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Standardizing Spatial Reconstruction Parameters for the Atom Probe Analysis of Common Minerals
Abstract
Well-defined reconstruction parameters are essential to quantify the size, shape, and distribution of nanoscale features in atom probe tomography (APT) datasets. However, the reconstruction parameters of many minerals are difficult to estimate because intrinsic spatial markers, such as crystallographic planes, are not usually present within the datasets themselves. Using transmission and/or scanning electron microscopy imaging of needle-shaped specimens before and after atom probe analysis, we test various approaches to provide best-fit reconstruction parameters for voltage-based APT reconstructions. The results demonstrate that the length measurement of evaporated material, constrained by overlaying pre- and post-analysis images, yields more consistent reconstruction parameters than the measurement of final tip radius. Using this approach, we provide standardized parameters that may be used in APT reconstructions of 11 minerals. The adoption of standardized reconstruction parameters by the geoscience APT community will alleviate potential problems in the measurement of nanoscale features (e.g., clusters and interfaces) caused by the use of inappropriate parameters.
... A technique commonly employed alongside APT is (scanning)-transmission electron microscopy ((S)TEM), which is used to access structural, crystallographic, and phase information [8][9][10] , probe the FE mechanism 11,12 , and calibrate reconstructions [13][14][15] . Compositional information can be obtained from (S)TEM-based spectroscopy methods (i.e., energy dispersive X-ray spectroscopy (EDS/EDX) or electron energy loss spectroscopy (EELS)), but these do not possess the required sensitivity to resolve the minute dopant concentrations in state-of-the-art devices 4 . ...
The following letter presents a novel, multi-length-scale characterization approach for investigating doping chemistry and spatial distributions within semiconductors, as demonstrated using a state-of-the-art CMOS image sensor. With an intricate structural layout and varying doping types/concentration levels, this device is representative of the current challenges faced in measuring dopants within confined volumes using conventional techniques. Focused ion beam-secondary ion mass spectrometry is applied to produce large-area compositional maps with nanoscale resolution, while atom probe tomography is used to extract quantitative dopant profiles. Leveraging the complementary capabilities of the two methods, this workflow is shown to be an effective approach for resolving nano- and micro- scale dopant information, crucial for optimizing the performance and reliability of advanced semiconductor devices.
... Peaks above twice the height of the background were manually ranged and assigned. A voltage-evolution reconstruction was performed using an atomic density average for corundum of 0.00847 nm 3 /atom and an empirically determined evaporation field of 34.18 V/nm (Fougerouse et al. 2022). ...
In natural corundum, a strong geochemical correlation is sometimes observed between Be and heavy high field strength elements (HHFSEs) such as Nb, Ta and W, and it has been hypothesized that trace elements are hosted in primary inclusions. However, no known mineral enriched in both Be and HHFSEs stable at these geological conditions can explain this correlation. To understand how Be and HHFSEs are distributed in natural corundum down to the atomic scale, two natural Be-bearing sapphire crystals from Afghanistan and Nigeria are studied using laser ablation inductively coupled plasma and time-of-flights secondary ion mass spectrometry, atom probe tomography and transmission electron microscopy. In addition to common trace elements such as Mg, Ti, and Fe, Be and W are detected in the metamorphic sapphire from Afghanistan, whereas Be, Nb and Ta are detected in the magmatic sapphire from Nigeria. Nanoclustering in both samples shows fractionation of Be and high field strength elements (including Ti) by atomic mass, suggesting a secondary process controlled by solid-state diffusion. The homogeneously distributed W and the secondary nano-precipitates bearing Nb and Ta indicates that HHFSEs can be incorporated into the corundum structure during crystallization, most likely through preferred adsorption on the growth surface. The strong correlation between Be and HHFSEs across the growth zones is probably due to Be being attracted by HHFSEs to partially balance the charge when incorporated into the corundum structure. The enrichment of high field strength elements by growth kinetics may result in supersaturated concentrations during crystallization, allowing them to precipitate out when the host corundum is heated above its formation temperature by basaltic magma. Comparison with previous transmission electron microscope studies suggests the same process for incorporating Be and HHFSEs also applies to other natural corundums from different localities.
... Voltage evolution reconstructions were performed using a detector efficiency of 0.36, an image compression factor of 1.65, and a kfactor of 3.3. For baddeleyite, the atomic volume was calculated at 0.01133 nm 3 per atom, and the electric field was empirically determined at 29.08 V nm (Fougerouse et al., 2022). Tomographic data for one specimen of each grain were successfully acquired, with 62 million ions for specimen M5 (Great Dyke of Mauritania) and 65 million ions for specimen M2 (Hart Dolerite). ...
