Microtexture Analysis Research Articles

Microtextural signatures in quartz grains and foraminifera from tsunami deposits of the Portuguese shelf

This study presents results from two sediment cores collected on the southern Portuguese shelf attempting to, partially, fill the knowledge gap of the offshore record of high-energy events. The results were obtained based on description of cores, microtextural analysis of quartz grains, and foraminiferal taphonomy. The lithostratigraphy corresponds to Late Holocene sedimentation, with interesting intercalations of medium sand rich with bioclastic fragments and erosive basal contact. In terms of microtextures, a high degree of mechanical marks on the grains associated with tsunami deposition was observed and reflects the high-energy hydrodynamic processes. In compositional terms, the higher presence of quartz grains in these units favors the increase of mechanical marks because grain-to-grain contact is more intense. Additionally, the geomorphological setting of the coring sites controlled the degree and type of mechanical microtextures observed. Furthermore, post-depositional changes and characteristics of the original sediment source contribute to explain the occurrence of dissolution in units of GeoB23513-02. The foraminiferal taphonomy displayed a predominance of dissolution alteration in the test surfaces that was more evident in the silty layers. On the other hand, similar to quartz grain microtextural signature, the sandy high-energy units exhibit a slight predominance of physical processes despite the still strong presence of dissolution. The sole presence of foraminifera species from the middle to the outer shelf in some units is an indication that there was little reworking of specimens. This work aims to increase the understanding of dynamics during Holocene high-energy events and to characterize their backwash phases through the different imprints left in the sedimentary offshore record of the Portuguese Algarve shelf.

Electrochemical dissolution behavior of phosphorus and boron doped Inconel 718 alloy in NaNO3 solution

As a promising material for the blisk of high-pressure compressors in aeroengines, modified Inconel 718 (IN718) alloy with phosphorus and boron additions is a new derivative of IN718 with higher temperature capabilities. Herein, electrochemical tests and electrochemical machining (ECM) experiments of the modified IN718 alloy in NaNO3 solution were conducted to investigate the electrochemical dissolution behavior. Microstructure and microtexture analysis revealed that modified IN718 fabricated by forging mainly comprises γ phase, carbide, and δ phase, and its grain size distribution is uneven. Furthermore, the anodic behavior of the modified IN718 alloy in NaNO3 solution was investigated by the polarization curve and electrochemical impedance spectroscopy. The results show that an uneven passivation film forms on the surface of modified IN718 due to the inhomogeneous grain size, which reduces the corrosion resistance. Then, ECM experiments at different corrosion times (1 s, 3 s,5 s, 7 s) and current densities (50 A/cm2, 100 A/cm2, 130 A/cm2, 160 A/cm2) were conducted. The results show that the δ phase existed like a needle-plate and the machined surface quality was poor. The corrosion resistance of the fine grains and δ phase was better than that of the coarse grains, which is the main factor influencing the surface quality. Finally, dissolution models were proposed to characterize the dissolution behavior of modified IN718 in NaNO3 solution.

Non-Cyanide Thick Silver Electrodeposition Base on Instantaneous Nucleation for 3D Microstructures with High Performance

To meet the diversified development of electronics, it is essential to develop 3D metal microstructures with high performance. However, 3D thick silver forming is still a challenge due to high internal stresses and various defects. Here we proposed a method for the thick Ag electrodeposition with low-stress and high chemical stability for 3D Ag-based microstructures. We carried out systematic studies through electrochemical measurements, morphological characterization, micro-texture analysis, internal and stability examination. The results show that the nucleation mechanism of electroplating Ag is transformed from progressive nucleation to instantaneous nucleation through the coordinated control of current density and temperature. Higher temperature and lower deposition potential are preferred to form the large-grained crystallization. Additionally, the external parameters for fabricating Ag coating were optimized, and the correlation between micro-texture and internal stress was also verified. The feasibility of thick Ag preparation process was demonstrated by extending the electroplating time, showing excellent performance. Finally, 3D Ag micropillar arrays with the height of 577 μm and the high aspect ratio close to 3 were fabricated by MEMS processing techniques, displaying satisfied 3D forming ability. This work presents a new approach for manufacturing the 3D Ag microstructures and lays foundation for the multi-functionalization Ag-based devices.

