Muscovite Research Articles

Thickness-Dependent Nonlinear Electrical Conductivity of Few-Layer Muscovite Mica

We report on the direct current (dc) current-voltage ($I$-$V$) characteristics of few-layer muscovite mica (MuM) flakes exfoliated and transferred onto $\mathrm{Si}\mathrm{O}$${}_{2}$/$\mathrm{Si}$ substrate, under different substrate dc bias voltages. Contrary to usual observations in conventional two-dimensional systems, we observe an increase in the in-plane electrical conductivity with a reducing thickness of MuM flakes. At a given voltage, the electrical conductivity of approximately five-layered MuM flake (${T}_{3}$) is 3 orders of magnitude larger than that in approximately ten-layered MuM flake (${T}_{2}$). The $I$-$V$ characteristics are used to analyze the mechanism of conduction. The model-based analysis reveals the hopping-conduction mechanism to be dominant as compared to the Poole-Frenkel effect. The thickness-dependent work function is measured using Kelvin probe force microscopy for a MuM flake on $\mathrm{Si}$ substrate. Assuming that the measured work function is correlated with the Fermi level, we report an upward movement of the Fermi level, toward the conduction band with the reducing thickness of MuM flakes, indicating an increase in the conduction-band carrier density. The observed increase in conductivity in ${T}_{3}$ when compared to ${T}_{2}$ may be attributed to surface doping due to the increased contribution from ${\mathrm{K}}^{+}$ ions and lattice relaxation. Our results show that there is a possibility of using few-layer mica as a wide-band-gap semiconductor and that it can open up different avenues for two-dimensional electronic devices.

Detecting Mineral Resources and Suggesting a Physical Concentration Flowsheet for Economic Minerals at the Northern Border Region of Saudi Arabia

There is a limited number of studies on sand deposit resources of Saudi Arabia, which cover nearly the one-third of the area of the country, whereas most of these studies deal with the environmental rather than the mineralogical or mining aspects. In this paper, and in the effort to detect the mineral resources of the Northern Border Region, the surficial Wadi sediments along the Ar'ar-Sakaka road are studied. The deposits of several Wadies (Al Aqra, Shiban al Hanzaliyat, and Arar) are mixed. The sediments of the collected samples are investigated to determine definite areas characterized by a relatively higher content of heavy minerals and a relatively lower content of carbonate minerals that are also friable enough to be investigated by some of the available physical concentration techniques. A large quantity of the surficial deposits, weighing 4.69 tons was collected from the stretch at the investigated area which is 3km long and 1.5km wide. Evaluation of the heavy minerals content, their types, and their ability for concentration and separation, was conducted. A suggested physical concentration flowsheet was concluded for concentrating and separating the contained economic minerals. The average heavy mineral content is 1.55 wt% and the identified economic minerals are magnetite, ilmenite, hematite, goethite, zircon, rutile, anatase, monazite, and xenotime. The other contained heavy minerals include monoclinic pyroxenes (diopside, and augite), monoclinic amphibole (winchite), and muscovite mica. Dolomite and calcite carbonate are also contained. The concluding results ensure that magnetite, zircon, TiO2 minerals, and monazite are mineable for separation in individual mineral concentrates. Most of the detected economic minerals are recorded in the area for the first time. Monazite, xenotime, and zircon are responsible for some recorded radioactivity in the area.

Vectors to ore in replacive volcanogenic massive sulphide deposits of the northern Iberian Pyrite Belt: Major and trace element mineral chemistry

