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Zircon grains preserve records of crystallization, growth, and/or deformation that can be envisaged from their internal structures and through the U–Pb isotope analysis. Electron backscatter diffraction (EBSD) is a non-destructive method for visualizing undeformed domains to differentiate them from those that are plastically deformed. In this study, we report EBSD analyses conducted on zircon grains, in thin sections with available textural information, from Himalayan eclogites. The studied eclogite samples show no petrographic evidence of shearing or mylonitization. However, several zircon grains preserve plastically deformed domains. These deformed domains display several degrees of misorientation relative to the undeformed domain and yielded geologically reset ages when analysed for U–Pb isotope ratios using nanoscale secondary ion mass spectrometry (NanoSIMS), in contrast to most undeformed domains which retained the protolith age. The degree of resetting is positively correlated with the extent of misorientation. These pieces of evidence indicate that plastic deformation in zircon grains, equilibrated at higher pressure–temperature conditions, affected the primary geochemical and geochronological records. Based on these observations, we assume that not only regional shearing/mylonitization in metamorphic rocks affects the geochemical records, but also that zircon grains in apparently unsheared high-grade metamorphic rocks behave plastically. The micro-scale intra-grain plastically deformed domains can easily be identified through EBSD analysis in the form of crystallographic misorientations. To extract meaningful geochronological results, it is necessary to identify undisturbed domains in zircon grains before applying any destructive analytical method.

We report a comprehensive data set characterizing and quantifying the geometrically necessary dislocation (GND) density in the crystallographic frame (ραc) and disclination density (ρθ) in fine-grained polycrystalline olivine deformed in uniaxial compression or torsion, at 1000 and 1200 ∘C, under a confining pressure of 300 MPa. Finite strains range from 0.11 up to 8.6 %, and stresses reach up to 1073 MPa. The data set is a selection of 19 electron backscatter diffraction maps acquired with conventional angular resolution (0.5∘) but at high spatial resolution (step size ranging between 0.05 and 0.1 µm). Thanks to analytical improvement for data acquisition and treatment, notably with the use of ATEX (Analysis Tools for Electron and X-ray diffraction) software, we report the spatial distribution of both GND and disclination densities. Areas with the highest GND densities define sub-grain boundaries. The type of GND densities involved also indicates that most olivine sub-grain boundaries have a mixed character. Moreover, the strategy for visualization also permits identifying minor GND that is not well organized as sub-grain boundaries yet. A low-temperature and high-stress sample displays a higher but less organized GND density than in a sample deformed at high temperature for a similar finite strain, grain size, and identical strain rate, confirming the action of dislocation creep in these samples, even for micrometric grains (2 µm). Furthermore, disclination dipoles along grain boundaries are identified in every undeformed and deformed electron backscatter diffraction (EBSD) map, mostly at the junction of a grain boundary with a sub-grain but also along sub-grain boundaries and at sub-grain boundary tips. Nevertheless, for the range of experimental parameters investigated, there is no notable correlation of the disclination density with stress, strain, or temperature. However, a broad positive correlation between average disclination density and average GND density per grain is found, confirming their similar role as defects producing intragranular misorientation. Furthermore, a broad negative correlation between the disclination density and the grain size or perimeter is found, providing a first rule of thumb on the distribution of disclinations. Field dislocation and disclination mechanics (FDDM) of the elastic fields due to experimentally measured dislocations and disclinations (e.g., strains/rotations and stresses) provides further evidence of the interplay between both types of defects. At last, our results also support that disclinations act as a plastic deformation mechanism, by allowing rotation of a very small crystal volume.

Earth's long-term cycling of carbon is regulated from mid-ocean ridges to convergent plate boundaries by mass transfers involving mantle rocks. Here we examine the conversion of peridotite to listvenite (magnesite + quartz rock) during CO2 metasomatism along the basal thrust of the Semail Ophiolite (Fanja, Sultanate of Oman). At the outcrop scale, this transformation defines reaction zones, from serpentinized peridotites to carbonated serpentinites and listvenites. Based on a detailed petrological and chemical study, we show that carbonation progressed through three main stages involving the development of replacive textures ascribed to early stages, whilst carbonate (± quartz) veining becomes predominant in the last stage. The pervasive replacement of serpentine by magnesite is characterized by the formation of spheroids, among which two types are identified based on the composition of their core regions: Fe-core and Mg-core spheroids. Fe zoning is a type feature of matrix and vein magnesite formed during the onset carbonation (Stage 1). While Fe-rich magnesite is predicted to form at low fluid XCO2 from a poorly to moderately oxidized protolith, our study evidences that the local non-redox destabilization of Fe oxides into Fe-rich magnesite is essential to the development of Fe-core spheroids. The formation of Fe-core spheroids is followed by the pervasive (over-)growth of Mg-rich spheroids and aggregates (Stage 2) at near-equilibrium conditions in response to increasing fluid XCO2. Furthermore, the compositions of carbonates indicate that most siderophile transition elements released by the dissolution of primary minerals are locally trapped in carbonate and oxides during matrix carbonation, while elements with a chalcophile affinity are the most likely to be leached out of reaction zones.

Melt transport mechanisms have an important impact on the chemical composition of the percolated host rock and the migrating melts. Melt migration is usually assumed to occur at grain boundaries. However, microstructural studies revealed the occurrence of polyphase inclusions along dislocations, subgrain boundaries and microcracks in single mineral grains. The inclusions are interpreted as crystallized melt pockets suggesting that melts can migrate within deformed crystals. Intracrystalline melt migration and diffusive re-equilibration can lead to significant mineral trace element enrichments when associated with dissolution–precipitation reactions. In this contribution, we study a body of replacive troctolites associated with the Erro-Tobbio ophiolitic mantle peridotites (Ligurian Alps, Italy). The replacive formation of the olivine-rich troctolite involved extensive impregnation of a dunitic matrix, i.e. partial dissolution of olivine and concomitant crystallization of interstitial phases. The olivine matrix is characterized by two distinct olivine textures: (i) coarse deformed olivine, representing relicts of the pre-existing mantle dunite matrix (olivine1), and (ii) fine-grained undeformed olivine, a product of the melt–rock interaction process (olivine2). Previous studies documented a decoupling between olivine texture and trace element composition, namely enriched trace element compositions in olivine1 rather than in olivine2, as would be expected from the dissolution–precipitation process. Notably, the trace element enrichments in deformed olivines are correlated with the occurrence of elongated 10 µm size polyphase inclusions (clinopyroxene, Ti-pargasite, chromite) preferentially oriented along olivine crystallographic axes. These inclusions show irregular contacts and have no crystallographic preferred orientation with the host olivine, and the phases composing the inclusions show similar chemical compositions to the vermicular phases formed at the grain boundaries during late-stage reactive crystallization of the troctolite. This suggests that the investigated inclusions did not form as exsolutions of the host olivine but rather by input of metasomatic fluids percolating through the deformed olivine grains during closure of the magmatic system. We infer that strongly fractionated volatile-rich melts were incorporated in oriented microfractures within olivine1 and led to the crystallization of the polyphase inclusions. The presence of intracrystalline melt greatly enhanced diffusive re-equilibration between the evolved melt and the percolated olivine1, in turn acquiring the enriched character expected in neoformed olivine crystals. Intracrystalline melt percolation can have strong geochemical implications and can lead to efficient re-equilibration of percolated minerals and rocks.