Factors influencing the development of microtextures on cold‐climate aeolian quartz grains revealed by experimental frost action

AbstractAeolian‐originated quartz grains of coarse‐sand size (0.5–1 mm) were subjected to experimental frost weathering. A total of 1,000 freeze–thaw cycles with temperature ranges from −5 to +10°C were simulated under full water availability conditions. Scanning electron microscope microtextural analysis of grain surfaces conducted after 0, 50, 100, 300, 700, and 1,000 freeze–thaw cycles resulted in different‐sized conchoidal fractures and breakage blocks as frost‐induced microtextures. The vast majority of these microtextures were encountered on the most convex parts of aeolian grains and their number increased with ongoing freeze–thaw cycles. However, the number of recorded frost‐originated microtextures remained relatively small up to 700 freeze–thaw cycles and increased after 1,000 freeze–thaw cycles. Transmission electron microscope microstructural analysis of grains after 0, 100, and 1,000 freeze–thaw cycles showed both primary (e.g., inclusions, grain boundaries) and secondary (e.g., cracks) defects in quartz crystals. The frequency of the latter remained unexpectedly low. The susceptibility of aeolian‐originated sand‐sized quartz grains to frost‐induced modifications is interpreted here to depend mainly on their internal characteristics. These include aeolian‐driven development of a subsurface impact zone that determines the depth to which frost‐originated microtextures develop. The outer impact zone consists of a thin layer of surficial crust and a series of more or less parallel ridges arranged into mechanically upturned plates. The inner impact zone consists of intact or cracked quartz crystals. The susceptibility of aeolian‐originated quartz grains to frost‐induced modifications depends therefore on a combination of internal (i.e., original crystallography of quartz grains) and external (i.e., aeolian and frost processes acting upon the grains) factors.

PRELIMINARY PETROMAGNETIC STUDY OF 1849, 1926, 1963, 1968 AND 1974 SURFACE LAVAS FROM BATUR VOLCANO, BALI, INDONESIA: INSIGHT ON THE MAGMATIC PROCESS OF SOURCE AND ROCK MAGNETIC NATURE

Geochemical and petrographic characterizations were carried out on five episodic effusive eruptions from Batur Volcano on the Island of Bali, Indonesia, and revealed that these lavas are basaltic to andesitic in composition. Various micro-textures were identified, reflecting pre-eruptive magmatic processes, magma mixing, and adiabatic decompression. Apart from XRF analyses, the five lava flows (L849, L926, L963, L968 and L974) were subjected to magnetic analyses in the form of magnetic susceptibility and magnetic hysteresis measurements. The presence of cyclicity in the magmatic process is suggested by the variation of micro-texture types, CaO content, and magnetic susceptibility values. Two possible models of this cyclicity are presented. In the first model, cyclicity is driven by the variation of influx from the lower magma chamber that affects the interaction of magma with the surrounding rocks. In the second model, cyclicity is affected by the appearance of a new magmatic vent, which causes intense interaction with surrounding rocks. This study infers that combined micro-texture, composition, and magnetic susceptibility analyses might provide insight into the cyclicity of lava episodes observed in active volcanos such as Batur.

Graphene Oxide Induced Sub-structures of Bi-tilted Smectic CG in Dimer Liquid Crystals

A nanocomposite built of the hydrogen-bonded (HB) dimer liquid crystal 4-heptyloxybenzoic acid (7OBA) and Graphene oxide (GO) exhibiting a cascade of phase transitions and phases not typical for pristine 7OBA, was investigated by optical polarization microtexture analysis and Raman spectroscopy. We use the 7OBA/GO nanocomposite beacause of the effective functionalizing properties of GO to study the generation and development of the ferroelectric smectic CG phase in its bi-tilted configuration. For the appearance of the CG phase with its substructures we propose an explanation based on the HB formation between 7OBA dimers in closed and open conformations and the GO sheets, aided by the π–π interaction with graphene hexagons which are free from functional groups. We propose molecular and macroscopic models for the substructures CGcl and CGln and indicate their triclinic C1 symmetry, characterized by a polar vector directed oblique to the layers. This confirms that the bi-tilted CG and its substructures are ferroelectric in the bulk similarly to solid ferroelectrics, thus showing potential as material for controllable photonic functional devices.