As part of a broader characterization and study of vectors to ore in VMS systems of the Iberian Pyrite Belt (IPB, Spain), we have investigated in detail mineral chemistry vectors in a representative replacive deposit hosted in felsic volcanic rocks. At the Aguas Teñidas deposit (northern IPB) the hydrothermal system affected rocks of an originally homogeneous composition extending hundreds of metres beyond its footprint. The major and trace element chemistry of white micas, chlorite and carbonates have been analysed by electron microprobe (EMP) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to characterize geochemical vectors across the extent of the hydrothermal system at Aguas Teñidas.White micas are dominated by muscovite in both regionally altered lithologies and within the hydrothermal system, with coexisting paragonite occurring in a halo beyond the first disseminated pyrite surrounding the stockwork and in the proximal hanging wall. Systematic variations have been observed in FeO/(FeO + MgO) and Na2O/(Na2O + K2O) across the alteration zone.Chlorite is predominantly clinochlore in composition, with chamosite restricted to the centre of the hydrothermal system. In regionally altered lithologied it is characterized by a constantly low Al and AlIV at variable Fe/(Fe + Mg); in rocks influenced by the hydrothermal system it presents higher AlIV and total Al, and a progressive increase in Al, AlIV and Fe/(Fe + Mg) towards its core.Regional carbonates consist of calcite, with additional dolomite forming in proximal rocks. Iron content in dolomite increases towards the centre of the hydrothermal system, with ankeritic compositions becoming dominant.Systematic variations in Ba, Cs, Li, Pb, Rb, Sn, Sr, Tl and Zn have been observed in white micas consistent with a proximal-distal transition (vector). Trends in chlorite have been less well characterized due to analytical limitations, although variations have been observed in the contents of As, Co, Li and Zn.A schematic model has been proposed which accounts for the observed mineralogical and mineral chemistry trends across the hydrothermal footprint of the Aguas Teñidas VMS deposit. Major element signatures in white micas and chlorite are interpreted as having been controlled by the upwards and outwards flow of hot reducing Fe-rich hydrothermal fluids producing a coupled increase in temperature and fluid Fe/(Fe + Mg), and a decrease in fO2, with increasing fluid/rock ratios towards the centre of the stockwork.The data presented herein are not only applicable to VMS exploration in the IPB, but on a broader scale improve our general understanding of vectors to ore in VMS deposits in general.

The impact of strain on growth mode in chemical vapor deposited mono- and few-layer MoS2

-The development of high-quality chemical vapor-deposited mono- and few-layer MoS2 is of high relevance for future applications in functional devices. Consequently, a detailed understanding of the growth mode and the parameters affecting it is important. Here, we show for the case of mono- and few-layer MoS2 grown on Muscovite mica, how strain and temperature impact the growth mode. We show how misleading the determination of the number of MoS2 layers is, solely based on Raman spectroscopy due to the occurrence of strain and changes in the growth mode. A combination of atomic force microscopy, Raman spectroscopy, and ab initio calculations reveal that that the growth at 500 °C synthesis temperature exhibits a strained layer-by-layer growth of up to three mono-layers, whereas at 700 °C, a strain release occurs and layer-by-layer growth is confined to the first mono-layer only. We relate the occurrence of strain to the formation of gas bubbles below the MoS2 film, escaping the mica sheets during high temperature synthesis. Our analysis shows that mica substrates can be used to study strain in 2D materials without the need to apply external stress and that a detailed knowledge of the MoS2 morphology is necessary to correctly interpret the Raman results.

Adsorption of caesium and cobalt ions on the muscovite mica clay-graphene oxide-γ-Fe2O3-Fe3O4 composite

The muscovite mica clay-graphene oxide-maghemite-magnetite (γ-Fe2O3-Fe3O4) composite was first used for the adsorption of caesium(I) and cobalt(II). The presence of clay minerals, graphene oxide, maghemite, and magnetite was detected in the prepared composite by XRD, WD-XRF, Mössbauer spectroscopy, and ATR-FTIR. The SEM and TEM results show that the composite has a layered structure with irregularly shaped pores on the surface. It was found that the adsorption of ions depends on the initial concentration, pH (except for caesium), mass of adsorbent, temperature, and contact time. The maximum adsorption capacity for Cs(I) and Co(II) was 2286mg/g and 652mg/g, respectively, and was obtained at concentrations (Cs(I) = 12,630mg/L; Co(II) = 3200mg/L), adsorbent mass of 0.01g, pH (Cs(I) = 7; Co(II) = 5), temperature of 20 ± 1°C, and contact time of 24h. The high adsorption capacity of the composite could be due to a diversity of functional groups, a large number of active sites or the multilayer adsorption of caesium and cobalt ions on the surface of the composite. The Freundlich, Langmuir isotherms, and the pseudo-second-order kinetic model better describe the adsorption of these ions on the composite. The adsorption was non-spontaneous endothermic for Cs(I) and spontaneous endothermic for Co(II). The proposed mechanism of adsorption of Cs and Co ions on the composite is complex and involves electrostatic interactions and ion exchange. The ANFIS model proved to be quite effective in predicting the adsorption of Cs(I) and Co(II), as shown by the obtained values of R2, MSE, SSE, and ARE.

