Application of Raman imaging and scanning electron microscopy techniques for the advanced characterization of geological samples

Raman is an important tool for diagnosing minerals in geoscience. However, smaller magnification of optical microscope assembled in conventional Raman spectroscopy limits the application of Raman in sub-micro and nano scale. Raman imaging and scanning electron microscopy (RISE) combine the advantage of scanning electron microscope and Raman spectroscopy, which can collect the morphology, composition, and structure information in the same micro region of the geological sample in situ. In this paper, we introduce the development and working mechanism of RISE, and carried out some typical applications in different research of geoscience. The purpose of this review is to allow readers to understand the basic principles and application potential of RISE in geoscience. Finally, we briefly point out current challenges faced by this technology and some research directions in the future.

Calibration of a multi-anvil high-pressure apparatus to simulate planetary interior conditions

This paper presents the setup and pressure calibration of an 800-ton multi-anvil apparatus installed at the Vrije Universiteit (Amsterdam, the Netherlands) to simulate pressure-temperature conditions in planetary interiors. This high-pressure device can expose cubic millimeter sized samples to near-hydrostatic pressures up to ~ 10 GPa and temperatures exceeding 2100 °C. The apparatus is part of the Distributed Planetary Simulation Facility (DPSF) of the EU Europlanet 2020 Research Infrastructure, and significantly extends the pressure-temperature range that is available through international access to this facility.

Tracing ultrahigh-pressure metamorphism at the catchment scale

Finding traces of ultrahigh-pressure (UHP) metamorphism in the geological record has huge implications for unravelling Earth’s geodynamic evolution, such as the onset of deep subduction. Usually, UHP rocks are identified by specific mineral inclusions like coesite and characteristic petrographic features resulting from its (partial) transformation to the lower-pressure polymorph quartz in thin sections of crystalline rocks. This approach relies on very small sample size and is thus limited to a few points within large regions. Here we present the first findings of coesite inclusions in detrital mineral grains. The intact monomineralic inclusions were detected in garnets from a modern sand sample from the Western Gneiss Region, SW Norway. They represent the first known intact monomineralic coesite inclusions in the Western Gneiss Region, and their presence is suggested to indicate the erosion of UHP rocks in the sampled catchment area. The novel approach introduced here allows for tracing UHP metamorphic rocks and their erosional products at the catchment scale instead of being limited to outcrops of crystalline rocks. It opens new avenues for the prospective exploration of UHP metamorphism in Earth’s geological record.

Tiny timekeepers witnessing high-rate exhumation processes

Tectonic forces and surface erosion lead to the exhumation of rocks from the Earth's interior. Those rocks can be characterized by many variables including peak pressure and temperature, composition and exhumation duration. Among them, the duration of exhumation in different geological settings can vary by more than ten orders of magnitude (from hours to billion years). Constraining the duration is critical and often challenging in geological studies particularly for rapid magma ascent. Here, we show that the time information can be reconstructed using a simple combination of laser Raman spectroscopic data from mineral inclusions with mechanical solutions for viscous relaxation of the host. The application of our model to several representative geological settings yields best results for short events such as kimberlite magma ascent (less than ~4,500 hours) and a decompression lasting up to ~17 million years for high-pressure metamorphic rocks. This is the first precise time information obtained from direct microstructural observations applying a purely mechanical perspective. We show an unprecedented geological value of tiny mineral inclusions as timekeepers that contributes to a better understanding on the large-scale tectonic history and thus has significant implications for a new generation of geodynamic models.

Newly discovered Late Triassic Baqing eclogite in central Tibet indicates an anticlockwise West-East Qiangtang collision

The Triassic eclogite-bearing central Qiangtang metamorphic belt (CQMB) in the northern Tibetan Plateau has been debated whether it is a metamorphic core complex underthrust from the Jinsha Paleo-Tethys or an in-situ Shuanghu suture. The CQMB is thus a key issue to elucidate the crustal architecture of the northern Tibetan Plateau, the tectonics of the eastern Tethys, and the petrogenesis of Cenozoic high-K magmatism. We here report the newly discovered Baqing eclogite along the eastern extension of the CQMB near the Baqing town, central Tibet. These eclogites are characterized by the garnet + omphacite + rutile + phengite + quartz assemblages. Primary eclogite-facies metamorphic pressure-temperature estimates yield consistent minimum pressure of 25 ± 1 kbar at 730 ± 60 °C. U-Pb dating on zircons that contain inclusions (garnet + omphacite + rutile + phengite) gave eclogite-facies metamorphic ages of 223 Ma. The geochemical continental crustal signature and the presence of Paleozoic cores in the zircons indicate that the Baqing eclogite formed by continental subduction and marks an eastward-younging anticlockwise West-East Qiangtang collision along the Shuanghu suture from the Middle to Late Triassic.