Geochemical characterization of altered rocks and element dispersion around an abandoned mining site in an arid zone: A case study of Fatira gold mine area, Eastern Desert, Egypt

Environmental impacts related to mining are widespread due to poor waste management. Fatira abandoned gold mine area in Egypt was studied for the geochemical/mineralogical properties of alterations haloes, mine tailings, stream sediments under dry conditions and impacts to nearby environment. Results of this study indicated that stream sediments and soil-tailings mixture were derived from felsic sources, while tailings, weathered and old tailings were obtained from intermediate-felsic sources. The Pb load was much higher in weathered rocks, soil-tailings mixture and old tailings, while metals like As, Co, Cr, Cu, Ni, V, and Zn show elevated values in weathered rocks nearby Fatira mine area and in downstream sediments. The tailings have high average concentrations of heavy elements, Sr (439), Zn (389), Cu (310), and Cr (50) (in mg/kg), higher than the toxicological standard values for contaminants indicating adverse effects to the neighbouring environment, whereas, soil-tailings mixture have higher As (263), Pb (56) and Al2O3 (17 wt%) reflecting the metavolcanics and granite composition of country rocks. At lower depths of tailings, escalated concentrations of Al2O3, K2O, Na2O, and Fe2O3, and little CaO, MgO, As, Pb, Zn, Cu, and Ni are indicate variable input from intermediate rocks, grey granites, felsite dykes and pink granites. Chemically altered granites showed replacement of hornblende into actinolite, calcite, pitted ilmenite and in-situ formation of Fe-oxides/hydroxides. Low hydraulic conductivity of the rocks increased the intensity of weathering. Gypsum and goethite are recorded due to repetitive sulfides decomposition. The element enrichment that exceeds the standard toxicological values in tailings, soil-tailings mixture, weathered rocks and stream sediments have severe effects on health. Integrating the chemistry, mineralogy and microtextural analyses in the study area is required for accurate geoenvironmental forecasting that facilitate mine rehabilitation with an effective management plan.

Morphological and multifractal properties of Cr thin films deposited onto different substrates.

In this study, the morphological properties and micro-roughness of chromium thin film prepared by thermal evaporation technique and confirmed via EDS analysis are examined on different substrates of BK7, Silicon (Si), and glass using atomic force microscope analysis (AFM). Analysis of amplitude parameters, Minkowski functionals, and films' spatial microtexture extracted from AFM analysis showed the difference between glass substrate and the other two (BK7 and Si) substrates for the growth of chromium thin films. In addition, we observed robust signatures of multifractality of the Cr thin films deposited on all substrates we studied. Moreover, we highlight that the Glass substrates displayed the strongest multifractality indicating that such samples present space filling properties distributed over more spatial scales than the samples of BK7 and Si.

Strain rate and microtexture analysis of rapid-quenched AA6061 friction stir welds produced with tool eccentricity

The paper investigates the effect of tool eccentricity on the strain rate and microtextures produced in rapid-quenched AA6061 friction stir welds. Thermocouples are embedded in the tool and the thermal cycle are recorded via a data acquisition system. Stir zone grain refinement is observed in all samples, with the eccentric sample having the finest grains (3.18 μm). The strain rate is measured to be between 0.17 and 51.94 s−1, with the eccentric sample experiencing the highest strain rate. The pole figures indicate crystallographic textures shifted approximately 90° when tool eccentricity is employed. The aligned samples have a strong {11-2}<1-10> B simple shear texture component, while the eccentric sample shows morphological changes towards a strong {11-1}<1-10> A simple shear texture component.

