Abdeltif Lahfid

Exhumation kinematics of the Cycladic Blueschists unit and back‐arc extension, insight from the Southern Cyclades (Sikinos and Folegandros Islands, Greece)

Current models for the Oligo-Miocene post-orogenic back-arc extension of the Aegean domain suggest that stretching is accommodated by two bivergent detachment systems of opposing shear sense. The co-existence in the Eocene of a top-to-the-S thrust at the base of the Cycladic Blueschists unit and top-to-the-N extensional shear zones at the roof raises the problem of differentiating synorogenic and post-orogenic deformations with similar directions and shear senses. Based on structural field data, this study shows that the post-orogenic deformation recorded in the Southern Cyclades is extremely asymmetric as the Cycladic Blueschists unit is pervasively affected by top-to-the-N shearing deformation distributed on four main shear zones. All activated in greenschist-facies conditions, some of these shear zones operated in the brittle regime during the final part of the exhumation. The Cycladic Blueschists/Cycladic Basement contact displays clear polyphased deformation with the preservation of top-to-the-S thrust kinematics. Thermal structure of the Cycladic Blueschists unit with regards to position of ductile shear zones was retrieved using the Raman Spectroscopy of Carbonaceous Material peak-metamorphic temperatures. This study shows a series of major metamorphic gaps accommodating an upward and stepwise decrease of more than 200 °C within the Cycladic Blueschists unit. Pressure-temperature estimates show that only lower parts of the Cycladic Blueschists unit recorded ca. 18–20 kbar for 530 °C peak-conditions. While flanking the West Cycladic Detachment System, which shows a top-to-the-S shear sense, the Southern Cyclades are dominated by a top-to-the-N non-coaxial shearing. Deformation is therefore genuinely asymmetric in the center of the Aegean domain.

Characterization of alunite supergroup minerals by Raman spectroscopy

Raman spectroscopy has been used to study the molecular structure of different natural minerals of the alunite supergroup (AB(3)(XO(4))(2)(OH)(6)), with A=K(+), Na(+), Ca(2+), Sr(2+), Ba(2+), B=Al(3+), Fe(3+) and X=S(6+), P(5+). The influence of the ions, in A-, B- and X-sites, is highlighted in the Raman spectra by variations in the position of certain vibrations and is discussed in association with published crystallographic data in order to describe the observed differences. It was found that A-site substitutions are characterized by wavenumber shifts of the vibrations involving hydroxyl groups. The positions of these vibrational bands vary linearly with the ionic radius of the ions in this site. B-site substitutions induce shifts of all bands due to structural modifications that lead to differences in the chemical environment around the hydroxyl and XO(4) groups and changes in B-O bond lengths. A correlation showed that these shifts correlate well with the ionic radii of the B-ions. The spectra of compounds containing both sulfate and phosphate groups are described by numerous vibration bands caused by a complex elemental composition and a symmetry change of the XO(4) groups. This study has also made it possible to generalize substitution effects on the wavenumbers of several vibrations and show that Raman spectroscopy could be a powerful tool for identifying and distinguishing minerals of the alunite supergroup.

Raman-in-SEM, a multimodal and multiscale analytical tool: Performance for materials and expertise

The availability of Raman spectroscopy in a powerful analytical scanning electron microscope (SEM) allows morphological, elemental, chemical, physical and electronic analysis without moving the sample between instruments. This paper documents the metrological performance of the SEMSCA commercial Raman interface operated in a low vacuum SEM. It provides multiscale and multimodal analyses as Raman/EDS, Raman/cathodoluminescence or Raman/STEM (STEM: scanning transmission electron microscopy) as well as Raman spectroscopy on nanomaterials. Since Raman spectroscopy in a SEM can be influenced by several SEM-related phenomena, this paper firstly presents a comparison of this new tool with a conventional micro-Raman spectrometer. Then, some possible artefacts are documented, which are due to the impact of electron beam-induced contamination or cathodoluminescence contribution to the Raman spectra, especially with geological samples. These effects are easily overcome by changing or adapting the Raman spectrometer and the SEM settings and methodology. The deletion of the adverse effect of cathodoluminescence is solved by using a SEM beam shutter during Raman acquisition. In contrast, this interface provides the ability to record the cathodoluminescence (CL) spectrum of a phase. In a second part, this study highlights the interest and efficiency of the coupling in characterizing micrometric phases at the same point. This multimodal approach is illustrated with various issues encountered in geosciences.

Structural inheritance in the Central Pyrenees: the Variscan to Alpine tectonometamorphic evolution of the Axial Zone

Estimating structural inheritance in orogens is critical to understanding the manner in which plate convergence is accommodated. The Pyrenean belt, which developed in Late Cretaceous to Paleogene times, was affected by Cretaceous rifting and Variscan orogeny. Here we combine a structural and petrological study of the Axial Zone in the Central Pyrenees to discuss structural inheritance. Low-grade Paleozoic metasedimentary rocks were affected by a Variscan transpressional event that produced successively: (1) regional-scale folds; (2) isoclinal folding, steep pervasive cleavage and vertical stretching, synchronous with peak metamorphism; (3) strain localization into ductile reverse shear zones. The persistence of a relatively flat envelope for the Paleozoic sedimentary pile and Variscan isograds, and the absence of Alpine crustal-scale faults in the core of the Axial Zone, suggests that the Axial Zone constitutes a large Variscan structural unit preserved during Pyrenean orogeny. This configuration seems to be inherited from Cretaceous rifting, which led to the individualization of a large continental block (future Axial Zone) against a hyper-extended domain along the North Pyrenean Fault zone. This study places the currently prevailing model of Pyrenean belt deformation in a new perspective and has important implications for crustal evolution and inheritance in mountain belts more generally. Supplementary materials: Raman spectroscopy of carbonaceous materials data and a figure illustrating peak-fitting of the Raman spectrum of carbonaceous material and Raman spectra from the various samples of the Pallaresa cross-section are available at https://doi.org/10.6084/m9.figshare.c.3906247

