peak P-T conditions. This suggests that the subducting slab potentially experienced multiple periods of seismic activity immediately prior to detachment and subsequent exhumation from intermediate depths. The presence of similar healed fractures in garnets from eclogites is relatively common in the rock record, implying that similar transient and periodic processes may be relatively common during subduction worldwide and through time

A XANES and EPMA study of Fe 3+ in chlorite: substitutions, importance of oxychlorite and implications for cation site distribution and thermobarometry

International audienceChlorite is a ubiquitous product of metamorphism, alteration of magmatic rocks and hydrothermal processes owing to its large stability field and wide compositional range. Its composition is governed by several substitutions and has been used as a geothermometer, on the basis of empirical, semi-empirical, and thermodynamic models. As in some other phyllosilicates of petrological interest, the oxidation state of iron in chlorite may differ from the usually assumed divalent state. However, the crystal chemistry of trivalent iron in chlorite remains poorly known, and the thermodynamic properties of ferric chlorite are missing from databases used for petrological modeling. As part of an attempt to fill this gap, we present results from in situ, micrometer-scale measurements of the oxidation state of iron in various chlorite-bearing samples. X-ray absorption near-edge spectroscopy (XANES) was combined with electron probe microanalysis (EPMA) on the same crystals. Results show iron oxidation states varying from ferrous to ferric; iron is in octahedral coordination in all ferromagnesian chlorites but to ~25% tetrahedral in the lithian chlorite cookeite (1.0 wt% Fe2O3(total)). Absolute amounts of ferric iron cover an unprecedented range (0 to ~30 wt% Fe2O3). For highly magnesian, ferric chlorite, Fe concentrations are low and can be accounted for by Al = Fe3+ substitution. In Fe-rich samples, Fe3+ may exceed 2 atoms per formula unit (pfu, 18 oxygen basis). When structural formulas are normalized to 28 charges corresponding to the standard O10(OH)8 anionic basis, these measurements define the exchange vector of a di-trioctahedral-type substitution: 3 VI(Mg, Fe2+) = VI☐ + 2 VIFe3+, as described in earlier studies. However, structural formulas calculated on the basis of the oxygen contents actually measured by EPMA show that this trend is an artifact, due to the neglect of variations in the number of protons in the structure. Our measurements indicate increasing hydrogen deficiency with increasing Fe3+ content, up to ~ 2 H+ pfu in the Fe3+-rich chlorite samples, corresponding to a net exchange vector of the type R2+ + H+ = Fe3+. These results do not support substitutions toward di-trioctahedral ferric end-members, and highlight the need for considering substitution toward an “oxychlorite” (i.e., H-deficient) ferric component, close to tri-trioctahedral, with an O12(OH)6 anionic basis, even in green, pristine-looking chlorite. The effects of iron oxidation and H deficiency on chlorite geothermometers were explored. They are deterring if H deficiency is ignored but, given the sensitivity of most thermometers to octahedral vacancy, the assumption FeTotal = Fe2+ is still safer than using high measured Fe3+ contents and the standard 28 charge basis, which artificially increases vacancies. In such ferric chlorites, EPMA measurement of oxygen allows a fair estimate of H content if Fe3+/Fe2+ is known; it should be more systematically implemented. For the same reasons, literature data reporting Fe3+-rich chlorite with vacancy content along the possibly artificial di-trioctahedral-type substitution should be verified. With the help of constraints from thermodynamic models, charge balance, crystal symmetry, and proton loss, a new cation site distribution is proposed for di-tri- to tri-trioctahedral chlorites in the Fe2+-Fe3+-Mg-Al-Si-O-H system, allowing a more realistic thermodynamic handling of their solid solutions

From static alteration to mylonitization: a nano- to micrometric study of chloritization in granitoids with implications for equilibrium and fluid percolation length scales

International audienceStrain accommodation in upper crustal rocks is often accompanied by fluid-mediated crystallization of phyllosilicates, which influence rock strength and shear zone formation. The composition of these phyllosilicates is commonly used for pressure-temperature-time constraints of deformation events, although it is often highly heterogeneous. This study investigates the reactions producing a phyllosilicate, chlorite, in and below greenschist-facies conditions and the variations in chlorite composition, along a strain gradient in the Bielsa granitoid (Axial Zone, Pyrenees). Compositional maps of chlorite (including iron speciation) are compared to nanostructures observed by transmission electron microscopy in increasingly-strained samples and related to mechanisms of fluid percolation and scales of compositional homogenisation. In the Bielsa granitoid, altered at the late Variscan, Alpine-age shear zones are found with high strain gradients. The undeformed granitoid exhibits local equilibria, pseudomorphic replacement and high compositional heterogeneities in chlorite. This is attributed to: (i) variable element supply and reaction mechanisms observed at nanoscale and (ii) little interconnected intra- and inter-grain nanoporosity causing isolation of fluid evolving in local reservoirs. In samples with discrete and mm-sized fractures, channelized fluid triggered the precipitation of homogeneous Alpine chlorite in fractures, preserving late-Variscan chlorite within the matrix. In low-grade mylonites, where brittle-ductile deformation is observed, micro-, nano-cracks and defects allows the fluid percolating into the matrix at the scale of hundreds of µm. This results in a more pervasive replacement of late-Variscan chlorite by Alpine chlorite. Local equilibria and high compositional heterogeneities in phyllosilicates as chlorite are therefore preserved according (i) matrix-fracture porosity contrasts at nanoscale and (ii) the location and interconnection of nanoporosity between crystallites of phyllosilicates that control reaction mechanisms and element mobility. In low grade mylonites, mineral and compositional replacement remains incomplete despite the high strain