GMPV1.1

Grain boundaries affect bulk properties of polycrystalline materials, such as electrical conductivity, melting or bulk viscosity. In the past two decades, observations of marked bulk material property changes have been associated with changes in the structure and composition of grain boundaries. This led to the term “grain boundary complexions” to mark the phase-like behaviour of grain boundaries while differing from phases in the sense of Gibbs (Cantwell 2014).

Here we introduce the principles of grain boundary structure to property relations and potent methods to study these. The focus is on the combination of structural, chemical and statistical analysis as obtainable using transmission electron microscopy and electron backscatter diffraction. Data from these complementary methods will be discussed on two systems; garnet and olivine polycrystals.

Past elasticity measurements showed that the Youngs modulus of garnet polycrystals changes as a function of sintering pressure (Hunt et al. 2016). Here we used high resolution transmission electron microscopy to study the structure of grain boundaries from polycrystals synthesized at low (4-8 GPa) and high (8-15) GPa sintering pressure. The HRTEM data were acquired using an image-corrected JEOL ARM 300 to achieve the highest resolution at low electron doses using a OneView camera. Our data indicate a grain boundary structural change occurs from “low-pressure” to “high pressure” grain boundaries, where the grain boundary facets change from >100 nm – 20 nm to 3-7 nm length scale, respectively. We conclude that sintering pressure affects grain-boundary strength and we will evaluate how this may influence anelastic energy loss of seismic waves through elastic or diffusional accommodation of grain-boundary sliding.

Polycrystalline olivine samples show different viscosity related to grain boundary segregation of impurities. To investigate if the distribution of grain boundaries is affected by grain boundary chemistry, we analysed grain orientation data from over 4x10⁴ grains, corresponding to more than 6000 mm grain boundary length per sample. Using stereology, we extract the geometry of the interfacial network. The thus obtained grain boundary character distribution (GBCD) is discussed in relation to bulk viscosity.

How to cite: Marquardt, K., Dobson, D., Hunt, S., and Faul, U.: Grain boundary character information via individual imaging or statistical analyses: complexion transitions and grain boundary segregation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14614, https://doi.org/10.5194/egusphere-egu21-14614, 2021.

Here we report Atom Probe Tomography (APT) analyses of grain and phase boundaries of laboratory-deformed, fine-grained mixtures of clinopyroxene and olivine (Zhao, et al., 2019).  The experiments show that the mixtures deform much more rapidly than either mineral endmember.  This enhanced deformation in the two-phase material is due to stress-driven reactions at the phase boundaries. Lower effective viscosities of phase mixtures may be critical to the initiation of plate tectonics and the formation of mantle shear zones.

The hypothesis presented here is that the ‘bulk rock’ – a wehrlite – deforms rapidly because conversion of one phase to the other occurs at phase boundaries (e.g., Sundberg & Cooper, 2008).  In this model, grain-scale transport of the shared (slowly-diffusing) mineralogical component Si⁴⁺ is not required.  The near-boundary gradients of olivine-insoluble ions are presented as evidence of the phase transformation which either dissolves olivine into clinopyroxene or vice versa.  

The resolving power of the APT makes it a promising tool for investigating the microphysics of rock deformation, bridging the atomic scale all the way to the plate-tectonic scale.

References:
Sundberg M, Cooper RF (2008) Crystallographic preferred orientation produced by diffusional creep of harzburgite: effects of chemical interactions among phases during plastic flow. J Geophys Res Solid Earth 113(12):B12208.
Zhao N, Hirth G, Cooper RF, Kruckenberg SC, Cukjati J (2019) Low viscosity of mantle rocks linked to phase boundary sliding. Earth Planet Sci Lett 517:83–94.

How to cite: Cukjati, J., Cooper, R., Parman, S., Zhao, N., Akey, A., and Laiginhas, F.: Atom probe as a tool for understanding mineral physics and rock deformation: a case study of deformed wehrlite, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6581, https://doi.org/10.5194/egusphere-egu21-6581, 2021.

Magnetite (Mt) is the foremost carrier of rock natural remanent magnetization (NRM). Needle- and lath shaped Mt micro-inclusions in plagioclase (Pl) from gabbro often have systematic crystallographic- and shape orientation relationships (CORs, SORs) with the Pl host. The SORs of Mt leads to magnetic anisotropy which may bias the NRM of the Mt-Pl inclusion-host assemblage. Thus, the origin of the CORs and SORs between Mt and Pl is important for paleomagnetic reconstructions. In this context, the atomic structures of Mt-Pl interfaces are of particular interest.

