Displays

The earliest known physical records of Mars and Earth lie in microscopic grains of zirconium-rich geochronology minerals such as zircon and baddeleyite. The reconstruction of the pressure and temperature histories of these phases is one of the few ways in which we can bracket the onset of conditions permissive of microbiota survival, and requires an integration of several nanoscale measurement techniques. This presentation will overview a recent, detailed investigation of zircons and baddeleyite from Mars [1], the earliest known from planets to date, as well as comparator studies of thermally and/or shock metamorphosed samples from the Earth and Moon. The approach is to spatially correlate measurements of the chemical and orientation microstructure of individual grains in order to characterize thermal, shock and diffusion history and better interpret U-Pb geochronology data. Also revealed are proxies for high temperature metamorphism such as nanoclusters of Pb and trace elements and nanoveins of impact melt as well as trace elements introduced through subsequent lower-temperature hydrothermal metamorphism. The techniques required include electron microscopy and cathodoluminescence (CL), Electron Backscatter Diffraction (EBSD), Transmission Kikuchi diffraction (TKD), mass spectrometry, and Atom Probe Tomography (APT). The Mars records were collected from a population of zircon and baddeleyite grains within five meteoritic fragments of polymict breccia (e.g. NWA 7034, NWA 7475). These data were compared to those from analogue sites of heavily bombarded Archean crust such as the central uplift of the Vredefort structure of South Africa, the Earth’s largest and oldest recognized impact crater, the Sudbury impact structure in Canada, and Apollo samples of the lunar regolith. The Mars population of grains reveals little evidence of the nanofeatures of heavily bombarded and heated crust, and no exposure to life-limiting pressures or temperature since crystallization 4.48 billion years ago. The conclusion is that global, planet-shaping bombardment effects on Mars, such as those which created its distinctive hemispheric dichotomy, had ceased by the time these grains and their associated crust crystallized. It follows that Mars entered a window of habitable conditions very early in solar system history, a pathway likely mirrored by the Earth. In this way nanoscale measurements, required to investigate microscopic mineral grains, serve as important tools for reconstructing important time periods in planetary evolution and abiogenesis.

Reference:

[1] DE Moser, GA Arcuri, DA Reinhard, LF White, JR Darling, IR Barker, DJ Larson, AJ Irving, FM McCubbin, KT Tait, J Roszjar, A Wittmann, C Davis (2019) Decline of giant impacts on Mars by 4.48 billion years ago and an early opportunity for habitability. Nature Geoscience 12,  522–527.

How to cite: Moser, D.: Dating habitability with nanoscale measurements of Early Mars, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11454, https://doi.org/10.5194/egusphere-egu2020-11454, 2020.

The field of planetary mineralogy has greatly benefited from recent studies of accessory minerals that utilise µm- and nm-scale analytical techniques such as EBSD, APT, TEM and SIMS. Apatite and merrillite have been of particular interest, as they record vital information on the volatile content, U-Pb ages and trace-element composition of various planetary materials. However, the extent to which shock-deformation, pervasive among all planetary materials, affects the distribution of these valuable geochemical tracers is still poorly understood. Here we focus on exploring the U-Pb and Pb-Pb ages of apatite and merrillite in a set of variably shocked lunar rocks, building on previous nanostructural analyses of the phosphates.

We carried out U-Pb and Pb-Pb analyses of phosphates in Apollo 17 samples of the Mg-suite rocks (76535, 76335, 76255, 72255, 78235 and 78236) using the CAMECA 1280 ion microprobe at the NordSIMS facility (Stockholm). In addition, we applied a recently developed approach of conducting high-precision U-Pb and Pb-Pb analyses by ID-TIMS of extracted phosphate grains (Jack Satterly Lab, University of Toronto). For this purpose, individual ~50x50x30 µm crystals of apatite and merrillite were extracted directly from thin sections using a Xe⁺ plasma FIB.

As determined by SIMS, ²⁰⁷Pb/²⁰⁶Pb systematics of the unshocked or weakly shocked apatite in 76535 and 76335 is undisturbed, implying cooling of the rock below the closure temperature of Pb diffusion in apatite (~450°C) at ~4.2 Ga, ~100 Ma younger than what is interpreted as the rock’s crystallization age. Phosphates that experienced similar levels of deformation but were in proximity or in direct contact with the impact melt in samples 76255 and 72255 show almost complete age resetting (~3.92 Ga). The SIMS determined age of 16 phosphates in sample 76255 is 3922.2 ± 6.7 Ma (2σ) and agrees with the previously published ²⁰⁷Pb/²⁰⁶Pb phosphate ages of impact melt breccias found within the same boulder and was interpreted as the timing of the Imbrium impact. These recrystallized phosphates yield comparable TIMS Pb-Pb ages (3917.8 ± 1.8 Ma and 3921.0 ± 1.3 Ma, 2σ) with significantly lower internal uncertainties than that of the individual SIMS measurements and may represent multiple impact-events close to the Imbrium event.

SIMS U-Pb analyses of highly shocked phosphates (78235 and 78236) reveal a discordia line with an upper intercept of ~4.2 Ga and a lower intercept of ~0.5 Ga. We interpret this new, younger age as a minor thermal event that reactivated existing shock-induced nm-scale grain boundaries, as visualised by APT, within the apatite population to allow for Pb-loss at ~0.5 Ga. We propose a small crater located near the Apollo 17 landing site as a possible source of this sample.

By correlating micro- to nanostructural characterization with in-situ age systematics we show that apatite and merrillite are powerful thermochronometers that provide a new approach to dating which has the potential to discriminate between temporally similar events. This can greatly aid in unravelling the bombardment record of solar system and be helpful when dealing with samples of limited availability (e.g. space return missions).

How to cite: Cernok, A., White, L., Tait, K., Anand, M., Kamo, S., Whitehouse, M., and Darling, J.: Lunar Phosphates Record Impact Cratering Events at Micro to Nano Scales , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1169, https://doi.org/10.5194/egusphere-egu2020-1169, 2020.

Introduction Primitive extraterrestrial materials like carbonaceous chondrite matrices and interplanetary dust particles contain tiny dust grains that were formed in the winds of red giant branch, or asymptotic giant branch stars (AGB) and in the ejecta of novae and supernovae (SNe) explosions before the formation of our solar system. Presolar grains survived all the processes that created our solar system and carry the signatures of their parent stellar sources. Correlating isotopic data of individual presolar silicates with microstructural and chemical analyses obtained by STEM, provides a unique opportunity to provide better insights into physiochemical conditions of grain formation in stellar environments, grain alteration in the interstellar and parent body processes and also helps constraining various astrophysical grain condensation models. In this work, isotopic, structural and chemical analysis of nine presolar silicate grains from the CH3/CBb3 chondrite Isheyevo and CR2 chondrite NWA801 are reported.