Atom probe tomography (APT) of ²³⁸U and ²⁰⁶Pb has been applied to baddeleyite crystals from the Hart Dolerite (1791±1 Ma) and the Great Dyke of Mauritania (2732±2 Ma) in an effort to constrain the average nuclear recoil distance of ²³⁸U series daughter nuclei and correct alpha-recoil-induced Pb loss on U–Pb ages from small baddeleyite crystals. The Hart Dolerite sample showed no variations in Pb concentrations near the edge and is interpreted to represent a cleaved surface instead of the original crystal surface. The Great Dyke sample shows U zoning, and the associated ²⁰⁶Pb zoning is affected by alpha recoil, apparently adjacent to a natural grain surface. This sample also shows primary clusters of U atoms at a scale of 10 nm that contain about 40 % of the total U. 207Pb/206Pb nanogeochronology suggests that the clusters are primary in origin; however, they are too small to constrain alpha recoil distance beyond a few nanometres. To constrain alpha recoil distance, a forward-modelling approach is presented where ²⁰⁶Pb redistribution functions were determined for a range of possible recoil distances. Synthetic 206Pb/238U profiles were determined from the convolution of the observed U profile with the redistribution functions for different alpha recoil distances. These were compared to the observed 206Pb/238U profile to determine the recoil distance that gives the best fit. The observed U zoning was extrapolated to account for the full range of possible alpha recoil redistribution effects, which is larger than the 40 by 400 nm size of the APT field of view. Any reasonable extrapolation constrains the average alpha recoil distance to over 70 nm, which is much larger than previous estimates using other methods. This could be because recoil distances can be highly anisotropic within small crystal samples or because laterally non-uniform U zoning was a factor that modified the recoiled Pb distribution. APT is a potentially useful approach for determining average alpha recoil distance but requires sampling of primary smooth crystal faces with demonstrably uniform zones.
... Voltage evolution reconstructions were performed using a detector efficiency of 0.36, an image compression factor of 1.65, and a k-factor of 3.3. For reconstruction of zircon, an atomic volume of 0.01076 nm 3 per atom was used, and the electric field was empirically determined at 32 V/nm (40). The mass resolving power (m/∆m, where the peak width ∆m is the full peak width at half its maximum height) of zircon APT data is ~1000, which provides good separation of peaks with differing mass numbers while not fully resolving isobaric interferences. ...
Finding direct evidence for hydrous fluids on early Mars is of interest for understanding the origin of water on rocky planets, surface processes, and conditions essential for habitability, but it is challenging to obtain from martian meteorites. Micro- to nanoscale microscopy of a unique impact-shocked zircon from the regolith breccia meteorite NWA7034 reveals textural and chemical indicators of hydrothermal conditions on Mars during crystallization 4.45 billion years ago. Element distribution maps show sharp alternating zoning defined by marked enrichments of non-formula elements, such as Fe, Al, and Na, and ubiquitous nanoscale magnetite inclusions. The zoning and inclusions are similar to those reported in terrestrial zircon crystallizing in the presence of aqueous fluid and are here interpreted as primary features recording zircon growth from exsolved hydrous fluids at ~4.45 billion years. The unique record of crustal processes preserved in this grain survived early impact bombardment and provides previously unidentified petrological evidence for a wet pre-Noachian martian crust.
... For the 3D reconstruction, voltage-based models were applied. The detector efficiency was set at 36 %, k-factor at 3.3, image compression factor at 1.65, atomic volume computed at 0.01190 nm 3 /atom for xenotime, and the field evaporation estimated at 28.98 V/nm as determined empirically (Fougerouse et al., 2021c). ...
... Voltage evolution reconstructions were performed using a detector efficiency of 0.36, an image compression factor of 1.65 and a k-130 factor of 3.3. For baddeleyite, the atomic volume was calculated at 0.01133 nm 3 /atom and the electric field was empirically determined at 29.08 V/nm (Fougerouse et al., 2022). Tomographic data for one specimen of each grain were successfully The U and Pb isotopic compositions were quantified from the atom probe data by using a narrow range (0.1 Da) for each 135 isotopic ionic specie. ...