Technological continuity and change in Late Bronze and Iron Age Plain Ware pottery from Sirkeli Höyük (Cilicia, southern Anatolia)

The period from the Late Bronze Age (c. 1550–1190 BCE) to the Iron Age (c. 1190–330 BCE) in the eastern Mediterranean is characterized by upheavals in political and socio-economic structures, accompanied by changes in material culture such as pottery production. Plain Ware is one of the most common pottery types found in Plain Cilicia in southern Anatolia during the Late Bronze and the Iron Ages and displays typological and stylistic variations during these periods. In this work, we examined the Plain Ware from Sirkeli Höyük, one of the key settlements in Plain Cilicia, using petrographic, mineralogical (XRPD), microtextural (SEM-EDX), and chemical analysis (LA-ICP-MS). The main objective is to determine the source of raw materials and to investigate Plain Ware production including clay procurement, clay processing, and firing techniques. Our archaeometric results suggest that the vessels were produced from calcareous clay available in the Ceyhan Plain. We have observed continuity in the methods of Plain Ware production from the Late Bronze Age to the Iron Age with a minor change in the Iron Age.

Multi‐stage evolution of frost‐induced microtextures on the surface of quartz grains—An experimental study

AbstractCoarse sand‐sized (0.5–1.0 mm) grains of vein quartz were subjected to frost‐induced stress under controlled laboratory conditions. A total of 1,000 freeze–thaw (FT) cycles, simulated under different (low, high) water mineralization conditions in the temperature range from −5°C up to +10°C, were used to test effects on collected samples. Scanning electron microscopic (SEM) microtextural analysis of grain surfaces was performed at 0 (start) and after 50, 100, 300, 700, and 1,000 FT cycles. The results indicate that variable frost‐induced microtextural imprints encountered on quartz grain surfaces prior to and following analysis depend largely on the mineralization (dissolved solute content) of water involved in the weathering process. The higher the water mineralization, the greater the intensity of mechanical weathering. Two predominant outcomes in the course of these micro‐scale frost weathering tests have been identified: a physical (mechanical) aspect manifested by the occurrence of conchoidal fractures and breakage block microtextures dominating up to 300 FT cycles, and a chemical aspect resulting in the occurrence of precipitation crusts and obliteration of grain microrelief. Moreover, three additional stages of microtexture development may be distinguished with the evolution of frost‐induced microrelief on the surface of quartz grains: (i) initial cracks of large‐sized conchoidal fractures, (ii) increasing frost cycles yielding additional small‐sized conchoidal fractures, and (iii) advanced breakage blocks. Frost‐induced exposure of fresh, unweathered grain surfaces leads to refreshing of the grain surface.

Multiproxy study of 7500-year-old wooden sickles from the Lakeshore Village of La Marmotta, Italy

The lakeshore site of La Marmotta is one of the most important Early Neolithic sites of Mediterranean Europe. The site is famous for the exceptional preservation of organic materials, including numerous wooden artefacts related to navigation, agriculture, textile production, and basketry. This article presents interdisciplinary research on three of the most complete and well-preserved sickles recovered from the site, yet unpublished. All the components of the tools are analysed: the stone inserts, the wooden haft and the adhesive substances used to fix the stones inside the haft. Our innovative methodology combines use-wear and microtexture analysis of stone tools through confocal microscopy, taxonomical and technological analysis of wood, gas chromatography–mass spectrometry analysis of the adhesive substances, and pollen, non-pollen palynomorphs, and phytolith analysis of the remains incorporated within the adhesive. This multiproxy approach provides a significant insight into the life of these tools, from their production to their use and abandonment, providing evidence of the species of harvested plants and the conditions of the field during the harvesting.

Effect of grain boundary constitution on corrosion behaviour in cobalt-carbon nanotube composite coatings

