Katharina Marquardt

Metallic lead nanospheres discovered in ancient zircons

Significance Metallic lead nanospheres have been discovered in ancient (>3.4 Ga) zircon grains from an Archean (2.5 Ga) high-grade metamorphic terrain in East Antarctica. Native Pb is present as 5–30 nm nanospheres, commonly in association with an amorphous silica-rich phase, along with titanium and aluminium-bearing phases. Together, these phases form nanoinclusions generated during the recovery of crystallinity in radiation-damaged zircon under high-grade metamorphic conditions. Once formed, the entrapment of nanospheres in annealed zircon effectively arrests Pb loss, explaining why zircon that has experienced such extreme conditions is not completely reset to its metamorphic age. The heterogeneous distribution of Pb can, however, affect isotopic measurement by microbeam techniques, leading to spurious age estimates. Metallic Pb is extremely rare in nature and has never previously been observed in high temperature rocks. Zircon (ZrSiO4) is the most commonly used geochronometer, preserving age and geochemical information through a wide range of geological processes. However, zircon U–Pb geochronology can be affected by redistribution of radiogenic Pb, which is incompatible in the crystal structure. This phenomenon is particularly common in zircon that has experienced ultra-high temperature metamorphism, where ion imaging has revealed submicrometer domains that are sufficiently heterogeneously distributed to severely perturb ages, in some cases yielding apparent Hadean (>4 Ga) ages from younger zircons. Documenting the composition and mineralogy of these Pb-enriched domains is essential for understanding the processes of Pb redistribution in zircon and its effects on geochronology. Using high-resolution scanning transmission electron microscopy, we show that Pb-rich domains previously identified in zircons from East Antarctic granulites are 5–30 nm nanospheres of metallic Pb. They are randomly distributed with respect to zircon crystallinity, and their association with a Ti- and Al-rich silica melt suggests that they represent melt inclusions generated during ultra-high temperature metamorphism. Metallic Pb is exceedingly rare in nature and previously has not been reported in association with high-grade metamorphism. Formation of these metallic nanospheres within annealed zircon effectively halts the loss of radiogenic Pb from zircon. Both the redistribution and phase separation of radiogenic Pb in this manner can compromise the precision and accuracy of U–Pb ages obtained by high spatial resolution methods.

Focused ion beam preparation and characterization of single-crystal samples for high-pressure experiments in the diamond-anvil cell

Abstract We show that the focused ion beam (FIB) technique is well suited to prepare single-crystal samples with defined dimensions and shape and excellent surface qualities for use in high-pressure experiments carried out in the diamond-anvil cell. The method allows for cutting and polishing delicate samples, including tiny, brittle, or metastable phases and thereby extends the range of materials that can be routinely probed at extreme pressures and temperatures. In addition, the technique is capable of producing electron-transparent foils from the same sample material that can be characterized on the nanometer scale by transmission electron microscopy (TEM). The application of the method to the preparation of various geomaterials is discussed with a focus on the preparation of double-side polished, transparent sample platelets for the use in Brillouin scattering experiments at extreme conditions. Using one of our FIB-prepared samples, we were able to perform direct experimental measurements of acoustic wave velocities of antigorite along crystallographic directions, which were previously inaccessible to direct Brillouin scattering measurements. At 0.6 GPa, we measure a 39% anisotropy of compressional wave velocities.

The most frequent interfaces in olivine aggregates: the GBCD and its importance for grain boundary related processes

AbstractRocks consist of crystal grains separated by grain boundaries that impact the bulk rock properties. Recent studies on metals and ceramics showed that the grain boundary plane orientation is more significant for grain boundary properties than other characteristics such as the sigma value or disorientation (in the Earth’s science community more frequently termed misorientation). We determined the grain boundary character distribution (GBCD) of synthetic and natural polycrystalline olivine, the most abundant mineral of Earth’s upper mantle. We show that grain boundaries of olivine preferentially contain low index planes, in agreement with recent findings on other oxides (e.g. MgO, TiO2, Al2O3 etc.). Furthermore, we find evidence for a preferred orientation relationship of 90° disorientations about the [001] direction forming tilt and twist grain boundaries, as well as a preference for the 60° disorientation about the [100] axis. Our data indicate that the GBCD, which is an intrinsic property of any mineral aggregate, is fundamental for understanding and predicting grain boundary related processes.

Quantitative electron backscatter diffraction (EBSD) data analyses using the dictionary indexing (DI) approach: Overcoming indexing difficulties on geological materials