Atom probe tomography (APT) of 238U and 206Pb has been applied to baddeleyite crystals from the Hart Dolerite (1791 ± 1 Ma) and the Great Dyke of Mauritania (2732 ± 2 Ma) in an effort to map U and Pb concentration at the nanometre scale. The purpose was to constrain the average nuclear recoil distance of 238U-series daughter nuclei in order to correct U-Pb ages determined on small baddeleyite crystals for alpha-recoil loss of Pb. Both crystals were thought to expose natural crystal surfaces providing a boundary where maximum effects of recoil loss could be observed. The Hart Dolerite sample showed no variations in Pb concentrations near the edge. The Great Dyke sample shows U zoning and the associated 206Pb zoning is affected by alpha recoil, apparently adjacent to a natural grain surface. The sample also shows 10 nm-scale apparently primary clusters of U atoms that contain about 40 % of the U. These are too small to constrain alpha recoil distance beyond a few nm but are apparently primary and their formation mechanism poses a dilemma. To constrain alpha recoil distance, a forward modelling approach is presented where 206Pb redistribution functions were determined for a range of possible distances and synthetic 206Pb/238U profiles were determined from the convolution of the observed U profile with the redistribution functions that were compared to the observed 206Pb/238U profile. A complication is the fact that the 40 by 400 nm size of the sample is lower than the range of possible alpha recoil redistribution effects so it was necessary to extrapolate the observed U zoning. An oscillatory pattern gives the best fit to the observed profile but any reasonable extrapolation constrains the average alpha recoil distance to be close to 80–90 nm, which is much larger than previous estimates using other methods. Either recoil distances can be highly anisotropic within small crystal samples or surface roughness was a factor that modified the recoiled Pb distribution. APT is a potentially useful approach to determining average alpha recoil distance but requires sampling of primary, smooth crystal faces.
... The laser was pulsed at 125 kHz with a laser energy of 300 pJ and at an automated detection rate of 0.02 ion/pulse and 60 K base temperature. The 3D reconstruction was performed using the Cameca IVAS 3.8 software using a voltage curve reconstruction and 32 V/nm evaporation field following a standardized approach (Fougerouse et al. 2022;Saxey et al. 2019). ...
The mineral zircon is used widely to constrain the age of rocks and the processes that formed them. Although zircon is robust to a range of physical and chemical processes, it may show evidence for rapid re-equilibration that is generally considered to reflect interaction with hydrous fluids. Here, we show that zircon grains that crystallized from melt produced during the catastrophic meltdown of the Chernobyl nuclear reactor exhibit re-equilibration textures that occurred in an environment without free water. The process of re-equilibration involved a melt-mediated interface-coupled dissolution-reprecipitation that took place over a few days to produce textures that are commonly observed in igneous and anatectic systems. Thus, the composition of zircon can be modified even in the absence of hydrous fluids in a short time frame. Through this process, zircon crystals may track the timing of the last silicate melt they interacted with.
... The detection rate was kept at 1%. 3D reconstruction and data analysis were performed using AP Suite 6.1 software. Reconstruction parameters for zircon can be found based on a standardized approach in Fougerouse et al. (2022). SEM images before and after the final milling of each tip were used to assist in the reconstruction. ...
... Hence, the standardization of APT mineral analysis is extremely dif cult since no specimen is equal to another. At the same time, suggestions for common practices of data reporting and analysis routines have been proposed (Blum et al., 2018;Exertier et al., 2018;Saxey et al., 2018a;Fougerouse et al., 2021), and indicative ranges for parameter settings have been de ned for some minerals (La Fontaine et al., 2017;Verberne et al., 2019;Joseph et al., 2021). ...
The use of atom probe tomography (APT) for mineral analysis is contributing to fundamental studies in Earth Sciences. Meanwhile, the need for standardization of this technique is becoming evident. Pending the use of mineral standards, the optimization of analysis parameters is needed to facilitate the study of different mineral groups in terms of data collection and quality. The laser pulse rate and energy are variables that highly affect the atom evaporation process occurring during APT analysis, and their testing is important to forecast mineral behavior and obtain the best possible data. In this study, five minerals representative of major groups (albite, As-pyrite, barite, olivine, and monazite) were analyzed over a range of laser pulse energies (10–50 pJ) and rates (100–250 kHz) to assess output parameter quality and evaluate compositional estimate stoichiometry. Among the studied minerals, As-pyrite, with the higher thermal conductivity and lower band gap, was the most affected by the laser pulse variation. Chemical composition estimates equal or close to the general chemical formula were achieved for monazite and As-pyrite. The analysis of multihit events has proved to be the best strategy to verify the efficacy of the evaporation process and to evaluate the best laser pulse setting for minerals.
... Both atomic and molecular ions, with charge states of 1+ to 3+ were identified in the time-of-flight spectrometry mass/charge spectra. For reconstructing the atom probe data an evaporation field of 27.02 V/nm and an atomic average for monazite of 0.01245 nm 3 /atom were used following the recommendations of Fougerouse et al. (2022). More details are available in Table S2. ...