Corrosion behavior and micro-texture of electrodeposited cobalt coatings containing different volume fractions of carbon nanotubes (CNTs) were investigated and correlated. For lower CNT volume fractions, a compact and smoother coating morphology were observed. The morphology however, became coarser for higher CNT volume fractions. Corrosion properties of the coatings were determined using the electrochemical impedance spectroscopy and Tafel polarization techniques. Pristine cobalt coating showed corrosion potential (Ecorr), corrosion current density (icorr), and polarization resistance (Rp) values of -0.574 V, 21.48µA/cm2 and 2071 ohms.cm2 respectively. With the incorporation of CNTs, the coating corrosion resistance increased up to Co-CNT3 coating (produced by 20mg/L of CNT dispersion). Co-CNT3 coating showed Ecorr, icorr and Rp values of -0.521V, 5.33µA/cm2 and 3525 ohms.cm2 respectively. However, for higher volume fractions of CNTs, the coating corrosion resistance dropped to values lower than pristine cobalt coating. For Co-CNT6 coating (produced from 80 mg/L of CNT dispersion) the Ecorr, icorr and Rp values were -0.581V, 23.45µA/cm2 and 1409 ohms.cm2 respectively. Micro-texture analysis was conducted for pristine Co and Co-CNT3 coatings to investigate the correlation between the observed corrosion behavior and coating micro-texture. Both the coatings showed strong {211¯0} texture along the coating growth direction. When compared to printine Co coating, the Co-CNT3 coating showed lesser fraction of high angle grain boundaries as well as higher fraction of low energy special boundaries like STGBs (32°/ [211¯0], 62°/ [211¯0] angle axis pair) leading to better corrosion resistance performance for Co-CNT3 coatings.

Microtexture analysis of copper-doped iron oxide thin films prepared by air pneumatic spray.

The stereometric and fractal concepts are crucial tools to analyse, to verify, to report 3-D microtexture of thin film surfaces on the nanometre scale and thereby to generate useful topographic characteristics for better understanding and steering them toward further improvements and rational use in modern applications. At first, the present work aimed to prepare hematite α-Fe2 O3 thin films with (0, 2, 4, 6 and 8 wt%) of Cu doping by using the air pneumatic spray method. Subsequently, the obtained pure α-Fe2 O3 and Cu-doped α-Fe2 O3 thin films were characterised by XRD device, which determines their polycrystalline nature with the rhombohedral hematite structure. Analysis by UV-VIS absorption showed that the transmittance of the thin films is extinct in the wavelength from approximately 500 to 800 nm, revealing that the films have good optical absorbance in the visible region. The obtained bandgap values varied between 2.23 and 2.21 eV. At second stage, the stereometric and fractal analysis are applied on 3-D image data of pure α- Fe2 O3 and Cu-doped α-Fe2 O3 thin films, which in prior generated using AFM device. Accordingly, the obtained statistical parameters such as surface roughness, density distribution of peaks, depths etc. were used to understand the influence of Cu doping on the 3D microtexture of pure α- Fe2 O3 and Cu-doped α- Fe2 O3 thin film surfaces.

Amphibolite facies metamorphism in lower Witwatersrand Supergroup rocks exposed in the Vredefort Dome – a Ventersdorp LIP connection

Abstract Metamorphism in the lower Witwatersrand Supergroup exposed in the Vredefort Dome has previously been proposed to be related to elevated heat flow linked to the 2.06 Ga Bushveld magmatic event; however, there are no unambiguous chronological data to confirm this timing. Microtextural and mineral compositional analysis of a garnet-bearing metapelite in the northwestern collar of the Vredefort Dome suggests at least two metamorphic events with distinctly different P-T conditions preceding the Dome-forming meteorite impact at 2.02 Ga. THERMOCALC mineral equilibrium calculations yield P-T conditions of 500°C, 3.1 kbar for the M1 mineral assemblage garnet1-plagioclase-muscovite-biotite-chlorite-ilmenite. Thin, discontinuous, garnet2 overgrowths on the garnet1 porphyroblasts define a subsequent, M2, event with P-T conditions of 530°C, 5 kbar. Garnet Sm-Nd chronology yields an isochron age of 2 796.0 ± 1.5 Ma, indicating an early Ventersdorp (Klipriviersberg) timing of M1 metamorphism. Although the M2 garnet overgrowths are volumetrically too small to date, the calculated M2 pressure is consistent with the predicted overburden thickness above the lower Witwatersrand Supergroup during emplacement of the Bushveld Complex. While elevated, the M2 apparent geotherm (30°C/km) is significantly lower than the M1 apparent geotherm (46°C/km); however, thermal modelling suggests both events benefitted from local perturbations caused by contemporaneous sill emplacement. Our results thus show that the initial garnet-forming, mid-amphibolite facies metamorphism in the collar of the Vredefort Dome is not related to the emplacement of the Bushveld Complex, but rather to the early stages of magmatism associated with the Ventersdorp Large Igneous Province (LIP). Nonetheless, elevated heat flow associated with the Bushveld LIP also reached comparable amphibolite facies conditions.