Experimental Presolar oxygen anomalous grain search using oxygen isotope imaging was done in-situ using NanoSIMS50 ion microprobe and five grains from AGB and four grains from SNe, were selected for (S)TEM investigations. The TEM lamellas were prepared using a TESCAN LYRA3 FIB-SEM at Curtin University. Structural and chemical analysis of presolar grains were performed by combining high-resolution scanning TEM imaging, spatially-resolved electron energy-loss spectroscopy (EELS) and spatially-resolved energy-dispersive X-ray spectroscopy (EDS) by using a FEI Titan Cubed Themis 60-300 microscope at Cádiz University which was operated at 200 kV. EDS quantification was corrected by using a standard reference sample of known composition and density and by taking into account the thickness of the probed area by using low-loss EELS. EELS spectrum images for fine structures (mostly, O-K, Si-L_2,3 and Fe-L_2,3 edges) analyses were acquired with the monochromator excited allowing an energy resolution of about 0.4 eV. After denoising using principal components analysis and removal of the multiple scattering, we were able to map the heterogeneities related to the Fe oxidation state and to the oxygen local chemical environment. This allowed us to compare the degree of aqueous alteration of the grain with the surrounding rim and matrix grains.

Results TEM and STEM data have revealed a strong heterogeneity and a broad range of structural and chemical compositions of the grains that enabled us to compare the stellar grain condensation environments (e.g. AGB stars and SNe), and suggest widely varying formation conditions for the presolar silicates identified in this study. Only one of the grains originally condensed as an amorphous grain has shown preferential sputtering of Mg, indicating that Mg-rich amorphous grains are not preferentially destroyed. Several grains are found with signatures that represent interstellar, nebular and parent body alteration. An oldhamite-like grain within a presolar enstatite grain is probably the first observation of an oldhamite grain as a seed grain for the condensation of an enstatite grain in stellar atmospheres. All these results, which will be discussed in detail, point out the importance of coordinated isotopic, microstructural and chemical studies of presolar silicates to investigate the processes that may have played a role in shaping our solar system.

How to cite: Lajaunie, L., Sanghani, M. N., Rickard, W. D. A., Calvino, J. J., Marhas, K. K., and Bizzarro, M.: Combined multi-isotopic and (S)TEM study of pre-solar silicates to probe the solar system’s prenatal history, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11139, https://doi.org/10.5194/egusphere-egu2020-11139, 2020.

Clinopyroxenes (Na,Mn,Ca,Fe²⁺)_1-2(Mg,Al,Cr,Fe³⁺)_1-2[(Al,Si)₂O₆] are common inclusions in natural diamonds of either peridotitic or eclogitic paragenesis. The variety of the composition of pyroxene inclusions in diamonds records the chemical and the physical conditions of the mantle. New approach based on Raman spectroscopy data on 43 pyroxene inclusions in diamonds from Yakutian province (the Siberian craton) and their chemical analyses are provided in this study.

Raman spectroscopy is a high-resolution and non-destructive method used to detect the compositional and the structural characteristics of materials and minerals including high-pressure crystal inclusions in diamonds. Raman spectra of clinopyroxene inclusions in diamonds were collected by Horiba LabRAM HR800 Raman spectrometer with 532-nm laser. In addition, the compositional analyses were obtained by EPMA (JEOL JXA-8100) to correlate chemical variations with specific spectral features and Raman shifts.

Clinopyroxene inclusions show variations of chemical content in the wide ranges: SiO₂ 54.1-55.9 wt.%, Al₂O₃ 0.31-4.11 wt.%,  Cr₂O₃ 0.32-5.73 wt.%, FeO 1.81-3.49 wt.%, MgO 13.4-18.3 wt.%, CaO 16.1-22.9 wt.%, Na₂O 0.28-3.82 wt.% for peridotitic type and SiO₂ 52.4-56.8 wt.%, Al₂O₃ 4.46-17.8 wt.%,  Cr₂O₃ <0.29 wt.%, FeO 2.22-11.4 wt.%, MgO 5.65-15.1 wt.%, CaO 9.81-17.1 wt.%, Na₂O 3.118-8.121 wt.% for eclogitic type.

Generally, the inclusions yield Raman spectra with four high-intense modes (ν₃, ν_3’, ν₄, ν₁₁, ν₁₇). Observed relative intense of most of these modes (except ν₁₁) depend on changing of crystal orientation. The ν₁₁-mode belongs to the Si-O stretching vibrations of bridging oxygen atoms (Si-O_br). The recorded position of this mode varies in the ranges 665.6-675.1 cm^-1 for peridotitic type inclusions and 673.7-688.2 cm^-1 for eclogitic type inclusions. One of the factors controlling the shifts of position frequencies of n11-mode is composition.

Peridotitic clinopyroxenes display strong linear correlations between the shifts of position of the ν₁₁-mode and contents of Al₂O₃ (correlation coefficient r = 0.94), FeO (correlation coefficient r = 0.68), MgO (correlation coefficient r = -0.52), CaO (correlation coefficient r = -0.69), Na₂O (correlation coefficient r = 0.92). Eclogitic clinopyroxenes show linear correlations between the shifts of position of the n₁₁-mode and contents of Al₂O₃ (correlation coefficient r = 0.95), FeO (correlation coefficient r = -0.64), MgO (correlation coefficient r = -0.85), CaO (correlation coefficient r = -0.59), Na₂O (correlation coefficient r = 0.84). The most expressed correlations can be used for estimation of composition of inclusions in diamonds only by Raman spectroscopy data without destruction of diamond-host and for identification of clinopyroxenes from potentially diamondiferous mantle rocks.

Acknowledgments: Russian Science Foundation (16-17-10067) supported this work.

How to cite: Kalugina, A. and Zedgenizov, D.: The Raman Estimation of the Composition of Clinopyroxene Inclusions in Natural Diamonds , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-826, https://doi.org/10.5194/egusphere-egu2020-826, 2020.