Abstract Electron backscatter diffraction (EBSD) data yield plentiful information on microstructure and texture of natural as well as experimentally produced mineral and rock samples. For instance, the characterization of microstructures and textures by EBSD allows for the evaluation of phase equilibria. Furthermore, determination of the preferred orientations of crystals using EBSD yields constraints on deformation mechanisms and history of the minerals/rocks. The latter affects bulk-rock properites, for example, through the relation between lattice preferred orientation and electrical conductivity and seismic anisotropy. EBSD is also applied to advance our understanding of various phenomena such as seismic wave attenuation in the Earth deep interior and its relation to the presence of interfacial small degrees of melt fractions, or free fluid phases. In standard EBSD software solutions, the original EBSD patterns are rarely saved and indexing routines result in many artifacts, such as pseudo-symmetry or unindexed pixels at interfaces that may be misinterpreted as amorphous material, such as a melt. Here we report the first application of the dictionary indexing (DI) approach proposed by Chen et al. (2015), an alternative indexing routine, which we extended to be applicable to multiphase geologic materials. The DI method is independent of the EBSD system, and thus of the used detector/software. The DI routine generates simulated EBSD patterns for all possible crystal orientations, taking the sample composition and experimental setups into account. The resulting pattern database is called a dictionary. The experimental electron backscattering pattern (EBSP) images are indexed by comparing them to the dictionary using a dot-product algorithm. We evaluate the new DI method in comparison to standard routines and highlight advantages and disadvantages. To test and compare the DI’s reliability and performance, we apply the routine to two scientifically challenging samples: (1) A nominally anhydrous (“dry”) residual eclogite composed of garnet (cubic), clinopyroxene (monoclinic) and an amorphous melt, where the different degrees of hardness of the phases cause surface topology; and (2) a pure forsterite (olivine) polycrystalline sample produced by vacuum sintering. The acquired EBSD patterns are of low quality for the latter as a result of fast data acquisition to reduce the on-line machine time. We conclude that the new DI method is highly precise and surpasses the performance of previously available methods, while being computer time and computer memory consuming. We find that the DI method is free of pseudo-symmetry-related problems. Interpolation of data becomes obsolete and high reproducibility is obtained, which minimizes the operator impact on the final data set. The latter is often caused by applying several cleaning steps on EBSD maps with low indexing fraction. Finally, much higher scientific integrity is ensured by image collections as described above, which requires that all patterns are saved. This in turn allows later re-analyses if required. The DI routine will help to achieve more reliable information on interface properties of geological samples, including amorphous materials, and thus in the long run help to improve the accuracy of large-scale Earth mantle process models.

Structural insights and elasticity of single-crystal antigorite from high-pressure Raman and Brillouin spectroscopy measured in the (010) plane

Abstract We report high-pressure Raman and Brillouin spectroscopy results measured in the (010) plane of a natural antigorite single crystal. We find that structural changes at >6 GPa lead to (1) an intensity crossover between Raman modes of the Si-O-Si bending vibrations, (2) changes of the compression behavior of Raman modes related to the SiO4 tetrahedra, (3) changes of the pressure derivative of the Raman shifts associated with OH stretching vibrations, (4) the emergence of a new Raman band in the OH spectral region, (5) a softening of the elastic constants c33 and c11, and (6) a directional change of the slowest compressional wave velocity in the a-c plane. In addition to the structural insights at high-pressure, the unique characteristics of our single-crystal sample allows for first direct measurements of the acoustic velocity anisotropy in a plane perpendicular to the basal a-b plane. Comparison to previously published data indicates that the elastic anisotropy of antigorite strongly depends on the FeO and/or Al2O3 content. In contrast, it seems not to be affected by increasing temperature as inferred from an additional high-temperature experiment performed in our study. These constraints are important for the interpretation of seismic anisotropy observations in subduction zone environments.

STEM EDX Nitrogen Mapping of Nanoinclusions in Milky Diamonds from Juina, Brazil, Using a Windowless Silicon Drift Detector System

Energy-dispersive X-ray spectroscopy (EDX) performed using scanning transmission electron microscopy (STEM) in combination with a windowless detector setup allows high-resolution imaging and chemical composition mapping even of light elements present in low concentrations. The used TEM-system combines a field emission electron source with four silicon drift detectors allowing for high detection sensitivity. We used this enhanced system to investigate 20 to 200 nm sized inclusions in milky diamonds from Rio Soriso, Juina area, Brazil. The diamonds act as a chemical inert container and therefore protect their inclusions from further chemical reactions with their surroundings. We visualize the presence and distribution of nitrogen within focused ion beam (FIB) slices containing these nanoinclusions. The investigation of these specific diamonds may open a new window to deeper parts of the Earth (>660 km) as they represent pristine material of this deep mantle environment.

Pressure, temperature, water content, and oxygen fugacity dependence of the Mg grain-boundary diffusion coefficient in forsterite

Abstract The Mg grain boundary diffusion coefficients were measured in forsterite aggregates as a function of pressure (1 atm and 13 GPa), temperature (1100–1300 K), water content (<1–350 wt. ppm bulk water), and oxygen fugacity (10–18–10–0.7 bar) using a multi-anvil apparatus and a gas-mixing furnace. The diffusion profiles were analyzed by secondary ion mass spectrometer, whereas the water contents in the samples were measured by Fourier transform infrared spectrometer. The activation volume, activation enthalpy, water content exponent, and oxygen fugacity exponent for the Mg grain-boundary diffusion coefficients are found to be 3.9 ± 0.7 cm3/mol, 355 ± 25 kJ/mol, 1.0 ± 0.1, and –0.02 ± 0.01, respectively. By comparison with the Mg lattice diffusion data (Fei et al. 2018), the bulk diffusivity of Mg in forsterite is dominated by lattice diffusion if the grain size is larger than ~1 mm under upper mantle conditions, whereas effective grain-boundary and lattice diffusivities are comparable when the grain size is ~1–100 μm.

Accurate Grain and Phase Boundary Location by Dictionary-based Indexing of Geological EBSD Data

Dictionary-based indexing (DI) is a recent approach [1] to the indexing of electron diffraction pat-terns, including electron backscatter diffraction (EBSD), in which a library of precomputed patterns is compared to a series of experimentally acquired patterns. The library is generated using an accurate physics-based forward model that incorporates the geometry of the sample-detector configuration as well as the dynamical and stochastic nature of the generation of backscattered electrons (BSEs) in the sample. Combined with a uniform sampling of orientation space, SO(3), DI uses a similarity metric (normalized dot product between pattern vectors) to select from the dictionary the pattern that best matches the experimental pattern; the orientation corresponding to the best match is assigned as the orientation of the experimental pattern. Since a similarity measure is obtained for each dictionary en-try, one can rank the top N matches and store them for further analysis; below we show that an analysis of the common matches in this set provides an accurate location of grain and phase boundaries, which are often incorrectly indexed using Hough transform based methods due to pattern overlaps.