Understanding radionuclides mass transfer mechanisms in monazite (LREEPO4) and the resulting features, from the micro- to the nanoscale, is critical to its use as a robust U–Th–Pb geochronometer. A detailed multiscale characterisation of discordant monazite grains from a granulite which records a polymetamorphic history explores the mechanisms of Th and Pb mobility in crystals. Some monazite grains display Th-rich linear features (0.1–1 µm thick) forming a regular network throughout the grain. They are interpreted as resulting from fluid ingress along crystallographically controlled pathways. Nanoscale features termed ‘clusters’ (Ø < 10 nm) are composed of radiogenic Pb (Pb*) ± Si ± Ca and are localised within monazite lattice defects. Their formation results from the competition, over millions of years, of both radiation damage production allowing element mobility (by diffusion) and accumulation in defects and α-healing inducing their trapping. Nanophases (Ø = 0.02–1 µm) containing Pb* are present in all grains and correspond to galena (PbS) or sesquioxide of Pb (Pb2O3). They are associated with a chemically varied suite of amorphous silicate (± Al, Mg, Fe) phases or sulphur (e.g. FeS). They are interpreted as precipitates within monazite crystals. They are formed during replacement mechanism of monazite through fluid interactions. Two generations of Pb*-bearing nanophases exist supported by previous geochronological data. The shielding effect of garnet and rutilated quartz (host minerals), limiting fluid access, induces plentiful Pb*-bearing nanophases precipitation (fluid saturation enhanced) and limits Pb*-loss at the grain scale. This multiscale study provides new insights for interpretations of meaningless geochronological information, thanks to nanoscale investigations.
... In the mass spectrum, peaks at least twice above the background were identified and reconstructed in 3-dimensions using Cameca's AP Suite 6 software. Following the recommendations of Fougerouse et al. (41), an average atomic volume of 0.01290 nm 3 /atom and an electric field value of 34.20 V/nm were used for the spatial reconstruction. ...
Rubble piles asteroids consist of reassembled fragments from shattered monolithic asteroids and are much more abundant than previously thought in the solar system. Although monolithic asteroids that are a kilometer in diameter have been predicted to have a lifespan of few 100 million years, it is currently not known how durable rubble pile asteroids are. Here, we show that rubble pile asteroids can survive ambient solar system bombardment processes for extremely long periods and potentially 10 times longer than their monolith counterparts. We studied three regolith dust particles recovered by the Hayabusa space probe from the rubble pile asteroid 25143 Itokawa using electron backscatter diffraction, time-of-flight secondary ion mass spectrometry, atom probe tomography, and 40Ar/39Ar dating techniques. Our results show that the particles have only been affected by shock pressure of ca. 5 to 15 GPa. Two particles have 40Ar/39Ar ages of 4,219 ± 35 and 4,149 ± 41 My and when combined with thermal and diffusion models; these results constrain the formation age of the rubble pile structure to ≥4.2 billion years ago. Such a long survival time for an asteroid is attributed to the shock-absorbent nature of rubble pile material and suggests that rubble piles are hard to destroy once they are created. Our results suggest that rubble piles are probably more abundant in the asteroid belt than previously thought and provide constrain to help develop mitigation strategies to prevent asteroid collisions with Earth.
... Details of the APT approach for geologic materials are given elsewhere . APT data were reconstructed in three dimensions using garnet-specific reconstruction parameters (Fougerouse et al., 2021b). To estimate the volatile composition of the boundary, OH (17 Da) was quantified as a proxy for H distribution. ...
Chemical heterogeneities along grain boundaries in garnet occur across a wide range of metamorphic conditions, yet the processes underlying their development remain poorly understood. Here we integrate electron backscattered diffraction (EBSD) and atom probe tomography (APT) to evaluate the mechanisms driving nanoscale trace element mobility to deformation microstructures in a granulite-facies garnet. This approach shows that low-angle boundaries can be enriched in Ca, Ti, P, Cu, K, Na, Cl, and H. Based on the correlation between EBSD and APT data, we propose that solute ions (Ca, Ti, P, and Cu) were segregated to the interface during the migration of dislocation associated with ductile deformation of the grain. In contrast, elements such as K, Na, Cl, and H are interpreted to reflect diffusion along the low-angle boundary from an externally derived fluid source. These results provide the missing link between chemical heterogeneity and deformation-related microstructures in garnet. Our approach shows that a combination of microstructural and nanoscale geochemical analyses can provide unprecedented insights into mechanisms of element transfer within minerals.
... Post-processing was done using CAMECA Integrated Visualization and Analysis Software (IVAS) 3.8.0 and standard background corrections (Larson et al., 1999). 3D volume reconstruction or done in IVAS 3.8 (Larson et al., 1999;Gault et al., 2012) following recommendation in Fougerouse et al. (2021d). Peaks in the mass spectra were labelled per individual isotope for specific ionization states and ranged with a constant width of 0.2 Da, slightly off centre of the peak maximum and a consistent bin width of 0.01 Da. ...