Indium-bearing sphalerites from the Hämmerlein skarn deposit, located in the western Erzgebirge (Germany), show complex distribution patterns of major and minor elements on a micrometer to sub-micrometer scale. However, with the spatial resolution of traditional analytical methods, such as SEM-based image analysis and field emission electron probe microanalysis (FE-EPMA), many features in these spalerites cannot be resolved. It remains unclear whether Cu, In and Fe are in solid solution in the sphalerite, are concentrated in nanoparticles or form discrete phases.

Atom probe tomography combined with transmission kikuchi diffraction has been used to resolve both the compositional heterogeneity and the nanostructure of these complex In-Cu-Fe-sphalerites. The obtained data indicate a complex structure with micro- to nanometer sized, plate-shaped inclusions of chalcopyrite in the sphalerite. In addition, a nanometer scale In-Cu-sulfide phase forms plate-like segregations in the sphalerite. All types of segregations have similar crystal structure and record the same crystal orientation indicating that they likely formed by exsolution.

The results indicate that complex sulfides containing cations of more than one element as minor or major constituents may represent discrete, exsolved phases, rather than solid solutions or being concentrated in nanoparticles. This heterogeneous nature will affect the nanoscale properties of the sphalerite, which may have implications for the economic extraction of precious elements such as In, when processing these minerals for beneficiation. Furthermore these nanoscale properties will open up new perspectives on formation processes of In-Cu-Fe-sphalerites, which might be relevant for other chemically complex minerals as well.

How to cite: Krause, J., Reddy, S. M., Rickard, W. D. A., Saxey, D. W., Fougerouse, D., and Bauer, M. E.: Nanoscale compositional segregation in complex In-bearing sulfides: Results from atom probe tomography and transmission kikuchi diffraction, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3473, https://doi.org/10.5194/egusphere-egu2020-3473, 2020.

Large Geometry Secondary Ion Mass Spectrometry (LG-SIMS), operating in multicollection mode, allows high precision light isotope ratio measurements at high lateral resolution (tens of μm down to sub-μm range). For some challenging applications involving fine scale analysis of low abundance isotopes (i.e. ¹⁷O or ³⁶S) or low-concentration elements (i.e. nitrogen in diamonds) measurement of low signal intensities is required. Traditionally, count rates between the upper level of pulse counting systems ~10⁵ c/s and the lower level of Faraday Cup (FC) measurements ~10⁶ c/s are considered to be in a “gap area” where neither detection protocol can achieve performance better than the 1‰ level.

Faraday Cup detectors (FC) offer high precision with no need for gain monitoring, however the uncertainty of FC measurements depends on the signal to noise ratio. One approach for measuring low signal intensities is to use FCs coupled to electrometers with high ohmic resistors. CAMECA LG-SIMS can now be equipped with low noise 10¹² Ω resistor FC preamplifier boards for measuring signal intensities down to the ~ 3 x 10⁵ c/s range with precision better than the 0.5‰ level (1SD).

For measurement of low-abundance isotopes, a complementary approach consists of using discrete-dynode pulse counting electron multiplier (EM) detectors, for which drift and aging effects are minimized using a fast automated EM high voltage adjustment routine.

During this PICO presentation, we will discuss the relevance of the detector choice (FC 10¹² Ω vs EM) for few examples of innovative applications.

Example of mass independent fractionation:

In addition to classical isotopic ratio measurements (e.g. δ¹³C, δ¹⁵N, δ¹⁸O or δ³⁴S), for which the instrumental mass fractionation (IMF) correction is mostly limited by the natural heterogeneity (chemical and isotopic) of the reference material, SIMS is particularly well suited for the measurement of mass independent fractionation (MIF, e.g. ∆³³S, ∆³⁶S and ∆¹⁷O). Along with classical geochemical processes, the degree of isotopic fractionation scales with the difference in mass of the isotopes involved (i.e. δ³³S ≈ 0.515 * δ³⁴S). MIF refers to non-conventional ratios that depart from these mass dependent rules. As instrumental mass fractionation has been shown to be strictly mass dependent, MIF measurements are not subject to IMF correction and are therefore measured directly. The use of SIMS in this specific case is particularly well suited and allows to fully explore the rich phenomenology of MIF source processes. We will discuss the advantages and disadvantages of using FC 10¹² Ω for the minor Sulphur isotope (³⁶S) measurement.

Carbon and Nitrogen in diamond:

We will also show a recent analytical development aiming to measure δ¹³C in diamonds at mass resolution of ~5000 (allowing the full separation of ¹³C- and ¹²CH-) as well as N-content and N-isotopes in diamonds at a mass resolution of ~9000 (full separation of ¹²C¹⁴N- and ¹³C¹³C-).  For this purpose, the use of FC 10¹² Ω greatly improves the data quality and allows the simultaneous measurement of N-content and δ¹⁵N.

How to cite: Peres, P., Thomassot, E., Deloule, E., Bouden, N., and Fernandes, F.: Innovative Detection Strategies on Large Geometry SIMS open new challenging applications for light isotope ratio analyses, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20427, https://doi.org/10.5194/egusphere-egu2020-20427, 2020.

Monazite((Ce,Y,La,Th)PO₄) is an important phosphate mineral and is one of the widely used minerals for U-Th-Pb dating in geochronology. In this study, we have examined the crystallinity, the valence and coordination of radiogenic Pb in a natural RW-1 monazite standard (ThO₂ up to13.5 wt% and Pb up to ~5000 ppm) with a ²⁰⁷Pb/²³⁵U age of 904.15 ± 0.26 Ma from a Norwegian pegmatite by using laser Raman spectroscopy (LRS), X-ray photoelectron spectroscopy(XPS) and transmission electron microscopy (TEM). The Raman spectrum analysis revealed that this monazite is well crystalline and is not damaged by α-particles. The results of XPS and TEM suggest that the radiogenic Pb produced by the α-decay of U and Th is divalent and radiogenic Pb atom substitutes the Ce-site within the monazite crystal lattice. The qualitative analyses conducted on the HAADF-STEM data reveal heterogeneous distribution of radiogenic Pb within the monazite crystal lattice. This is the first work on the determination of the oxidation state, the atomic location and distribution of radiogenic Pb in a natural monazite (CePO₄). The deeply study of radiogenic Pb in monazite at the nanoscale and atomic scale provides a good insight for us to understand the mechanisms of nano-isotopic mobility and the nano-geochronology  that has been poorly understood so far.

How to cite: Tang, X., Li, Q., and Gu, L.: The study of the valence and distribution of radiogenic Pb in monazite by using XPS and TEM, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12962, https://doi.org/10.5194/egusphere-egu2020-12962, 2020.