The CORs and SORs between Mt and Pl were reported earlier and the underlying systematics was revealed from correlated optical and scanning electron microscopy (SEM) including electron back scattered diffraction (EBSD) analyses [1] (and references therein). The so-called plane normal type Mt micro-inclusions extend parallel to the Mt<111> direction, which is perpendicular to the densely packed Mt{222} oxygen layers that are parallel to one of seven Pl lattice planes with nearly identical d-spacings, namely Pl(112), Pl(-312), Pl(1-50), Pl(150), Pl(100), Pl(31-2) and Pl(1-12). Direct imaging of Mt-Pl interfaces has rarely been reported due to the beam sensitivity of Pl. Here we present the microscopic structure of a Mt-Pl interface along the inclusion elongation direction using high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and integrated differential phase contrast STEM (iDPC-STEM) techniques.

The TEM foil was prepared using a focused Ga-ion beam (Ga-FIB) from a lath-shaped Mt micro-inclusion of 23 μm x 17 μm x 0.1 μm extending perpendicular to Mt{111}/Pl(-312). The foil is oriented so that the Mt<111>/Pl(-312)-pole are parallel and Mt{110}/Pl(150) planes are perpendicular to the foil.

The STEM images show that the Mt-Pl interface is perfectly straight and parallel to Mt{110}/Pl(150) and that it is devoid of steps. Electron diffraction patterns confirm that the elongation direction of the micro-inclusions is determined by the good fit of oxygen layers across the Pl-Mt interface. A 2.4% difference in the d-spacings between Pl(-312) and Mt{222} is likely accommodated by every about 42'nd Mt{222} plane forming an edge dislocation at the Mt-Pl interface. In addition, elastic strain is indicated by a deviation of d₁₁₁/d₁₁₀ of Mt from the strain free reference lattice. Moreover, lattice fringes in iDPC-STEM images reveal coherence between Pl(22-1) and Mt{111} planes without misfit dislocations. This additional coherence may explain the particularly strong alignment of Mt{111} and Pl(-312) reflected by the EBSD data.

In summary, the elongation directions of the Mt inclusions are determined by the alignment of important oxygen layers of both phases across the Mt-Pl interface, which is parallel to oxygen-rich lattice planes in both phases. Misfit dislocations are presumably introduced to compensate the 2.4% lattice misfit along the elongation direction. The well-organized interface structure ensures a low interfacial energy and is a viable explanation for the observed Mt-Pl CORs and SORs.  

Acknowledgement

Funding by FWF project I 3998-N29 and RFBR project 18-55-14003 is acknowledged.

Reference

[1] Ageeva et al (2020) Contrib. Mineral. Petrol. 175(10), 1-16.

How to cite: Bian, G., Ageeva, O., Habler, G., Roddatis, V., and Abart, R.: Atomic scale structure of a plagioclase – magnetite interface, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2432, https://doi.org/10.5194/egusphere-egu21-2432, 2021.

The trace-element composition of rutile is commonly used to constrain P-T-t conditions for a wide range of metamorphic systems. Recent studies have highlighted the importance of micro- and nanostructures in the redistribution of trace elements in rutile via high-diffusivity pathways and dislocation-impurity associations. In this contribution, we investigate the effect of crystal-plastic deformation of rutile on its composition by combining microstructural and petrological analyses with atom probe tomography. The studied sample is from an omphacite vein of the ultrahigh-pressure metamorphic Lago di Cignana unit, Western Alps, Italy. Zr-in-rutile thermometry and inclusions of quartz in rutile and of coesite in omphacite constrain rutile deformation to around the prograde HP-UHP boundary at 500–550 °C. Crystal-plastic deformation of a large rutile grain resulted in low-angle boundaries that generate a total misorientation of ~25°. Dislocations constituting the low-angle boundary are enriched in common (Fe, Zr) and uncommon trace elements (Ca). The Ca is interpreted to be derived from the grain exterior, suggesting diffusion of trace elements along the dislocation cores. The potential for dislocation microstructures to act as fast diffusion pathways must be evaluated when applying traditional geochemical analyses as compositional disturbances caused by the presence of dislocation might lead to erroneous interpretations.