Characterization of the deformation bands in AA1100 deformed by plane forging and return-plane forging

In this study, the deformation bands (DBs) in AA1100 after plane forging (PF) and return-plane forging (R-PF) during multi-axis diagonal forging (MADF) were observed using microtexture analysis. Typical texture components (plane-strain texture components) observed in face-centered cubic (FCC) materials deformed by plane-strain compression were developed in the PF specimen. Blurry bands with an orientation relationship of mainly LAGBs were developed in the deformed grains in the PF specimen. Slip trace analysis and Schmid factor (SF) calculations revealed the correlation between the blurry bands and the slip behavior. Regions of matrix and DBs inside the deformed grains of the R-PF specimen were distinguished mostly by their HAGBs. The heterogeneity of the Taylor factor (M) values that occurred within the blurry bands and their surrounding regions induced the development of the DBs in the R-PF specimen. The heterogeneities of the deformation behavior between the inside and the outside of the blurry bands that developed in the grains of the PF specimen during the R-PF were a direct reason for the generation of DBs in the deformed grains of the R-PF specimen. The transmission-EBSD (t-EBSD) technique revealed that the average misorientation angle inside the DBs was higher than that inside the matrix.

Unraveling the formation mechanism of deformation bands in AA1100 alloy during plane forging and return-plane forging

In this study, the formation mechanism of deformation bands (DBs) observed in AA1100 after plane forging (PF) and return-plane forging (R-PF) during multi-axis diagonal forging (MADF) was unraveled via microtexture analysis using a rate-sensitive model and crystal plasticity finite element method (CPFEM). Blurry bands with an orientation relationship of mainly LAGBs were developed in the deformed grains in the PF specimen. The heterogeneity of the orientation stability (Q) that occurred within the blurry bands and their surrounding regions contributed to the development of the DBs in the deformed grains in the R-PF specimen. Microstructure-based CPFEM successfully simulated the development of DBs within deformed grains during R-PF. CPFEM revealed that heterogeneity of the crystallographic orientation that occurred within the blurry bands and their surrounding regions inside the PF specimen was the main cause of the formation of DBs in the deformed grains during R-PF.

Magmatic-hydrothermal evolution of the El Laco iron deposit revealed by trace element geochemistry and high-resolution chemical mapping of magnetite assemblages

The Plio-Pleistocene El Laco magnetite ore bodies in the Chilean Altiplano represent an unusual subvolcanic/aerial type of an iron oxide-apatite (IOA) deposit. The textures of these magnetite ore bodies have sustained a long-standing geological controversy on the origin of iron oxide deposits with models spanning the spectrum from purely igneous to magmatic-hydrothermal processes. In particular, the link between the geochemical processes taking place in the source magma and the subsequent evolution of the overlying magmatic-hydrothermal system are still not well understood. Here we address this problem by focusing on microtextural and geochemical features of iron ore and silicate gangue minerals retrieved from drill core samples from El Laco ore bodies, as well as from the andesite volcanic host rocks. We report a comprehensive geochemical dataset of magnetite assemblages at El Laco obtained using a combination of EPMA, LA-ICP-MS, and synchrotron radiation µ-XRF methods. Microtextural and geochemical data of magnetite reveal consistent variations with depth. Magnetite in the andesitic host rocks and from deep/intermediate levels of the ore bodies have a high concentration of trace elements including Ti, V, Ni, Mn, Zn, Cr, Al, Ga, Cu and Co in comparison with magnetite from the upper sections of the ore bodies. Clinopyroxene is present in the andesite host rocks as well as the ore bodies, showing chemical differences between both types, with Mn and Fe contents that are higher in the former, and Na and Ca concentrations enriched in the latter. We interpret the enriched-to-depleted compositional trends in magnetite and clinopyroxene as resulting from a transition from purely igneous conditions to a fluid-dominated, cooling magmatic-hydrothermal system. Detailed microtextural analysis of magnetite and clinopyroxene from the ore bodies support this notion, revealing multiple growth stages punctuated by dissolution-reprecipitation processes. Our data further supports a genetic model that explains the formation of the El Laco iron oxide deposit through the injection, upward migration and venting of magmatically-derived Fe-rich fluids.