In recent years, increasing developments in microscopy and microanalysis have allowed for the direct observation of nanoscale crystalline defects (i.e. dislocations). These defects are particularly important in naturally deformed materials yet this avenue of research remains understudied within the Earth Sciences. Dislocations can now be documented through the use of new and innovative structural and chemical analytical techniques such as electron channeling contrast imaging (ECCI), transmission electron microscopy, and atom probe tomography (APT). The presence and migration of dislocations in crystalline materials, including their role in trace element mobility, play a vital function in the way these materials respond to an applied stress. However, the mechanisms by which dislocations nucleate in minerals remain poorly understood. Prevailing models for dislocation nucleation include generation by Frank-Read sources, stress localization at crack-tips, atomic segregation, and free surface nucleation by critical stress-gradient criterion. Based on recent APT data from naturally-deformed pyrite, combined with electron backscatter diffraction (EBSD) mapping and ECC imaging, we propose a new nucleation mechanism where dislocations are generated by the local stress field in the vicinity of fluid inclusions. The investigated sample consists of a polycrystalline pyrite aggregate within a black shale host rock that has witnessed a peak temperature of 300°C. The combined EBSD and ECCI results reveal crystal plasticity in the form of lattice misorientation up to 8.5° and low-angle grain boundary development. APT data reveals nanoscale fluid inclusions enriched in As, O (H₂O), Na and K as well as As- and Co-rich dislocations linked by fluid inclusions. This new model is the first documentation with APT methods of fluid inclusions (voids) in minerals, nanoscale features that are commonly misinterpreted as element clusters or chemically-enriched crystal-defects. The combined data has significant trans-disciplinary implications to the geosciences (structural geology, geochemistry, economic geology, geochronology), the material sciences (metals, ceramics, polymers), and analytical microscopy. Within geochronology voids and dislocations such as these in dated minerals may host elements or isotopes that negatively affect their age. Within ore deposit geology, voids in precious metal-hosting minerals may act as the necessary traps to structurally prevent the metals (gold, silver, copper) from migrating or diffusing out of the host mineral. In material sciences, the presence of such crystalline features can either limit or enhance the performance of engineering materials. Thus, performing APT analysis on crystalline material can help us better understand and predict their physical properties.

How to cite: Dubosq, R., Rogowitz, A., Schweinar, K., Gault, B., and Schneider, D.: Dislocation nucleation at nanoscale fluid inclusions: Direct observation from atom probe tomography data of naturally deformed pyrite, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-64, https://doi.org/10.5194/egusphere-egu2020-64, 2020.

Natural and synthetic iron oxides and iron hydroxides are important minerals for many industrial sectors (e.g. steel making, colors, pigment for coating, electronics, catalysis, soil, waste water and gas treatments, and medicine). In natural environments, such as iron ore mines or iron rich soils (laterites or bauxites), iron oxy-hydroxide associations are complex and evolve related to varying physico-chemical conditions, including. biological interactions. For efficient resource use, unambiguous multiscale characterization is indispensable. Synthetic iron oxides, produced for medical and electronic sector, needs to be failure-free pure phases, thus a continuous quality control is required. Complex iron oxy-hydroxide association can be related to various processes, topotactic transition, pseudomorphosis by substitution and alteration paramorphosis, and corrosions, leading to massive, porous, fibrous and acicular textures or poorly crystalline crusts.

We present examples from iron ore deposits, where coupling of X-Ray diffraction (XRD) with scanning electron microscopy (SEM) and micro-Raman spectroscopy is a powerful tool to distinguish hematite, maghemite and magnetite at grain scale. Oxygen analyses by electron microprobe at (EMPA) fixed carbon coating thickness help to distinguish magnetite and hematite, and contribute with quantitative trace element analyses to chemically differentiate both oxides. At micro- and nano-scale, Transmission Electron Microprobe analyses coupled to X-Ray Diffraction (XRD) and Electron Energy Loss Spectroscopy (EELS) on nanometric inclusions can unambiguously identify various iron oxy-hydroxide phases. In Nickel-laterite and bauxite profiles, iron oxy-hydroxides (e.g. lepidocrocite, ferrihydrite, goethite…) are abundant and may form complex intergrowth with various types of phyllosilicates. Part of it host valuable metals such as Nickel. Combined XRF-XRD and Raman spectroscopy allow phase mapping and differentiation at micron scale of these phases, and even detect solid solutions (e.g. Ni-rich and Ni-poor goethite; El Mendili et al., 2019). Results from coupled laboratory analyses are necessary for building up data bases. They allow calibrating recently developed combined XRF-XRD-Raman benchtop systems. For industrial applications coupled and combined analyses will increase resource efficiency, and ensure a quality control for natural and synthetic iron oxide products. Such systems are recently developed by EU projects, such as SOLSA (www.solsa-mining.com).

El Mendili, Y., Chateigner, D., Orberger, B., Gascoin, S, Bardeau, JF., Petit, S., Le Guen, M., Pillière, H. (2019). Combined XRF, XRD, SEM-EDS, and Raman analyses on serpentinized harzburgite (Nickel Laterite Mine, New Caledonia): Implications for Exploration and Geometallurgy. ACS Earth and Space Chemistry. 3, 10, 2237-2249; DOI: 10.1021/acsearthspacechem.9b00014

How to cite: Orberger, B., Wagner, C., El Mendili, Y., Chateigner, D., Gascoin, S., Pillière, H., Fialin, M., Rividi, N., Boudouma, O., and Wirth, R.: Coupled and combined analyses for unambiguous iron-oxy hydroxides characterization: from laboratory to industrial use , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2019, https://doi.org/10.5194/egusphere-egu2020-2019, 2020.

Quantitative modal analysis of rock thin sections or liberation analysis of minerals processing plant materials can be very complex as grain sizes can vary by more than 7 orders of magnitude: Thin sections of rocks may contain extremely coarse grains (mm-sized crystals) down to glassy material with no long-range order (ordered domains <1 nm).

Material characterisation and modal analysis have traditionally been carried out with a combination of solid-state, microscopic and spectroscopic techniques (e.g., optical / scanning electron microscopy, powder X-ray diffraction, X-ray fluorescence spectroscopy). These techniques require different sample preparation routines, data acquisition and evaluation - a time-consuming process that may be considered too complex to implement in mineral processing plants despite requiring the relevant sample preparation equipment. Scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS provides an opportunity to carry out this characterisation in a more rigorous and, in certain cases, automated way. This process includes image thresholding (setting of grey levels of present phases by the analyst) and X-ray data collection with EDS. EDS is an ideal analytical technique for this work as it offers high acquisition speeds and the collection of the whole energy spectrum with a single detector, not requiring the selection of a fixed element list prior to data acquisition. Characterisation of coarse-grained rocks requires larger areas to be scanned in order to ensure representativity.