Element mobility is a critical component in all geological processes and understanding the mechanisms responsible for element mobility in minerals is a fundamental requirement for many geochemical and geochronological applications. Volume diffusion of elements is a commonly assumed process. However, linear defects (dislocations) are an essential component of the high-temperature creep of minerals. These defects are commonly inferred to form fast-diffusion pathways along which trace elements can more rapidly migrate. In contrast, dislocations in minerals are also energetically favourable sites of trace element segregation, which counters the notion that they enhance bulk diffusion rates by a pipe diffusion mechanism. In this paper we characterize the trace-element composition of dislocations on twin boundaries in rutile by combining atom probe tomography with transmission electron microscopy. First, morphology and correlative microstructural data are used to demonstrate that the linear compositional features in the atom probe tomography dataset represent dislocations. Assessment of dislocation composition indicates that segregation is trace element specific. The data show that dislocations in rutile act as both, fast-diffusion pathway and trace-element traps which potentially leads to erroneous estimations of the composition.
The study of the structure and geochemistry of olivine crystal defects is important but difficult because of their nanometer size and the analytical limitations of most techniques. Laser-assisted atom probe tomography (APT) is capable of sub-nanometer resolution, quantitative geochemical analysis and 3D reconstruction of olivine defects, but optimal analytical conditions and data reconstruction strategies have not been sufficiently studied. Here, we investigate the effect of different laser pulse energy (LPE) and crystal orientations on the quality and reconstruction parameters of APT data using specimens from two San Carlos olivine grains. Our findings show that increased LPE reduces the background noise, percentage of multiple hit events, and applied electric field, as shown by the Mg2+/Mg+ ratio, but increases the peak tails. The major element compositions show inaccuracies under all LPEs but exhibit higher consistency for higher LPEs. We determine that a LPE of 150pJ is the best compromise for optimal data quality in olivine. Using scanning electron microscopy imaging before and after APT analyses, we suggest that the Mg2+/Mg+ ratio can be used as a guide to estimate the electric field parameter and results in more accurate reconstructions.
Trace elements in sulfides are commonly used to determine the physicochemical conditions of ore deposit formation. The thermodynamic models underpinning these studies rely on the assumption that trace elements are incorporated into the mineral's crystal structure, however recent atomic-scale investigations suggest that this assumption may be erroneous, especially in metamorphosed environments. Here, in primary undeformed colloform sphalerites from two Pb-Zn deposits in South-China, we study the microstructural, geochemical, and nanoscale distribution of trace elements. Our results show that colloform sphalerite hosts trace elements such as Ge (up to 5671 ppm) and Ga (up to 16307 ppm) in nanoscale polyphase inclusions (mainly 10-20 nm), comprising an aqueous solution and solid phases such as galena and pyrite. These Ge(-Ga) polyphase inclusions are rich in light elements and halogens (H, Li, Na, Cl, K) and heavier metals such as Mn and Pb, accounting for 5%-78% of the trace element budget in bulk sphalerite. We propose a model whereby the rapid crystallization of colloform sphalerite favors the preservation of elevated trace element concentrations in nanoscale fluid inclusions (i.e., Ga, Ge, Pb, Mn) that are in apparent thermodynamic disequilibrium with sphalerite. A nucleation mechanism is proposed involving the entrapment of dense liquid composed of an intermediate high-density disordered state under supersaturation conditions. Based on a global geochemical data compilation of colloform sphalerite, we show significant enrichment of Pb in colloform sphalerite and multiple positive correlations between Pb and Ge. This suggests that Pb-Ge-rich nanoscale dense-liquid inclusions may be a prevalent carrier for trace elements observed in colloform sphalerite textures. Similar colloform textures resulting from supersaturated solutions in minerals such as pyrite or quartz may also contain trace element-rich nanoscale inclusions. Presence of these nanoscale inclusions appears to have a minimal effect on the estimated formation conditions derived from sphalerite chemistry (temperature, fS2). This study highlights the value of chemical mapping in revealing temperature variations in sphalerite.
Understanding elements uptake and release from minerals in source rocks is crucial for comprehending critical metals accumulation, yet the mechanisms and kinetics of element mobilization at the atomic scale remain mostly unknown. Here, we analyzed the distribution of cobalt (Co) in natural pyrite from a Cu-Co ore deposit and found that metals distribution is best described by steady-state diffusion with constant flux and concentration-dependent diffusivities, rather than transient-state diffusion with time-evolving concentrations. First-principles calculations and diffusion modelling further demonstrate that this diffusion is accelerated by vacancy pathways and is far more efficient than traditional vacancy-mediated lattice diffusion, with element transfer rates higher by almost two orders of magnitude. We conclude that steady-state lattice diffusion induced by vacancies in the presence of fluid can be an efficient mechanism promoting the preferential release of metals into ore fluids and the accumulation of metals during ore formation.