Temperature-induced oligomerization of polycyclic aromatic hydrocarbons at ambient and high pressures

Temperature-induced oligomerization of polycyclic aromatic hydrocarbons (PAHs) was found at 500–773 K and ambient and high (3.5 GPa) pressures. The most intensive oligomerization at 1 bar and 3.5 GPa occurs at 740–823 K. PAH carbonization at high pressure is the final stage of oligomerization and occurs as a result of sequential oligomerization and polymerization of the starting material, caused by overlapping of π-orbitals, a decrease of intermolecular distances, and finally the dehydrogenation and polycondensation of benzene rings. Being important for building blocks of life, PAHs and their oligomers can be formed in the interior of the terrestrial planets with radii less than 2270 km.

Synthesis, crystal structure, and luminescence properties of a new microporous europium silicate: Na3EuSi6O15·1.47H2O

A new microporous europium silicate, Na₃EuSi₆O₁₅·1.47H₂O (denoted as 1), was synthesized under high-temperature and high-pressure conditions, and structurally characterized by single-crystal and powder X-ray diffraction (XRD) analysis.

Novel open-framework europium silicates prepared under high-temperature and high-pressure conditions

Two new europium silicates, Na15Eu3Si12O36 (denoted as 1) and K2EuSi4O10F (denoted as 2), were successfully synthesized under high-temperature and high-pressure conditions, and structurally characterized by single-crystal and powder X-ray diffraction (XRD). The single-crystal XRD analysis of 1 reveals that its structure is based on [Si6O18]n(12n-) cyclosilicate anions that are built from six SiO4 tetrahedra sharing two of their four O corners with each other. Such [Si6O18]n(12n-) cyclosilicate anions are linked via EuO6 octahedra to form a three-dimensional (3D) framework containing 6-membered ring channels delimited by the SiO4 tetrahedra and EuO6 octahedra along the [010] direction. The structure of 2 consists of infinite tubular chains of corner-sharing SiO4 tetrahedra, which are further linked together via corner sharing O atoms by infinite chains of EuO4F2 octahedra forming a 3-D framework that contains 8-ring and 6-ring channels along the [010] direction. The photoluminescence properties of 1 and 2 were also investigated.

Discovery of coesite and stishovite in eucrite

Howardite-eucrite-diogenite meteorites (HEDs) probably originated from the asteroid 4 Vesta. We investigated one eucrite, Béréba, to clarify a dynamic event that occurred on 4 Vesta using a shock-induced high-pressure polymorph. We discovered high-pressure polymorphs of silica, coesite, and stishovite originating from quartz and/or cristobalite in and around the shock-melt veins of Béréba. Lamellar stishovite formed in silica grains through a solid-state phase transition. A network-like rupture was formed and melting took place along the rupture in the silica grains. Nanosized granular coesite grains crystallized from the silica melt. Based on shock-induced high-pressure polymorphs, the estimated shock-pressure condition ranged from ∼8 to ∼13 GPa. Considering radiometric ages and shock features, the dynamic event that led to the formation of coesite and stishovite occurred ca. 4.1 Ga ago, which corresponds to the late heavy bombardment period (ca. 3.8-4.1 Ga), deduced from the lunar cataclysm. There are two giant impact basins around the south pole of 4 Vesta. Although the origin of HEDs is thought to be related to dynamic events that formed the basins ca. 1.0 Ga ago, our findings are at variance with that idea.

Kinetic and microstructural studies of the crystallisation of coesite from quartz at high pressure

The kinetics of the α-quartz to coesite transition have been studied in the pressure range of 3.2 GPa to 5.2 GPa and in the temperature range 950 K–1175 K by in situ X-ray diffraction using a MAX80 cubic anvil high-pressure apparatus at the Hamburger Synchrotronstrah-lungslabor (HASYLAB). During the phase transition, X-ray diffraction patterns were collected at intervals of 60 s. The transformed volume fraction has been calculated from the diffracted intensities of the respective phases as a function of time. By fitting a fundamental rate equation for grain boundary nucleation and interface-controlled growth to the transformation-time data, rates of nucleation and growth have been calculated. A discrepancy between the experimental determined and theoretical calculated phase boundary is discussed.

After quenching the samples, the reaction products could be investigated by TEM images. A specific feature of the synthesised coesite crystals was a concentrated microtwinning. Its influence on the crystal structure and thus on the thermodynamic behaviour of the α-quartz-coesite phase transition is illustrated.

High pressure single crystal X-ray diffraction study on α-quartz

Single crystal X-ray diffraction experiments on α-quartz (trigonal, space group P3121) were performed to pressures up to 19.3 GPa using synthetic samples.