The analytical workflow can be further optimised by combining SEM-based analytical techniques for in situ, non-destructive, and potentially simultaneous bulk analysis. Electron backscatter diffraction (EBSD) is an SEM-based technique which can be used to determine the crystallographic properties and orientation of mineral grains, as well as to perform fabric analyses on polycrystalline materials. EBSD allows for crystallographic data to be collected simultaneously with chemical data and does not require powdered samples. As a result, the texture of the material can be fully preserved. The sample preparation requirements of the technique are similar to those for standard SEM-EDS, with an additional final polishing step, essential for the removal of surface imperfections, as the EBSD signal is generated on the sample surface. The coupling of EDS and EBSD datasets permits the enhanced interpretation of feature analysis data, allowing for a deeper understanding of the compositional, structural and textural properties of the sample. This, highly-efficient, in-situ, bulk material characterisation, is key for the mining industry, as it provides insights for optimising downstream procedures thereby saving time and resources and bolstering throughput and efficiency.

How to cite: Stavropoulou, A., Hiscock, M., Kamber, B., and Rodriguez-Blanco, J.-D.: Combining SEM, EDS & EBSD: Challenges and considerations in the micro-analysis of rock thin sections, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3941, https://doi.org/10.5194/egusphere-egu2020-3941, 2020.

Cathodoluminescence (CL) microscopy is a well-known technique for imaging geological specimens, in which the light that is generated with an energetic electron beam is collected and analyzed with a CL detector. CL provides a unique imaging contrast that can be used for visualizing growth zonation, distinguishing cement and granular detrital material, detecting trace elements, and characterizing fractures and deformation features in a large range of rocks, to name a few examples. In its simplest form CL imaging is performed with a static electron beam in an optical microscopy system (optical CL) but for more advanced experiments CL imaging is performed in a scanning electron microscope (SEM). This enables high scan speeds, high spatial resolution (< 100 nm), and correlation with other SEM based techniques such as X-ray imaging (EDS), secondary electron (SE) and backscattered electron (BSE) imaging, and more.

Currently, SEM-based CL work is mostly performed on costly floor model SEMs that require large amounts of space, complex auxiliary support systems, and an experienced operator to run the machine. In contrast, compact, affordable, and user-friendly table-top SEMs have improved substantially in the last years but they typically lack (advanced) CL imaging capabilities. Here, we will present our progress in developing a table-top SEM based CL system that can be used for geological research amongst other applications.

In particular, we have integrated a CL collection and detection system in a Thermo Fisher Scientific/Phenom XL table-top microscope, which already is equipped with SE, BSE, and EDS imaging modalities. In this SEM, electron energies of 5 – 15 keV can be used which is appropriate for most CL imaging experiments. The CL is collected using a multimode fiber optic cable connected with a graded index lens to increase the numerical aperture of the collection. Subsequently, the light is send to a spectrometer where the CL emission spectrum can be measured for every excitation point on the sample; a technique known as hyperspectral CL  imaging. To synchronize electron beam scanning with data acquisition and for data analysis we have developed dedicated software control.

We assess the potential of table-top CL by imaging representative polished zircon and quartz samples for various beam and acquisition parameters. To benchmark the system performance we compare our experimental results with results obtained from a state-of-the-art floor model SEM (Thermo Fisher Scientific Quanta 650 SFEG) system equipped with a high-end Delmic SPARC CL system. In the future, these developments may lower the threshold for using CL imaging through cost reduction and workflow simplification, making it accessible to a larger range of users within the field of geology and beyond.

How to cite: Coenen, T. and Polman, A.: Table-top Cathodoluminescence Microscopy for Geology, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4515, https://doi.org/10.5194/egusphere-egu2020-4515, 2020.

The interaction of incident laser radiation and sample substrate is complex and difficult to predict. Natural zircons areoften both structurally and chemically heterogeneous in 3-dimensional space. Encountering growth-related, structural micro-heterogeneities, inclusions and chemical complexities is almost inevitable when employing ‘conventional’ static, high-frequency laser sampling protocols often lasting several tens of seconds at a time.

A multi-shot approach to laser ablation by contrast implements a minimal sample exposure time to incident laser radiation by applying multiple 1 Hz shots in delayed succession to a single sampling site. This process can be conceptualised as a “slowing down” of a high-frequency (10-20 Hz) static Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) analysis. Each laser pulse applied in this manner, produces signal peak which is distinct albeit transient. The ability to integrate and collate signal pulses for a small number of consecutive laser shots (10-30 shots), as opposed to continuously pulsing the laser, produces highly precise age determinations (<1% reproducibility, 2slevel) on small sample volumes (~695µm³ on 91500 zircon standard). The multi-shot LA-ICP-MS protocol employed here effectively eliminates ‘downhole’ fractionation as the resultant craters are extremely shallow (as shallow as ~553nm on 91500 zircon standard) and maintain an aspect ratio of <<1. Further benefits include a reduced probability of thermally induced effects (e.g., substrate melting), plasma loading, and the potential for signal mixing (with depth) in a heterogeneous sample.

How to cite: Corbett, E., Simonetti, A., Shaw, P., Corcoran, L., Crowley, Q., and Hoare, B.: Eliminating Laser Induced Elemental U-Pb Fractionation using low sample volume multi-shot ablation protocols, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4623, https://doi.org/10.5194/egusphere-egu2020-4623, 2020.

Uranium-Lead dating of zircon remains one of the most widely utilized and most reliable temporal records throughout Earth history. This stems from the mineral’s widespread occurrence, pristine zircon being both physically and chemically robust, and the ability to evaluate the presence of open system behavior (i.e. “concordance”) through comparison of the independent ²³⁸U→²⁰⁶Pb, ²³⁵U→²⁰⁷Pb, and ²³²Th→²⁰⁸Pb decay chains. The phenomenon of discordance is well documented in zircon, and is typically (though not always) associated with radiation damage accumulation and Pb-loss. Despite a long history of research, the nanoscale controls on Pb mobility and Pb loss (i.e. the relative rates of radiation damage, annealing, and Pb diffusion) remain poorly defined. The unique characterization capabilities of atom probe tomography (APT) provide a novel means to study U-Pb systematics on the scale of the radiation damage, annealing and diffusion processes. APT studies have documented nanoscale heterogeneity in trace elements, Pb, and Pb isotope ratios, and correlated the ²⁰⁷Pb/²⁰⁶Pb ratios within clusters to transient thermal episodes in the history of a zircon.