Understanding the complex interplay between the processes of mineral crystallization and the incorporation of trace elements, particularly in economically significant deposits like Carlin-type gold systems, is essential for unraveling geological processes. This study investigates the microscale to nanoscale texture and composition of weakly deformed arsenian pyrite from the Shangmanggang Carlin-type Au deposit in Southwest China, employing advanced techniques such as scanning transmission electron microscopy and atom probe tomography. Trace element−rich oscillatory zones in the pyrite are characterized by ∼30-nm-thick bands enriched in As, Au, and Cu. Cu, As, Sb, Pb, Hg, and Tl are distributed heterogeneously and form clusters and discontinuous planar features on the outer edge of As-rich oscillatory bands. Discontinuous planar features, nucleating from trace element−enriched clusters, are oriented approximately in line with the direction of epitaxial growth. The nanoscale epitaxial growth zones are likely the result of the incorporation of impurity defects coupled with diffusion-limited self-organization and fluctuations in fluid composition. Arsenic-induced lattice distortion facilitates surface adsorption of dopant trace metals, which leads to “unstructured” impurities (Sb, Pb, Hg, and Tl) clustering locally in misfit crystal defects. The transition from homogeneous element distribution in As-rich bands to clustered trace elements suggests a Stranski-Krastanov growth process. Discontinuous planar features may represent the propagation of crystal defects locally and the further incorporation of trace elements. Our study provides insights into the factors governing the heterogeneous incorporation of trace elements, particularly Au, into pyrite during epitaxial growth.
The causes of U-Pb isotopic discordance documented by Paquette et al. (2004) in monazite grains from the ultra-high temperature (UHT) granulite of the Andriamena unit of Madagascar are re-evaluated in the light of nanoscale crystal-chemical characterization utilising Atom Probe Tomography (APT) and state-of-the-art Scanning Transmission Electron Microscopy (STEM). APT provides isotopic (²⁰⁸Pb/²³²Th) dating and information on the chemical segregation of trace elements (e.g., Pb) in monazite at nanoscale. Latest generation of STEM allows complementary high-resolution chemical and structural characterization at nanoscale. In situ isotopic U–Pb dating with Secondary Ion Mass Spectrometry (SIMS) on 25 monazite grains and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) on zircon have been employed to refine the age spectra. Monazite and zircon grains located in quartz and garnet formed with the peak UHT metamorphic assemblage, which is partially overprinted by retrograde coronitic textures. Zircon grains hosted in garnet and in quartz yield concordant U–Pb ages at 2758 ± 28 Ma and 2609 ± 51 Ma, respectively whereas monazite grains hosted in quartz and garnet show a discordant Pb* loss trend on the Concordia diagram recording disturbance at 1053 ± 246 Ma that is not seen by the zircon, underlining the importance of combining the use of monazite and zircon to understand the history of polymetamorphic rocks. The Pb*-loss trend of monazite is related to petrographic position, with less Pb* lost from monazite hosted in quartz and garnet than monazite hosted in the coronitic reaction texture domains. STEM shows that the garnet- and quartz-hosted monazite grains contain more Pb-bearing nanophases than monazite grains located in the coronitic textures. An inverse correlation between the number of Pb-bearing nanophases and the percentage of Pb*-loss in monazite grains demonstrates that Pb* is retained in the grain in the form of nanophases. The formation of Pb-bearing nanophases limits Pb*-loss at the grain scale and therefore allows the preservation of early events. ²⁰⁸Pb/²³²Th ratios obtained with APT in monazite located in quartz and garnet and excluding Pb*-bearing nanophases indicate a mean age of 1059 ± 129 Ma corresponding to a disturbance event hitherto undetected in the geochronological record of the Andriamena unit. Thus, geochronology with APT allows access to information and the definition of events that may be blurred or obscured when examined at lower spatial resolution.