How to cite: Verberne, R., van Schrojenstein Lantman, H., Reddy, S., Alvaro, M., Wallis, D., Fougerouse, D., Langone, A., Scambelluri, M., Saxey, D., and Rickard, W.: Trace-element migration during crystal-plastic deformation in UHP rutile: dislocations in low-angle boundaries as high-diffusivity pathways., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9404, https://doi.org/10.5194/egusphere-egu21-9404, 2021.

The U-Pb system in titanite has been shown to be reset during a variety of high-temperature processes including high-temperature deformation, but post-deformation modification and recovery of crystal-lattice strain have so far made U-Pb equilibration mechanism from deformed titanites equivocal. Microstructures, including mechanical twinning and subgrain rotation recrystallization are more likely to be preserved at low-temperatures, but the systematics of chemical equilibration have not been established for these conditions. This study identifies progressive crystallographic misorientation and deformation twins in titanite porphyroclasts from the Wasatch Fault Zone, Utah, USA. The microstructures, mapped using electron backscatter diffraction (EBSD), developed at ~11 km depth during 300–400 ºC crystal-plastic deformation within the ductile fault zone. These microstructural maps were used to guide laser ablation-split stream ICP-MS analysis: U-Pb isotopes measured in tandem with major and trace element contents. Despite the low temperature, U-Pb and trace element contents in titanite equilibrated, at least partially, during deformation. Both major and trace elements in titanite also likely partitioned with a fluid and in response to the (re)crystallization of other mineral phases in the fault zone. Chemical zoning and crystal lattice recovery suggestive of fluid-aided recrystallization are absent, and the main mechanism for this resetting may instead be an enhancement of element mobility along microstructure dislocations. These processes are interpreted to record complex open-system behavior of titanite caused by crystal-plastic deformation during the initiation of the WFZ. This presentation will summarize the comparative analysis of microstructure by EBSD and titanite chemistry by LASS-ICP-MS, and how it bears on the understanding of elemental mobility in titanite during low-temperature crystal-plastic deformation.

How to cite: Udy, N. and Stearns, M.: Insights into chemical mobility in titanite driven by low-temperature crystal-plastic deformation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7327, https://doi.org/10.5194/egusphere-egu21-7327, 2021.

The proliferation of modern techniques available for petrologists has resulted in an explosion of detailed information on mineral compositions and processes over the last decade. One area of research that is undergoing dramatic advances is the non-destructive interrogation of samples in three dimensions through X-ray microscopy (XRM). Techniques such as ZEISS Versa and Ultra XRM can be performed at a variety of scales and resolutions, resulting in micro-to-nano scale information on geological samples. Such techniques can be correlated with each other i.e. expanding nanoscale resolution to a large sample or internal calibration of X-ray intensity to identify mineral assemblages, or even correlation with other techniques such as electron microscopy (EM). X-ray techniques are also particularly adaptable to digital resolution enhancements through software processes such as machine learning algorithms.

Collecting 3D information for petrological investigations can often require ground truthing of mineralogical and compositional interpretations. The more developed the 3D microscopy becomes, the more we are increasingly interested in features that are deeply buried within our samples. This means the corresponding techniques for excavating a region of interest also need to advance in both speed and accuracy.

The ZEISS Crossbeam-Laser (XBL) system provides a unique capability of rapidly excavating to a point of interest within a 3D sample volume. The XBL is already seeing use in material sciences, with a standard XB chamber with focussed Ion Beam (FIB) and Electron Microscopy (EM), and a correlated femtosecond laser chamber for rapid material removal. Sample data collected through XRM can be correlated to the XBL stage so that any internal features located by XRM have their coordinates automatically available in three dimensions. The femtosecond laser can excavate to a region of interest (RoI) within the sample within seconds or minutes, dramatically reducing preparation time compared to standard FIB/PFIB. The laser cut surface can be used for analysis techniques such as energy dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD), even prior to final polishing with the focussed ion beam (FIB).

Here we show the XRM-XBL workflow in high grade metamorphic rocks for identifying minerals in context for geochronology and micro-to-nano scale textures.

How to cite: Taylor, R.: Advances in 3D characterisation for correlative microscopy in geosciences, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9597, https://doi.org/10.5194/egusphere-egu21-9597, 2021.