This work seeks to provide a foundation for multi-scale U-Pb characterization, including how differential Pb mobility at the nanoscale can influence micron- to- grain-scale U-Pb systematics. Historically, concordia diagrams have used simple Pb-loss models to extract temporal information about the timing of Pb mobility/loss; however, these models assume ²⁰⁷Pb and ²⁰⁶Pb are uniformly disturbed within a grain and lost in equal proportions at the time of Pb loss. Our previous studies suggest that radiogenic Pb can be concentrated and immobilized in nanoscale clusters, leading to differential retention of Pb in clusters vs. matrix domains, and requiring a more complex treatment of isotopic shifts during any post-clustering Pb loss. This “multi-domain element (Pb) mobility” (MDEM or MDPM) influences subsequent Pb-loss trajectories on concordia diagrams, manifesting in systematic offsets for discordia as a function of the zircon crystallization age, the timing of cluster formation, and the timing of Pb mobility. These results highlight that (1) traditional interpretations of discordia in the presence of cryptic nanoscale clustering can lead to inaccuracies, and (2) multi-scale U-Pb characterization offers a means to both study discordance and to extract additional temporal information from zircon with otherwise ambiguous and/or complex Pb-loss patterns.

How to cite: Blum, T., Bonamici, C., and Valley, J.: Nanoscale Pb clustering and multi-domain Pb-mobility in zircon, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12779, https://doi.org/10.5194/egusphere-egu2020-12779, 2020.

The laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) is a unique method for local analysis that allows studying mineral grains in situ. The aims of these geochemical researches are to estimate concentrations and distributions of REE, Hf, U, Th, Y, Ti, PGE and other elements in accessory and ore minerals from complex deposits in the Arctic region (Fennoscandian Shield), using the LA-ICP-MS local analysis of trace elements. Accessory minerals of zircon and baddeleyite are much valued to study distributions of rare and rare earth elements (REE). Besides, pyrite, pentlandite, pyrrhotite and other sulfides are important for determining platinum-group elements (PGE), REE, etc.

The electron (LEO-1415) and optic (LEICA OM 2500 P, camera DFC 290) spectroscopy have been applied to study the morphology of the samples. Analytical points have been selected on baddeleyite, zircon crystals and sulfide minerals based on analyses of their BSE, CL and optical images. REE, PGE and other elements have been estimated in situ by ICP-MS, using an ELAN 9000 DRC-e (Perkin Elmer) quadrupole mass spectrometer equipped with UP-266 MAСRO laser (New Wave Research).

More than 19 elements were profiled during each measurement in zircon or baddeleyite. For the first time, LA-ICP-MS techniques have been applied to estimate PGE, REE and other (S, Cr, Fe, Cu, Ni, Co, As, Se, Mo, Cd, Sn, Sb, Re, Te, Tl, Hf, W, Bi, Pb, Th, U) elements in sulfide minerals. NIST 610, NIST 612 and tandem graduation (using solutions), considering sensitivity coefficients of isotopes have been used to check the accuracy of estimations. Fe, Ni and Cu have been used as internal standards, being most evenly distributed elements in minerals, when concentrations of elements in sulphides were calculated. The estimates have been carried out, using inter-laboratory standards of chalcopyrite, pentlandite and pyrrhotite, which had been preliminarily prepared and studied using micro probe analysis (Cameca MS-46).

These techniques had been used to estimate elements in zircon extracted from basic and acidic rocks of the Lapland belt (1.9 Ga), the Keivy zone (2.7 Ga), the Kandalaksha and Kolvitsa zone (2.45 Ga) and from the Cu-Ni deposit (Terrace, Mt. Nyud, 2.5 Ga). Novel techniques have been used to analyze baddeleyite from rocks of layered PGE intrusions of the Monchegorsk ore area (2.5 Ga) and carbonatites of Kovdor and Vuoriyarvi (380 Ma). Elaborated LA-ICP-MS techniques have been applied to provide in situ measurements of PGE, Au, Ag, siderophile and chalcophile elements in sulphide minerals from the Pechenga and Allarechka Cu-Ni deposits (1.98 Ga), Fedorova Tundra and Severny Kamennik PGE deposits (2.5 Ga).

The scientific researches are supported by RFBR Grant No 18-05-70082, scientific themes 0226-2019-0032 and 0226-2019-0053.

How to cite: Drogobuzhskaya, S., Bayanova, T., and Novikov, A.: Geochemical researches in situ (LA-ICP-MS) of accessory and ore minerals from multimetal (PGE, Cu-Ni) deposits in the Arctic zone (Fennoscandian Shield) of the Russian Federation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13123, https://doi.org/10.5194/egusphere-egu2020-13123, 2020.

The Scanning Electron Microscope (SEM) is the most prolific piece of analytical equipment in the Earth Sciences, therefore quantitative mineral chemistry obtained directly from the SEM has the potential to streamline many geological fields. Mineral chemistry provides direct constraints on geological processes that are used in a wide variety of Earth Science disciplines. As a result, major element analysis of rock forming minerals have been one of the major contributors to geochemistry for decades. Electron beam techniques have been the most widely used method of obtaining in situ major element chemistry, dominated by the quantitative Wavelength Dispersive Spectroscopy (WDS) employed by the Electron Probe Micro Analyser (EMPA). More rapid, and typically more qualitative Energy Dispersive Spectroscopy (EDS) major element measurements are often obtained on a standard SEM instrument.

The relative simplicity of the EDS technique saw the growth of automated mineralogy systems beginning in the 1980’s. The peaks of EDS spectra are characteristic of the major elements present, and therefore lookup tables can be used to match the spectra to known mineral compositions and provide a likely mineralogy in both grain mounts and mapped thin sections. The automated mineral analysis technique remained essentially unchanged for decades, with an experienced operator required for many of the analytical tasks, such as creating the files for matching spectra to known minerals, processing the data, and interpreting complex phases and solid solutions (e.g. Fe/Mg-bearing silicates).

The ZEISS Mineralogic automated quantitative mineralogy (AQM) takes a new approach, using EDS detectors, but following an analytical protocol more closely aligned with EPMA. A combination of matrix corrections, peak deconvolution, and standard calibration means that peak intensities are converted directly into wt% element directly at the time of analysis. The result is a data output that can be immediately interpreted, even for minerals not previously analysed, by both new and experienced users.