Volumina of the unit cell were determined to 13.1 GPa, by combining the data with data taken from the literature; the bulk modulus and pressure derivative calculate to 38.7(3) GPa and 5.2(1) respectively according to a Birch-Murnaghan equation of state.

Intensity data were collected at 10.9(1), 12.0(1), 12.1(1), 12.6(1) and 13.1(1) GPa. From the five intensity data sets, one was collected using synchrotron radiation at HASYLAB/DESY and the other four using a conventional X-ray tube. Results show that the SiO4 tetrahedra, the building blocks of the structure, become increasingly distorted as pressure is increased but no significant change in the polyhedral volume is observed. The volume reduction is mainly compensated by the decrease in the inter-tetrahedral Si–O–Si-angle, which is correlated to the increase in the tilt angle φ.

In addition to the five intensity data sets collected, X-ray diffraction experiments were carried out up to 19.3 GPa in order to address the question of when the structure undergoes amorphization. It was observed that up to 19.3 GPa, no pressure-induced amorphization takes place. This result is in agreement with studies carried out on powder quartz and theoretical studies.

Rare earth disilicates R 2Si2O7 (R = Gd, Tb, Dy, Ho): type B

The type B structure of the single rare earth element disilicates has been revised using room temperature and pressure single-crystal X-ray diffraction measurements on Gd2Si2O7, Tb2Si2O7 and Ho2Si2O7 synthesized at 2–2.5 GPa, 1400–1500 °C in a piston-cylinder apparatus and Dy2Si2O7 synthesized hydrothermally at 0.1 GPa. Crystal data are: triclinic, space group P1̅, Z = 4; Gd2Si2O7 – a = 6.6609(2), b = 6.7081(2), c = 12.1390(4) Å, α = 94.277(1), β = 90.577(1), γ = 91.441(1)°, R = 0.033, and Dx = 5.929 g/cm3; Tb2Si2O7 – a = 6.6331(3), b = 6.6799(2), c = 12.0967(2) Å, α = 94.128(1), β = 90.609(2), γ = 91.541(1)°, R = 0.038, and Dx = 6.041 g/cm3; Dy2Si2O7 – a = 6.6158(2), b = 6.6604(2), c = 12.0551(4) Å, α = 94.373(2), β = 90.836(2), γ = 91.512(2)°, R = 0.031, and Dx = 6.156 g/cm3; and Ho2Si2O7 – a = 6.5960(2), b = 6.6328(3), c = 12.0214(6) Å, α = 94.479(1), β = 90.856(1), γ = 91.749(2)°, R = 0.034, and Dx = 6.313 g/cm3. Substitution of Gd3+, Tb3+, Dy3+, and Ho3+ cations into the type B structure results in proportional linear decrease in size of the REEOn polyhedra and unit-cell parameters, and leaves the silicate stereochemistry essentially unchanged.

Fossilized high pressure from the Earth's deep interior: the coesite-in-diamond barometer

Mineral inclusions in diamonds provide an important source of information about the composition of the continental lithosphere at depths exceeding 120-150 km, i.e., within the diamond stability field. Fossilized high pressures in coesite inclusions from a Venezuela diamond have been identified and measured by using laser Raman and synchrotron x-ray microanalytical techniques. Micro-Raman measurements on an intact inclusion of remnant vibrational band shifts give a high confining pressure of 3.62 (+/-0.18) GPa. Synchrotron single-crystal diffraction measurements of the volume compression are in accord with the Raman results and also revealed direct structural information on the state of the inclusion. In contrast to olivine and garnet inclusions, the thermoelasticity of coesite favors accurate identification of pressure preservation. Owing to the unique combination of physical properties of coesite and diamond, this "coesite-in-diamond" geobarometer is virtually independent of temperature, allowing an estimation of the initial pressure of Venezuela diamond formation of 5.5 (+/-0.5) GPa.

Serpentine Stability to Mantle Depths and Subduction-Related Magmatism

Results of high-pressure experiments on samples of hydrated mantle rocks show that the serpentine mineral antigorite is stable to approximately 720 degrees C at 2 gigapascals, to approximately 690 degrees C at 3 gigapascals, and to approximately 620 degrees C at 5 gigapascals. The breakdown of antigorite to forsterite plus enstatite under these conditions produces 13 percent H(2)O by weight to depths of 150 to 200 kilometers in subduction zones. This H(2)O is in an ideal position for ascent into the hotter, overlying mantle where it can cause partial melting in the source region for calc-alkaline magmas at a depth of 100 to 130 kilometers and a temperature of approximately 1300 degrees C. The breakdown of antigorite in hydrated mantle produces an order of magnitude more H(2)O than does the dehydration of altered oceanic crust.