Exhumation of deep continental crust in a transpressive regime: The example of Variscan eclogites from the Aiguilles‐Rouges massif (Western Alps)

AbstractMafic eclogites are found in many orogens as lenses embedded in quartzofeldspathic migmatites. These high‐pressure relics are interpreted either as remnants of ancient sutures and thus formed during oceanic subduction or as fragments of lower crust exhumed from the root of orogenic thickened crust. It is critical to distinguish between these two endmember scenarios as the resulting palaeogeographic and geodynamic reconstructions may significantly differ. In this contribution, we investigated eclogite relics from Lac Cornu in the Aiguilles‐Rouges massif, one of the External Crystalline Massifs of the Western Alps. Phase equilibrium modelling suggests that these mafic rocks were buried along a prograde path (M1) from ~600°C/1.2 to 1.6 GPa to peak conditions of ~630–775°C and &gt;1.6 GPa. Zircon rims, with a rare earth element signature typical of eclogite facies zircon (no Eu anomalies, flat HREE spectrum), and rutile were dated by U–Th–Pb laser ablation inductively coupled plasma mass spectrometry (LA‐ICPMS) at c. 335–330 Ma. Prograde deformation has not been identified in the field and is only recognized thanks to the crystallographic preferred orientation (CPO) of inclusions of omphacite and rutile in garnet. Peak pressure conditions were followed by a decompression stage (M2) from ~760°C/1.4 GPa to ~600–650°C/0.9 GPa supported by the breakdown of omphacite into plagioclase–clinopyroxene symplectite and the crystallization of plagioclase–amphibole corona around garnet. The M2 retrogression stage is associated with the development of a main sub‐horizontal planar fabric and the CPO of minerals composing symplectite. This deformation stage is interpreted as the result of horizontal lower crustal flow. The final stage of exhumation (M3) is characterized by the replacement of symplectite and garnet by plagioclase and large euhedral amphibole and by the breakdown of rutile and ilmenite into titanite dated at c. 300 Ma. The CPO of titanite and amphibole are consistent with the development of vertical dextral shear zone in a transpressive regime. The combination of field observations and petrological, microtextural and geochemical analyses suggests that the mafic eclogites preserved in migmatitic rocks of the Aiguilles‐Rouges massif are remnants of a continental lower crust exhumed and juxtaposed with lower‐grade migmatites in crustal‐scale vertical transpressive shear zones.

The Biotic‐Abiotic Control of Si Burial in Marine Carbonate Systems of the Pre‐Eocene Si Cycle

AbstractThe silicon (Si) cycle in the modern ocean might still be representative of some of the processes that occurred in the Si‐depleted post‐Eocene oceans resulting after the expansion of diatoms. However, silicon‐rich pre‐Eocene seas, where sponges and radiolarians were major Si users before the emergence of diatoms, were radically different from modern ocean scenarios. The spatial and temporal evolution of Si cycling in Earth history is recorded in geological deposits and could be reconstructed by petrographic and mineralogical analyses. The thick successions of carbonate siliceous rocks deposited in marine environments during the Paleozoic and Mesozoic indicate the significant role of Si outflow from the Si cycle via burial in sediments. The aim of this study is to fill an important gap in knowledge concerning the functioning of the main silicon sink in the oceans: burial in sediments. Si outflow from the marine biogeochemical Si cycle occurs via early diagenetic silica crystallization within seabed mud. The mechanisms leading to Si binding in silica minerals are a complex process controlled by abiotic global events, biological activity of silicifiers and geochemical conditions mediated by microbes. This study concentrates on reconstructions of the mechanisms of Si outflow from the Si cycle by silica polymorph crystallization below the seabed surface and was realized through mineralogical and microtextural analyses of the main components of Upper Cretaceous carbonate–siliceous rocks.