Chemomechanical weakening of muscovite quantified with in situ liquid nanoindentation

Chemomechanical weakening of layered phyllosilicate muscovite mica was studied as a function of chemical environment via in situ liquid-nanoindentation under four conditions (dry, deionized water, and two NaCl solutions of different pH). While traditional indentation analyses of layered materials with extreme mechanical anisotropy have been limited due to displacement bursts (pop-ins), here the bursts were used as proxies for delamination, fracture, and spalling events. Since displacement bursts during an indent represent a stochastic process, 120 indents were conducted for each condition to generate statistically significant amounts of data. In total, over 9000 bursts were assessed using a load–displacement threshold criterion, classifying this as a high-throughput nanoscale fracture technique. For each burst, initiation load, initiation displacement, plastic zone volume at initiation, and energy dissipation were analyzed. A power-law relationship between the burst load and burst energy was noted which separated the bursts into two continuous distributions: (1) bursts due only to the mechanics of the indent and (2) bursts due to both the mechanics of the indent and the environment. By using a cumulative probability distribution, it was found that the NaCl solutions decreased the minimum plastic zone volume necessary to initiate a displacement burst by an order of magnitude relative to the dry condition. Finally, the underlying mechanisms explaining the trends in initiation volume as a function of environment were discussed, with a focus on the chemomechanical degradation processes via chemical attack and cation exchange.

Atomic resolution with high-eigenmode tapping mode atomic force microscopy

Atomic surface structure imaging is instrumental for the understanding of surface-related phenomena. Here, we show that conventional tapping mode atomic force microscopy with high cantilever eigenmodes and subnanometer amplitudes allow routine atomic imaging at atmospheric pressures. We identify the reasons for failure of atomic resolution imaging employing low eigenmodes. Strong tip-surface interactions cause significant differences between the oscillatory behaviors of the inclination of the cantilever as detected by conventional instruments and of the vertical position of the tip, which prevents correct functioning of instrumental feedback control loops. However, high effective spring constants of high eigenmodes make it possible to overcome the problem. Furthermore, the combination of high effective elastic constants of high cantilever eigenmodes with the high flexibility of the cantilever substantially enhances the imaging stability, thereby universally allowing atomic imaging of solid surfaces in gaseous environments and at elevated temperatures. Demonstrated imaging examples include single sulfur vacancies at the surface of ${\mathrm{MoS}}_{2}$ crystals imaged at temperatures ranging from room temperature to 250\ifmmode^\circ\else\textdegree\fi{}C and potassium ions on hydrophilic and highly adhesive muscovite mica surfaces. Moreover, the high imaging stability allows knocking atoms off the ${\mathrm{MoS}}_{2}$ surface by hard tapping, indicating the potential for ultrahigh resolution lithography.

Ordered and Disordered Carboxylic Acid Monolayers on Calcite (104) and Muscovite (001) Surfaces.

The adsorption of carboxylic acid molecules at the calcite (104) and the muscovite (001) surface was investigated using surface X-ray diffraction. All four investigated carboxylic acid molecules, hexanoic acid, octanoic acid, lauric acid, and stearic acid, were found to adsorb at the calcite surface. Whereas the shortest two carboxylic acid molecules, hexanoic acid and octanoic acid, showed limited ordering and a flexible, disordered chain, the two longest carboxylic acid molecules form fully ordered monolayers, i.e., these form highly structured self-assembled monolayers. The latter molecules are oriented almost fully upright, with a tilt of up to 10°. The oxygen atoms of the organic molecules are found at similar positions to those of water molecules at the calcite–water interface. This suggests that in both cases, the oxygen atoms compensate for the broken bonds at the calcite surface. Under the same experimental conditions, stearic acid does not adsorb to K+ and Ca2+-functionalized muscovite mica because the neutral molecules do not engage in the ionic bonds typical for the mica interface. These differences in adsorption behavior are characteristic for the differences of the oil–solid interactions in carbonate and sandstone reservoirs.