Accessory minerals like zircon, rutile and monazite are routinely studied to inform about the timing and nature of geological processes. These studies are underpinned by our understanding of the transfer processes of trace elements and the assumption that the isotopic systems remain undisturbed. However, the presence of microstructures or Pb-bearing phases in minerals can lead to the alteration of the Pb isotopic composition. To gain insight into the relationship between Pb isotopic alterations from inclusions and microstructures, this study focused on inclusions from an ultra-high-temperature metamorphic rutile. The studied inclusions are submicrometer monazites, a common mineral rich in Pb but normally not present in rutile. The sample is sourced from Mt. Hardy, Napier Complex, East Antarctica, an ultra-high-temperature (UHT) metamorphic terrane. By applying correlative analytical techniques, including electron backscatter diffraction mapping, transmission electron microscopy (TEM), and atom probe tomography, it is shown that monazite inclusions are often in contact with low-angle boundaries and yield no preferred orientation. TEM analysis shows the monazite core has a mottled texture due to the presence of radiation damage and nanoclusters associated with the radiation damage defects that are rich in U, Pb, and Ca. Some monazites exhibit a core-rim structure. The rim yields clusters composed of Ca- and Li-phosphate that enclose Pb nanoclusters that are only present in small amounts compared to the core, with Pb likely diffused into the rutile-monazite interface. These textures are the result of two stages of Pb mobility. Initial Pb segregation was driven by volume diffusion during UHT metamorphism (2500 Ma). The second stage is a stress-induced recrystallization during exhumation, leading to recrystallization of the monazite rim and trace element transport. The isotopic signature of Pb trapped within the rutile-monazite interface constrains the timing of Pb mobility to ca. 550 Ma.
This study focuses on the low-temperature mineralogical response of xenotime, a
phosphate mineral routinely used as a geochronometer, to fluid-assisted alteration.
The studied xenotime grain (z6413) comes from a ~1000 Ma pegmatite from the
Grenville Province, Canada, and is commonly used as reference material for U-Pb
analyses. At the microscale, the grain has a mottled texture, sub-micrometer porosity,
and small domains dark in backscattered electron (BSE) images that are characterised
by curviplanar, sharp boundaries. The small dark BSE domains are associated with Th-
U-rich inclusions and larger porosity (2-3 μm) and are interpreted to result from
localised fluid-assisted coupled dissolution-reprecipitation. Sensitive high-resolution ion
microprobe (SHRIMP) U-Pb analyses of unaltered and fluid-affected domains yield
concordant crystallisation dates, irrespective of the textural domains. The apparently
unaltered xenotime domain was characterised at the nanoscale to determine if the
grain was affected by fluids beyond the altered domains defined by BSE imaging.
Transmission electron microscopy (TEM) imaging results indicate the presence of
randomly distributed Ca+Pb nanoscale precipitates. Atom probe tomography (APT)
reveals the presence of spherical clusters (4 to 18 nm in size) enriched in radiogenic
Pb, Ca, and Si atoms, which, combined with TEM observations, are interpreted as
nanoscale inclusions of apatite. In addition to the inclusions, a dislocation enriched in
Ca and fluid mobile elements such as Cl, Li, Na, and Mn was imaged from APT data
indicating percolating of fluids further than the reaction front. APT 206Pb/238U
nanogeochronology indicates that the nanoscale inclusions of apatite formed at
863±28 Ma, 100–150 Ma after crystallisation of the host xenotime, with its formation
attributed to fluid metasomatism. This study shows that fluid-xenotime reaction caused
Pb* to be redistributed at the nanoscale, recording the timing of metasomatism.
However, at the scale of SHRIMP analytical spot (10 μm), xenotime is concordant,
indicating that Pb was not mobile at the microscale and fluid-altered xenotime can
preserve its crystallisation age. Although the studied grain shows a limited amount of
altered domains in BSE imaging, nanoscale analyses reveal a more pervasive reequilibration
of the minerals through the percolation of fluids along dislocations.
Tellurium (Te) is a critical commodity, essential for renewable energies and high-tech applications. Most Te is currently recovered from copper smelters, but rising demand requires sourcing from alternative supplies. However, the mineralogy of Te-rich ores is poorly understood and hinders their economic potential. Here, we investigate the distribution of Te in pyrite from a high-grade Au-Ag-Te epithermal-type ore to inform metallurgical extraction methods, and secure future Te supply. We identified three distinct modes of Te incorporation in this pyrite, which challenge previous solubility models. (1) Te solid-solution, at concentrations (up to 285 ppma) that significantly exceed previous solubility limits. (2) Nano-telluride inclusions along cracks that formed by intra-grain remobilization. (3) Crystal defects, enriched in Te through pipe diffusion hosting up to 0.5 at.% Te. Our results therefore provide new fundamental insights into the chemical and structural coordination of Te in pyrite, which may guide future efforts for its direct recovery.