Geochemical, petrographic, and spectroscopic indices that vary with compositional changes in petroliferous organic matter (OM) during thermal maturation are key petroleum system parameters used to understand petroleum generation. In unconventional shale source-rock reservoirs, where multiple, highly dispersed OM types may be present in intimate contact with surrounding mineral phases, OM molecular composition (e.g., aromaticity) is especially useful for informing structure-reactivity relationships representative of different OM types. Here, we employ microscale, in situ, and correlative Raman and reflectance approaches to evaluate aromaticity evolution for a suite of OM types (i.e., liptinite, micrinite, solid bitumen, vitrinite, and inertinite) at the single particle level across an artificial thermal gradient. Our samples include a marginally mature (vitrinite reflectance ~0.5%) Late Cretaceous Boquillas Shale from south Texas, United States, and two hydrous pyrolysis (HP) residues following reaction of the raw Boquillas Shale sample at 300°C and 330°C for 72 hours. Our data indicate that: (i) liptinite, micrinite, solid bitumen, vitrinite, and inertinite particles exhibit different aromatic signatures in the raw shale sample and (ii) these OM types, with the exception of inertinite, effectively experience similar changes in aromatic structure with thermal advance. Data also reinforce the concept that reservoir temperature may be a secondary factor in controlling the molecular composition of inertinite. These findings inform a broader understanding of how different petroliferous OM types evolve throughout thermal reactions and further demonstrate that correlative Raman spectroscopy and reflection analyses, combined with careful organic petrography, can provide complimentary estimates of OM molecular composition and thermal maturity.

How to cite: Jubb, A., Birdwell, J., and Hackley, P.: Evaluating aromaticity changes with thermal stress at the single particle level for a suite of organic matter types from the Boquillas Shale (Texas, United States) via correlative Raman and reflection analyses, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-259, https://doi.org/10.5194/egusphere-egu21-259, 2021.

Over the last two decades, Raman microspectroscopy has known a spectacular development in various research fields of petrology opening new avenues for studies in sedimentology, metamorphism or magmatism and cosmochemistry. This has been made possible thanks to major technological improvements (e.g., Raman hyperspectral mapping) and a better theoretical approach (e.g., data processing and interpretation). Raman spectra are actually sensitive to even minor (chemical or structural) perturbations within chemical bonds in (even amorphous) solids, liquids, and gases. They can, thus, help identify, characterize, and differentiate between individual minerals, fluid inclusions, glasses, carbonaceous materials, solid solution phases, strain in minerals, and dissolved species in multi-component solutions. Such sensitivity and versatility make Raman a unique tool for petrology. Yet, it relies on a weak and subtle signal and a cautious approach is required to avoid pitfalls during the analysis and/or the interpretation of data. Some recent scientific milestones will be presented and discussed in various fields like geothermobarometry of metamorphic rocks, geochemistry of meteorites, speciation of deep fluids involved in fluid-rock interactions or the characterization of organic/mineral assemblages of astrobiological interest. For the particular c       ase of petrology, Raman microspectroscopy has the immense advantage that it requires minimal sample preparation, thus it can be performed in situ preserving the original microtexture of the sample with a rather high spatial resolution for analysis, typically 1 mm at 532 nm for modern systems. Therefore, this technique is now increasingly used to study poorly crystalline and chemically heterogeneous materials involved for instance in geochemical processes occuring at Earth surface. But it faces numerous challenges due to the reactivity of such phases making them fragile under the laser beam, or due to the quasi-systematic presence of intense backgrounds in the spectra overwhelming the Raman signal. The source of this background can be multiple as it can be observed with fine-grained samples and/or it can be generated by the presence of luminescence/fluorescence emission centers. More generally such background is not well understood although it is a major issue for Raman spectroscopy in many petrological applications. However, there too, recent technological developments, sometimes based on old ideas, offer new possibilities to investigate safely and accurately such materials : time-resolved spectroscopy and surface-enhanced Raman spectroscopy (SERS) will be presented as well as some applications for petrology of complex samples.

How to cite: Beyssac, O.: Raman spectroscopy for petrology : recent scientific milestones, technological trends and challenges, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3416, https://doi.org/10.5194/egusphere-egu21-3416, 2021.