Chapter 3:Raman-in-SEM studies of inorganic materials

SEM-EDS and micro-Raman spectroscopy have been combined for material characterization in several recent studies. Switching from one to the other is frequently considered as a problem that cannot be solved using specific solutions. Although both techniques have followed a parallel but very different evolution since their introduction in the early 1930s, the concept of Raman-in-SEM first began in the 1980s and the first commercial systems were marketed in the early 2000s. The two main systems and techniques that have been developed and marketed by three manufacturers are presented and described in this chapter. An evaluation of their advantages and limitations is proposed. A metrological study is then proposed for one of these systems, based on the ‘on-axis’ technique using a curved mirror placed under the SEM pole piece. This study allows a discussion of the performance and limitations of Raman spectroscopy when performed in a SEM. A comprehensive review of published work is provided, although papers are rare in the open literature. The technique is essentially used for controls, expert assessments and high technology applications. Advanced techniques that allow the use of Raman-in-SEM spectroscopy are discussed in detail using application examples taken from different fields in geosciences, materials chemistry or from expert assessments. The conclusions of this study show that Raman-in-SEM spectroscopy is to date the first step in the combination of two well-known and mature techniques enabling the synergy between them to be maximised. Raman-in-SEM spectroscopy is relatively easy to set up and effectively complements the capabilities and efficiency of analytical SEM for material characterization. What are the most likely development perspectives that may be considered for this analytical coupling? Today, commercial systems are limited to only point-level micro-Raman analysis at the micrometre scale. In the near future, developments in both hardware and software will probably allow analysts to acquire Raman maps, or to employ multi-technique analyses based on a combination of data from SEM, EDS Raman, etc. New hardware developments may enhance the spatial resolution of both SEM and Raman spectroscopy.

Fluid–rock interactions related to metamorphic reducing fluid flow in meta-sediments: example of the Pic-de-Port-Vieux thrust (Pyrenees, Spain)

In orogens, shortening is mainly accommodated by thrusts, which constitute preferential zones for fluid–rock interactions. Fluid flow, mass transfer, and mineralogical reactions taking place along thrusts have been intensely investigated, especially in sedimentary basins for petroleum and uranium research. This study combines petrological investigations, mineralogical quantifications, and geochemical characterizations with a wide range of analytical tools with the aim of defining the fluid properties (nature, origin, temperature, and redox) and fluid–host rock interactions (mass transfers, recrystallization mechanisms, and newly formed synkinematic mineralization) in the Pic-de-Port-Vieux thrust fault zone (Pyrenees, Spain). We demonstrate that two geochemically contrasted rocks have been transformed by fluid flow under low-grade metamorphism conditions during thrusting. The hanging-wall Triassic red pelite was locally bleached, while the footwall Cretaceous dolomitic limestone was mylonitized. The results suggest that thrusting was accompanied by a dynamic calcite recrystallization in the dolomitic limestone as well as by leaching of iron via destabilization of iron oxides and phyllosilicate crystallization in the pelite. Geochemical and physical changes highlighted in this study have strong implications on the understanding of the thrust behavior (tectonic and hydraulic), and improve our knowledge of fluid–rock interactions in open fluid systems in the crust.

Exhumation of eclogite and blueschist (Cyclades, Greece): Pressure-temperature evolution determined by thermobarometry and garnet equilibrium modelling

High-pressure rocks such as eclogite and blueschist are metamorphic markers of paleo-subduction zones, and their formation at high-pressure and low-temperature conditions is relatively well understood since it has been the focus of numerous petrological investigations in the past 40 years. The tectonic mechanisms controlling their exhumation back to the surface are, however, diverse, complex and still actively debated. Although the Cycladic Blueschist Unit (CBU, Greece) is among the best worldwide examples for the preservation of eclogite and blueschist, the proposed P-T evolution followed by this unit within the Hellenic subduction zone is quite different from one study to another, hindering our comprehension of exhumation processes. In this study, we present an extensive petrological dataset that permits refinement of the shape of the P-T trajectory for different subunits of the CBU on Syros. High-resolution quantitative compositional mapping has been applied to support the thermobarometric investigations, which involve semi-empirical thermobarometry, garnet equilibrium modelling and P-T isochemical phase diagrams. The thermodynamic models highlight the powerful use of reactive bulk compositions approximated from local bulk compositions. The results were also

A New Deposit of Gem-Quality Grandidierite in Madagascar

A new deposit of grandidierite, considered one of the world’s rarest gems, has been discovered in southern Madagascar. The new deposit is outside the town of Tranomaro, near the original locality of Andrahomana. It occurs in the form of strong bluish green to greenish blue translucent to transparent crystals measuring up to 15 × 7 × 3 cm. Grandidierite is the magnesium end member in the solid-solution series with ominelite as the iron end member. The studied samples have a very low Fe/(Mg + Fe) ratio. This confirms that the Tranomaro deposit, together with Johnsburg in New York State, provides the purest grandidierite ever found. The crystals host inclusions of Cl-apatite, zircon, and monazite.The paragenesis also includes plagioclase, phlogopite, enstatite, diopside, dravite, and sapphirine (locally as gemquality crystals). Transparent crystals have been faceted, yielding small but eye-clean jewelryquality gems.