The timing of blueschist-facies metamorphism in the Makrotantalon Unit on Andros Island, Greece: Cretaceous and Eocene high-pressure/low-temperature events?

AbstractThis study corroborates interpretations suggesting that the Makrotantalon Unit on Andros represents a tectonic slice with Pelagonian affinity in the nappe stack of the Cycladic Blueschist Unit. Previously reported Cretaceous 40Ar–39Ar dates of a garnet-glaucophane schist from the Makrotantalon Unit could not be reproduced by Rb–Sr geochronology, but this is not an indication of contamination with excess Ar. Instead, the newly dated samples record disturbance of the Rb–Sr isotope system by partial recrystallization. Subsets of these phengite populations, representing the smaller grain-size fractions, yielded low-precision dates ranging from c. 21 to c. 15 Ma that document deformation-related resetting and recrystallization of a presumably Cretaceous white mica population. Although these Miocene dates cannot be linked with blueschist-facies metamorphism, they provide time constraints on the formation of shear zones that overprinted the original thrust contact during exhumation. The geological relevance of a Cretaceous high-pressure event is confirmed by a Rb–Sr date of c. 121 Ma for an epidote-glaucophane schist collected further away from the tectonic contact. The occurrence of a second blueschist-facies event in the Eocene is verified by Rb–Sr dates of two epidote-glaucophane schists (c. 40 Ma and c. 44 Ma) that can unambiguously be assigned to the Makrotantalon Unit.

Tracking mineral evolution and element mobility during albitisation and subsequent kaolinisation of phyllite rocks: A case study from the Verrucano of Monti Pisani, Tuscany, Italy

AbstractThis work describes the first occurrence of albitite rocks in the Middle Triassic Verruca Formation, Monti Pisani, Northern Apennines, northern Tuscany, Italy. The albitite formed by Na-metasomatism of phyllites (‘potassic white mica' + quartz + ‘chlorite' + hematite + albite) in an amagmatic environment. The albitisation process took place after the Miocene main phases of Apenninic deformation and was followed by the formation of veins of Fe-carbonate + quartz. Hydrothermal alteration progressed with the ingression, possibly favoured by the increase of permeability due to albitisation, of a slightly acidic, oxidising, aqueous fluid that led to the pervasive kaolinisation of the albitite and to the complete transformation of the Fe-carbonate of the veins into Fe-hydroxides. This stage was followed by supergene alteration that led to the formation of a pervasive network of halloysite veinlets and colloform (P–Al–Si)-bearing Fe-hydroxides. Finally, the hydrothermally altered rock underwent a localised brittle fracturing without new minerals being formed. The prominent compositional changes occurring during this multi-stage hydrothermal process were the inversion of the Na2O/K2O ratio of the whole rock (from 0.07 in the pristine phyllite to up to 200 for the kaolinised albitite), the loss of Fe and Mg, and the enrichment of Sb. The MREE were partially lost, whereas LREE and HREE behaved conservatively. Though pervasive hydrothermal alteration occurrences are common in central-southern Tuscany, mostly related to the post-collisional extensional regime, lithospheric thinning and emplacement of magmatic bodies in the crust, the rare Monti Pisani kaolinised albitite described in this investigation expands the effects of post-collisional hydrothermal activity in Tuscany northwards, far from potential magmatic sources.