Here we demonstrate the use of the ZEISS Mineralogic system for mapping thin sections from high grade metamorphic rocks. The bulk chemistry of the entire thin section, as well as individual mineral compositions can be used to constrain P-T conditions directly from the SEM, without the need for an additional step of obtaining mineral chemistry from an EPMA. With quantitative analysis at every pixel, major element profiles can be obtained at any point in the thin section, and P-T can therefore be determined from any domain within the mapped section. This approach makes the use of P-T pseudosections possible with greater speed and flexibility than has previously been possible.

How to cite: Taylor, R., Hill, E., Lanari, P., Clark, C., and Johnson, T.: Quantitative automated mineralogy to constrain metamorphic processes using ZEISS Mineralogic, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14000, https://doi.org/10.5194/egusphere-egu2020-14000, 2020.

With this work, we aim at exploring the extent to which valence-to-core X-ray emission spectroscopy (vtc-XES) can provide this first- and second-coordination-shell information from amorphous germanium oxides.

We measured the vtc-XES spectra of germanium oxides at ambient and high pressure. The Kbeta’’ emission line, part of the vtc-XES spectra, is sensitive to coordination and oxygen-germanium bond distance as first coordination shell effects. Furthermore, it reflects the different binding energies of bridging and non-bridging oxygen atoms. The Kb’’ emission line may thus allow for tracking the coordination and the state of polymerization of a germanium oxide glass under pressure in diamond anvil cells or in other confining environments.

Spiekermann et al. (2019) Persistent octahedral coordination in amorphous GeO₂ up to 100 GPa revealed by Kbeta'' X-ray emission spectroscopy, Physical Review X, 9, 011025.

How to cite: Spiekermann, G.: XPS-like evaluation of valence-to-core X-ray emission spectra of germanates, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20142, https://doi.org/10.5194/egusphere-egu2020-20142, 2020.

Over the past two decades, laser ablation coupled with the mass spectrometer has become a major analytical tool for the measurement of isotopic ratios and the determination of trace elements. The improvement of the sensitivity has provided new perspectives and permits to study new types of targets. For example, many questions remain open about the formation of supergene mineralization such as: what is exact timing for their deposition? What are the required associated physico-chemical conditions? To answer these questions, we focused on two copper deposits located in Chile (Mina Sur) and Burkina Faso (Gaoua) to develop U-Pb analysis and trace element profiles in pseudomalachite and chrysocolla. The analyses were carried out at the GET Laboratory (Toulouse). Different couplings between a femtosecond laser (fs-LA) or a nanosecond laser (ns-LA) and a HR-ICPMS or a MC-ICPMS were used. Trace elements determination and in situ U-Pb analysis present different challenges. For U-Pb analyses, matrix effects must be taken into account and the contribution of common lead (²⁰⁴Pb) must be subtracted. As there is no chrysocolla or pseudomalachite reference materials, zircon and apatite were used as the primary external standards and fs-LA was used as a matrix independent sampling method. No significant U-Pb fractionation was observed, whatever the structure of the ablated matrix (silicate, phosphate). The bias linked to common lead was calculated from fs-LA-MC-ICPMS measurements. The ²⁰⁶Pb / ²⁰⁴Pb intensity ratio gives a first approximation on the possibility to determine the U-Pb age. Three cases have been distinguished: 1) If ²⁰⁴Pb is low (²⁰⁶Pb / ²⁰⁴Pb ≥ 500) the U-Pb age obtained by this first analyze can be used. 2) If ²⁰⁴Pb is significant and the intensity ratio of ²⁰⁶Pb / ²⁰⁴Pb range between 500 and 5, a second step is necessary. In such a case, ²⁰⁴Pb must be determined more precisely using a MC-ICPMS to retrieve the common lead corrected U-Pb age. 3) If ²⁰⁴Pb is high (²⁰⁶Pb / ²⁰⁴Pb <5), then it is not possible to determine the U-Pb age of the sample. Trace element profiles were also performed on the same chrysocolla and pseudomalachite samples. These analyses have been carried out using a ns-LA coupled to HR-ICPMS and NIST SRM 610 was used as primary standard. The reproducibility and accuracy of the analyses were verified by the ablation of secondary standards (91500 zircon and Durango apatite) and comparison with EMPA analyses. In this study we demonstrate that supergene mineralization can be directly dated and the trace elements in pseudomalachite and chrysocolla can be determined. The combination of these methods provides a new tool to understand the physico-chemical and geological conditions that are required for the formation of supergene mineralization.

How to cite: Leisen, M., Kahou, Z. S., Brichau, S., Duchêne, S., d’Abzac, F.-X., and Choy, S.: U-Pb in situ dating and trace elements profiles in chrysocolla and pseudomalachite : Application to supergene copper mineralization. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20802, https://doi.org/10.5194/egusphere-egu2020-20802, 2020.

Phosphorus removal from wastewaters is a great interest to avoid eutrophication of natural waters (i.e. rivers, lakes). Additionally phosphorus recovery is also important due to increasingly limited nutrient resources. Struvite (MgNH₄PO₄*6H₂O) is of current interest as it is considered an alternative way for water remediation and potential use as a renewable fertilizer. Towards environmental sustainability, phosphorus and nitrogen removal and recovery from domestic, industrial, and/ or agricultural inputs, in the form of struvite may be an attractive alternative for the valorization of wastewaters (Mpountas et al., 2017).

Direct observations of crystal dissolution and growth process at the nano- level are possible through the use of Atomic Force Microscopy (AFM). A considerable number of studies have investigated mineral dissolution and growth by in-situ AFM imaging in a fluid-cell (Vavouraki et al., 2008; 2010). These direct observations and measurements allow the investigation of possible process mechanisms at the mineral-solution interface. Previous study on AFM imaging indicated struvite micro- to nanocrystal morphology. Hövelmann & Putnis (2016) investigated the interactions of ammonium-phosphate solutions with brucite (Mg(OH)₂) cleavage surfaces by AFM suggesting coupled brucite dissolution and struvite precipitation at the mineral-fluid interface. To the best of our knowledge, there are no records of nanoscale observations of dissolution and/ or growth of struvite surfaces. The aim of this study is to perform in situ AFM experiments using freshly cleaved struvite surfaces at flow conditions. Step retreat and/ or etch pit spreading were observed. Dissolution rates of struvite using doubly deionized water at different pH values were determined  whereas growth rates at different saturation values and pH using magnesium and ammonium-phosphate bearing solutions were also measured . Raman and SEM analyses were carried out to assess chemical structure and morphology of the obtained struvite crystals before and after AFM experiments.