The correct assessment of maximum temperatures experienced by rocks is an essential tool to unravel the evolution of the thermal structure of the crust during the main phases of an orogenesis. Given to broad P-T stability field of classical metamorphic mineralogical indicators, maximum temperatures derived from the analyses of carbonaceous material dispersed in rocks by means of Raman spectroscopy has shown to be a suitable alternative to classical geothermometer. Initially developed for high metamorphic rocks the use of this tools has recently been extended also at lower metamorphic degree and diagenesis. This allowed us to extend the analyses of paleotemperatures experienced by rocks from Ghomarides and Sebtides from the Internal Rif in North Morocco with respect to previous works. Ghomaride and Sebtides in this portion of the Rif-Betic-Tell chain, represent respectively the upper and lower plates of a metamorphic core complex  and are composed, the first, by Paleozoic rocks with a partially preserved Mesozoic-Cenozoic cover and the second by lower Paleozoic to Triassic deep-crustal mica-schists, migmatites and granulites associated with peridotites (Beni Bousera complex).

Our data suggest that the uppermost Tiszgarine Unit of the Upper Sebtides experienced warmer condition than previously observed. Moreover, we calculate the maximum temperatures experienced by the Ghomarides  during both the Eo and Late Variscan cycles showing that differences in temperature exist among the vary units that compose the complex. Finally, in the southern area our data suggest a less severe alpine heating related to the emplacement of the Beni Bousera peridotite, than previously calculated.

How to cite: Schito, A., Atouabat, A., Calcagni, R., Corrado, S., Muirhead, D., Romano, C., Pozzi, A., Galimberti, R., and Spina, A.: An insight on the polyphase thermal history of the Ghomarides and Upper Sebtides in the Internal Rif (North Morocco) by means of Raman spectroscopy on organic matter, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7459, https://doi.org/10.5194/egusphere-egu21-7459, 2021.

Amorphous carbonaceous materials are typically those that would be considered immature, at temperatures below 300°C, in a typical sedimentary basin burial setting. Only recently has there been much discussion on the efficacy of Raman spectroscopy on amorphous carbon at temperatures below 300°C. Here we present data from a variety of published sources alongside our own data reviewing the apparent trends in amorphous carbon with some discussion related to thermal regime (intensity, duration), with case studies including intruded host rocks, fold and thrust belts and wildfires. We conclude that Raman spectroscopy can be applied successfully to ‘low-temperature’ carbonaceous material whilst noting the challenges faced to fully understand the physio-chemical mechanisms at these temperature ranges.

How to cite: Muirhead, D.: Raman Spectral Trends in ‘low-temperature’ Carbonaceous Materials., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7939, https://doi.org/10.5194/egusphere-egu21-7939, 2021.

Carotenoid compounds such as β-carotene are some of the most prevalent organic molecules on Earth and are key biomarkers as there is no known abiogenic source. During diagenesis and thermal alteration, carbon undergoes well-documented changes in Raman spectra.

There has been little research into the transitional degradation of carotenoid spectra to where they are fully replaced by the carbon spectra as the main identifier of thermal maturity under Raman spectroscopy. This is an overlooked regime when discussing the search for life, terrestrial and extra-terrestrial, where current research (using Raman spectroscopy) either focuses on finding living organisms displaying common organic molecules, or looks for the elemental carbon evidence of extinct fossil life. The real world is not usually so polarised, so covering the transition between these modes of life detection will improve any detection analysis.

For this study the volcanic thermal spring system in Viterbo, Italy was used as a field-based laboratory. The high rate of carbonate precipitation in these thermal springs, the wide range of thermal regimes (58°C to 25°C), and the prevalence of fast-growing algae in the run-off streams, give excellent preservation of a range of organic matter states at directly measurable temperatures.

The results demonstrate how the Raman spectra of the carotenoid compounds change with hydration, death of the organism, and thermal alteration of the organic material. The relationship between the spectra of the carotenoid compound and the elemental carbon spectra in the transition zone is also shown.

This study expands on the use of Raman spectroscopy of carbon as a low-temperature geothermometer, and provides a framework for the spectral response of organic matter in an often-overlooked geological regime. It is anticipated that the fields of geothermal energy, climatology, geobiology and astrogeobiology, could all benefit from this study, incorporating this enhanced thermal alteration data into existing and future work.

How to cite: O'Donnell, A.: Searching for life in volcanic carbonate systems and the effect of temperature on organic molecules, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9659, https://doi.org/10.5194/egusphere-egu21-9659, 2021.