Lithné fosfáty a slídy v leukogranitu typu Homolka

Homolka granite forms intrusion of highly fractionated leucocratic magma which belongs to the youngest members of the Moldanubian Pluton. The age of the intrusion was estimated between 319 and 315 Ma. The Homolka granite is P-rich, peraluminous and strongly enriched in Li, Rb, Cs, Sn, Nb, Ta and F, showing high U (8–20 ppm) and low Th (< 3 ppm) contents. This study focuses on Li-bearing minerals – phosphates and white micas. Their modal proportions correspond to 1–3 vol. % and 5–15 vol. %, respectively. Both mineral groups occur in several generations in the rock. The first phosphate generation (for example, Al-bearing montebrasite or Fe-Mn-bearing triphylite) contains up to 11 wt. % Li2O. The first generation of mica (Li-phengite) contains about 7–8 wt. % F and up to 1 wt. % Rb2O with Li2O content ranging between 1.3 and 1.9 wt. %. The second generation of white mica, replacing Li-phengite, is depleted in F and Rb and shows decrease in Li2O to 0.5–0.9 wt. %. Despite the low total content of lithium (Li2O = about 0.1 wt. %) in the rock, the occurrence of the leucocratic Homolka granite is not negligible. Because of the high economic potential of lithium, these highly evolved peraluminous and lithium-bearing intrusions will deserve a special attention in the future.

Exhumation of the high‐pressure Richarddalen Complex in NW Svalbard: Insights from 40Ar/39Ar geochronology

Abstract40Ar/39Ar dating of white mica from foliated metamorphic rocks of the Caledonian orogen of NW Svalbard was performed using the single‐grain fusion technique. Two samples, an augen orthogneiss and a mica schist, were collected from the high‐pressure Richarddalen Complex. Three additional mica schist samples were collected from the tectonically underlying Montblanc Unit. 40Ar/39Ar analysis of white mica separates yielded weighted average 40Ar/39Ar dates of 446.1 ± 1.1, 438.4 ± 1.1, 439.4 ± 1.2, 439.8 ± 1.5 and 439.3 ± 1.0 Ma, with the oldest date provided by the Richarddalen mica schist. The single‐grain fusion 40Ar/39Ar dating was accompanied by geochemical mapping of white mica using a microprobe. We interpret the oldest date as the age of cooling after a Late Ordovician high‐pressure event and the four younger dates to represent the timing of subsequent deformation associated with assembly of a Caledonian nappe stack.

Rotational dynamics and transition mechanisms of surface-adsorbed proteins

Assembly of biomolecules at solid–water interfaces requires molecules to traverse complex orientation-dependent energy landscapes through processes that are poorly understood, largely due to the dearth of in situ single-molecule measurements and statistical analyses of the rotational dynamics that define directional selection. Emerging capabilities in high-speed atomic force microscopy and machine learning have allowed us to directly determine the orientational energy landscape and observe and quantify the rotational dynamics for protein nanorods on the surface of muscovite mica under a variety of conditions. Comparisons with kinetic Monte Carlo simulations show that the transition rates between adjacent orientation-specific energetic minima can largely be understood through traditional models of in-plane Brownian rotation across a biased energy landscape, with resulting transition rates that are exponential in the energy barriers between states. However, transitions between more distant angular states are decoupled from barrier height, with jump-size distributions showing a power law decay that is characteristic of a nonclassical Levy-flight random walk, indicating that large jumps are enabled by alternative modes of motion via activated states. The findings provide insights into the dynamics of biomolecules at solid–liquid interfaces that lead to self-assembly, epitaxial matching, and other orientationally anisotropic outcomes and define a general procedure for exploring such dynamics with implications for hybrid biomolecular–inorganic materials design.

El Álamo district (Baja California, México): A hint of a new Cordilleran orogenic gold belt?