Acknowledgments: The work has been supported by IKY-DAAD (2018-4) and KRHPIS II (Action PERAN).                                                                                

References: Hövelmann & Putnis, 2016. Env. Sci. Technol. 50, 13032−13041; Mpountas et al., 2017. J. Chem. Technol. Biotechnol. 92, 2075–2082; Vavouraki et al., 2008. Chem. Geol. 253, 243–251; Vavouraki et al., 2010. Cryst. Growth Des. 10, 60–69.

How to cite: Vavouraki, A., King, H., Putnis, C., and Koutsoukos, P.: In situ AFM imaging of dissolution and growth of struvite surface, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21866, https://doi.org/10.5194/egusphere-egu2020-21866, 2020.

The marine Fe-Mn polymetallic nodules contain relatively high concentrations of Mn, Cu, Ni, Zn, Co and rare earth elements plus yttrium (REY), with a growing economic potential interest in their exploitation. To determinate the metallogenic processes and occurrence phases of the economic metals in the Fe-Mn nodules, we have undertaken high resolution mineralogical and geochemical studies of Fe-Mn nodules collected from the South China Sea (SCS).
The whole-rock mineralogical and chemical compositions of the SCS Fe-Mn nodules indicate hydrogenetic origin. The Mn mineral phases mainly are composed of nanocrystalline vernadite with interlayered 10 Å and 7 Å phyllomanganates, such as todorokite, birnessite, and buserite. Fe(-Ti) oxides/hydroxides are intergrown and essentially X-ray amorphous feroxyhyte and goethite. But we recognize two main types of internal microlayers in the SCS Fe-Mn nodules: Layer type A of suboxic diagenetic precipitates with extremely high Mn/Fe ratio and concentrations of Cu, Ni, Zn, Ba, Li and Mg; Layer type B of oxic hydrogenetic accretions with low fractionation of Mn and Fe and high contents of Co, REY, Ti, Sr and Pb. Furthermore, the elemental mapping indicates that the enrichment of Co and REY mainly associated with Fe mineral phases rather than Mn mineral phases, which are enriched in Mg, Cu, Ni, Zn, Li and Ba. Two mineralization processes and distributions of metals in the individual microlayers respectively are controlled and occurred by the different mineral phases. The increasing occurrence of 10 Å and 7 Å phyllomanganates present in the Layer type A are typically enriched in trace metals such as Ni, Cu, Zn, Li, Ba, and Mg, whereas the metals associated with the Layer type B include Co, Ti, Pb, Sr, REY, which might be carried by the intergrown of Fe(-Ti) oxyhydroxides and vernadite. Thus, hydrogenesis is more beneficial to the enrichment of Fe, Co, Ti, Sr, Pb and REY, while diagenesis is more favorable for the enrichment of Mn, Ni, Zn, Cu, Li, Ba and Mg during the metallogenic processes of the SCS nodules.

How to cite: Guan, Y.: Fine scale study of major and trace elements in the Fe-Mn nodules from the South China Sea and constraints on their formation processes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21817, https://doi.org/10.5194/egusphere-egu2020-21817, 2020.

The recent introduction of a high-brightness RF plasma oxygen ion source (Hyperion H201, Oregon Physics) to large geometry secondary ion mass spectrometers (e.g. CAMECA IMS1280/1300) has increased the range of available primary beam options compared to the several decades old technology of the duoplasmatron it replaces. Notably, the new source provides considerably higher beam density (ca. 10x and 3x for O^- and O₂^- respectively), which in principle allows for higher spatial resolution and/or shorter analysis times, coupled with unprecedented long-term beam stability.

Incorporating the RF plasma into both conventional spot analysis and ion-imaging geochronology routines at the NordSIMS facility has, however, revealed that the source upgrade has consequences for data-acquisition and data reduction strategies, which need to be modified in order to avoid degradation in precision. The most significant difference using the new source for spot analyses is the significant change in aspect ratio (width/depth) of the analysed volume. During a comparable length analysis, a three times brighter O₂^- primary beam (still favoured for U-Th-Pb geochronology) will sputter a three times deeper crater that is half the width of a comparable intensity duoplasmatron beam, an effective aspect ratio change of six times, introducing “down-hole” inter-element and, to a lesser degree, isotope fractionation effects that SIMS has largely been free of. Depending on the target matrix, this can have a marked effect on the within-run ratio evolution during an analysis, particularly the inter-element ratios Pb/U and UO_n/U required for full U-Pb geochronology, with standard error of the mean values several times higher than counting statistics, compared to analyses with the lower beam density of the duoplasmatron where s.e. mean commonly closely approaches Poisson counting statistics during a ca. 10 minute analysis. In line with previous observations [1], some improvements can be made by using a Pb/UO vs. UO₂/UO calibration scheme instead of Pb/U vs. UO_n/U, but clearly this is not the complete answer. Shortening analyses via fewer cycles in a peak-hopping routine also means smaller √n, affecting s.e. mean; lower integration times can be introduced to permit more cycles, but magnet settling times between peak jumps cannot be reduced in proportion, so the duty cycle is less efficient.

Strategies developed to mitigate this degradation and take full advantage of the new RF source include: 1) rastering of critically focused primary beams to retain high aspect ratio (at the expense of improved spatial resolution); 2) use of a defocused aperture-projected (Köhler-mode) primary beam (effectively lower beam density); 3) modelling of within-run ratio evolution based on standard analyses in a manner similar to that employed by laser ablation methods [2]; and/or 4) introduction of multicollection capabilities [3] to increase duty cycle efficiency in a shorter analysis. Ultimately, the choice of which method(s) to use will depend upon the goal of a specific project.

References: [1] Jeon, H. & Whitehouse, M.J.., Geostds & Geoanal. Res. 2014, 39, 443-452]; [2] Paton, C. et al., Geochem. Geophys. Geosyst., 2010, 11, Q0AA06]; [3] Li et al., J. Anal. At. Spectrom., 2015, 30, 979-985

How to cite: Whitehouse, M. and Jeon, H.: Living up to the Hype(-rion)! – observations on ion microprobe geochronology using a high-brightness oxygen plasma source, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20107, https://doi.org/10.5194/egusphere-egu2020-20107, 2020.