The El Álamo is one of the most important historic gold mining districts in the Peninsular Ranges batholith. Gold mineralizations at El Álamo consist in fault-fill laminated quartz veins with small amounts of sulfides and oxides, and lesser amounts of gold. Veins are mainly emplaced along compressional brittle-plastic shear zones within the El Álamo quartz diorite (146.6 ± 1.6 Ma, U–Pb zircon), which is intruded by the San Marcos dike swarm (126.8 ± 1.7 Ma, U–Pb zircon). Several lines of evidence indicate that the dike swarm is older than quartz veins, and the formation of the later is related to the Peninsular Ranges orogeny (∼110−98 Ma). Gold bearing quartz veins formed during regional prograde metamorphism (greenschist facies), and the veins subsequently underwent plastic deformation, dynamic recrystallization and metamorphism. Gold remobilization and concentration seems to have been largely enhanced by metamorphism, probably accounting for very high grade, albeit sporadic, bonanza ores, typical of the district. 40Ar/39Ar dating of hydrothermal white micas from the auriferous veins yielded ages of 97.86 ± 0.17 Ma at the Ulises mine and 97.46 ± 0.17 Ma at the La Americana mine. These ages are interpreted as the minimum age of the regional metamorphism in the area. Based on their primary characteristics, the gold deposits from El Álamo district can be ascribed to the orogenic gold type. These deposits are similar to other known deposits in the same orogenic context (e.g., Julian district; southern California), and they probably conform a new Cordilleran orogenic gold belt that could extend over much of the exhumed axis of the Peninsular Ranges batholith.

Adsorption of Cr (III) and Cr (VI) ions on muscovite mica: Experimental and molecular modeling studies

The article explores the possibility of adsorption of Cr (III) and Cr (VI) ions on muscovite mica for the first time. The adsorption behaviors of both the ions on the mineral have been compared vide experimental and molecular modeling studies. Experiments have been carried out to find out the effects of different operating parameters on the adsorption of the metal ions, while Density Functional Theory (DFT) based simulations have been conducted to understand the adsorption mechanisms of various Cr (III) and Cr (VI) species, occurring at different pH, on muscovite (001) surface. The maximum metal removal efficiencies obtained for Cr (III) and Cr (VI) are about 15.6 mg/g and 1.74 mg/g, respectively. The trend of adsorption energies calculated using DFT supports this finding. The simulation studies depict the nature of bonding of various possible Cr ions, at different pH levels, on the muscovite surface and reveal that the patterns Cr-O bonds and hydrogen bonds define the adsorption mechanism. The Partial Density of States (PDOS) calculations provide a further understanding of the involvement of different orbitals of Cr, O, H of the Cr ion and O atom of muscovite surface in the metal-mineral interaction.

Structural and petrological characteristics of a Jurassic detachment fault from the Mont-Blanc massif (Col du Bonhomme area, France)

Necking zones, located between the proximal and distal domains of passive margins, are characterized by important crustal thinning and the occurrence of extensional detachment faults. Here we provide a petrographic, microstructural and geochemical study of an extensional detachment fault discovered in the Mont-Blanc massif, which was located in the necking zone of the European margin during Early Jurassic rifting. The footwall of the fault consists of basement gneisses, while its hanging wall presents pre-to syn-rift deposits. A careful observation of the basement/cover contact also reveals the presence of a strain gradient affecting the Variscan gneisses, which are progressively deformed to cataclasite and to black gouges at the top basement, and sealed by undeformed Jurassic syn-rift sandstones, the Grès Singuliers, which rework fault rock pebbles. Microscale observations also indicate fluid-assisted deformation, characterized by the total breakdown of feldspars and the coeval formation of white mica, leading to strain softening along the fault plane. The overall observations point toward a rift-related detachment fault responsible for the exhumation of the internal Mont-Blanc basement, an interpretation which challenges previous models of crustal exhumation in the External massifs of the Alps. Finally, the preservation of an undeformed syn-rift sedimentary cover sealing a tectonized top basement makes this area a fossil analogue for detachment faults in necking domains. It demonstrates that preservation of pre-Alpine, rift related structures is possible even in highly deformed areas, and emphasizes the role played by pre-Alpine inheritance on the building of the Alpine Mountain belt.

Boron isotopes of white mica and tourmaline in an ultra-high pressure metapelite from the western Tianshan, China: dehydration and metasomatism during exhumation of subducted ocean-floor sediments

Boron isotopes of white mica and tourmaline in an ultra-high pressure metapelite from the western Tianshan, China: dehydration and metasomatism during exhumation of subducted ocean-floor sediments

Flexible Plasmonics Using Aluminum and Copper Epitaxial Films on Mica.

We demonstrate here the growth of aluminum (Al), copper (Cu), gold (Au), and silver (Ag) epitaxial films on two-dimensional, layered muscovite mica (Mica) substrates via van der Waals (vdW) heteroepitaxy with controllable film thicknesses from a few to hundreds of nanometers. In this approach, the mica thin sheet acts as a flexible and transparent substrate for vdW heteroepitaxy, which allows for large-area formation of atomically smooth, single-crystalline, and ultrathin plasmonic metals without the issue of film dewetting. The high-quality plasmonic metal films grown on mica enable us to design and fabricate well-controlled Al and Cu plasmonic nanostructures with tunable surface plasmon resonances ranging from visible to the near-infrared spectral region. Using these films, two kinds of plasmonic device applications are reported, including (1) plasmonic sensors with high effective index sensitivities based on surface plasmon interferometers fabricated on the Al/Mica film and (2) Cu/Mica nanoslit arrays for plasmonic color filters in the visible and near-infrared regions. Furthermore, we show that the responses of plasmonic nanostructures fabricated on the Mica substrates remain unaltered under large substrate bending conditions. Therefore, the metal-on-mica vdW heteroepitaxy platform is suitable for flexible plasmonics based on their bendable properties.

HT overprint of HP granulites in the Oisans–Pelvoux massif: Implications for the dynamics of the Variscan collision in the external western Alps

The Oisans–Pelvoux massif belongs to the Paleozoic basement of the External western Alps and records high temperature (HT) metamorphism associated with intense migmatization during the syn to post-collisional stages of the Variscan orogeny. Metamorphic assemblages related to the early collision stages have been obliterated making it difficult to constrain the earlier evolution of the unit. In this study, we report the finding of new bodies of high pressure (HP) mafic granulites, which preserve relics of the prograde evolution. Relics of Mn-rich garnet cores containing inclusions of epidote, titanite, chlorite and rare white micas constrain an early prograde stage at 460–550 °C and 0.4–1.0 GPa. The HP assemblage consists in garnet + clinopyroxene+ quartz + rutile ± plagioclase ± amphibole ± biotite and yield peak-P conditions at 650–730 °C and 1.5–1.7 GPa. Decompression was associated with heating to HT granulite-facies conditions of 800–870 °C and 0.6–0.9 GPa, which led to the development of granoblastic polycrystalline mosaics over the resorbing HP assemblage. The U-Pb dating of magmatic zircon cores constrains the emplacement age of the mafic protoliths at 479 ± 5 Ma. Timing of the prograde to HP evolution is estimated around 345–330 Ma based on rutile U-Pb dating, which is contemporaneous with HP recorded in the other crystalline massifs of the western Alps. Zircon metamorphic rims yields U-Pb dates scattering between 337 and 294 Ma, which are interpreted to record crystallization during decompression and heating to granulite facies. Metamorphic conditions during the early prograde stage precludes scenarios involving subduction of a cold passive margin and are better reconciled with thickening of an orogenic wedge during collision. In contrast with other Variscan HP relics of the external Alps, the HP rocks of the Oisans–Pelvoux massif record heating to HT granulitic conditions during decompression, which may result from longer residence time in the lower crust before exhumation.

Enhanced scanning electron microscopy images using muscovite mica, an example with Rhizaria.

Muscovite mica sheets were used as a support to capture scanning electron microscopy pictures of marine biological samples. The physical properties of the cleaved muscovite mica provide a clean background, which greatly reduces the postprocessing of images, thereby enhancing them and resulting in impressive images. We chose siliceous Rhizaria for this investigation due to their morphological diversity and elaborate skeletons. RESEARCH HIGHLIGHTS: Muscovite mica sheet was used as a SEM support for biological sample Physical properties of muscovite mica sheet can provide native dark, gray, and white background of SEM images Muscovite mica sheet should be considered for image production in the context of art and science intersection.