The trace‐element composition of rutile is commonly used to constrain P‐T‐t‐conditions for a wide range of metamorphic systems. However, recent studies have demonstrated the redistribution of trace elements in rutile via high‐diffusivity pathways and dislocation‐impurity associations related to the formation and evolution of microstructures. Here we investigate trace‐element migration in low‐angle boundaries formed by dislocation creep in rutile within an omphacite vein of the Lago di Cignana unit (Western Alps, Italy). Zr‐in‐rutile thermometry and inclusions of quartz in rutile and of coesite in omphacite constrain the conditions of rutile deformation to around the prograde boundary from high pressure to ultra‐high pressure (~2.7 GPa) at temperatures of 500–565 °C. Crystal‐plastic deformation of a large rutile grain results in low‐angle boundaries that generate a total misorientation of ~25°. Dislocations constituting one of these low‐angle boundaries are enriched in common and uncommon trace elements, including Fe and Ca, providing evidence for the diffusion and trapping of trace elements along the dislocation cores. The role of dislocation microstructures as fast‐diffusion pathways must be evaluated when applying high‐resolution analytical procedures as compositional disturbances might lead to erroneous interpretations for Ca and Fe. In contrast, our results indicate a trapping mechanism for Zr.
The discordance of U–Th–Pb isotopic systems in geochronometers, and how such data are interpreted, are still major issues in the geosciences. To better understand the disturbance of isotopic systems, and how this impacts the derivation of geologically-meaningful ages, previously studied discordant monazite from the ultrahigh temperature paragneiss of the Archean Napier Complex (Antarctica) have been investigated. Monazite grains were characterized from the micro to the nanoscale using an analytical workflow comprising laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), secondary-ion mass spectrometry (SIMS), electron microprobe (EMP), transmission electron microscopy (TEM) and atom probe tomography (APT). Results reveal that the least discordant monazite, hosted in garnet and rutilated quartz, contain a large number of small Pb-bearing nanocrystals (Ø∼ 50 nm) while the most discordant monazite, hosted in the quartzo-feldspathic matrix, contain a smaller number of Pb-bearing nanocrystals bigger in size (Ø∼ 50 to 500 nm). The degree of the discordance, which was previously correlated with textural position is mechanistically related to the partial retention of radiogenic Pb (Pb⁎) in distinct Pb⁎-bearing nanocrystals (e.g. PbS) within the monazite grains. In-situ dating (U–Pb systems with LA-ICP-MS and SIMS), and isotopic information obtained by using APT (²⁰⁷Pb/²⁰⁶Pb isotopic signature of galena and ²⁰⁸Pb/²³²Th ages of the monazite matrix) allow the timing of Pb-disturbance and mobility to be constrained. Results show that monazite grains crystallized at ca. 2.44 Ga and were affected by two episodes of Pb⁎ mobility. The first episode (t1) at ca. 1.05 Ga, led to crystallization of a first generation of Pb⁎-bearing nanocrystals and a complete resetting of the monazite matrix at the nanoscale. The second episode (t2) at ca. 0.55 Ga was associated with the crystallization of a second generation of Pb⁎-bearing nanocrystals with a ²⁰⁷Pb/²⁰⁶Pb signature indicating a mixing of two Pb⁎ components: a component from the monazite matrix and remobilized Pb⁎ from the first generation of Pb⁎-bearing nanocrystals. This second event is characterized by a more localized resetting of the monazite matrix at the nanoscale compared to the t1 event. These results indicate the potential of nanoscale studies of Pb-rich nanocrystals within monazite to yield important details of the themal history of complex metamorphic terranes.
Glassy melt inclusions are unique geological repositories that preserve evidence of the formation and evolution of mantle and crustal-derived magmas. However, the mechanisms responsible for their preservation in slowly cooled crustal rocks remain contentious, in some part due to their small size (commonly < 10 µm) and the technical difficulty in quantifying composition and microstructures. In this work, time-of-flight secondary ion mass spectrometry, transmission electron microscopy and atom probe tomography are used to characterize glassy melt inclusions found in peritectic garnets of a migmatite from the Spanish Betic Cordillera. The glassy melt inclusions coexist in a close spatial relationship with partially to totally crystallized melt inclusions (nanogranitoids). Analyses of the glassy inclusions show a heterogeneous, patchy distribution of Na and K within the glass and along inclusion walls. Nanoscale spherical domains of Al, Fe, K, Na, Cl and Li are also found systematically distributed at inclusion edges, and are interpreted to represent pre-nucleation clusters. The location and compositional similarity of these clusters with micas and feldspars in nanogranitoids indicate that the glassy inclusions represent former nanogranitoids “captured” at an earlier stage of crystallization, suggesting a likely common origin for both the glassy inclusions and nanogranitoids. A comparison between the composition of melt inclusions with previously published data reveals that preserved glassy inclusions contain significant less H2O (av. 2.72 wt%) than nanogranitoids (average of 6.91 wt%). This suggests the low-H2O content representing a further impediment to crystallization, along with the very small volume of these cavities, favouring the coexistence of glassy inclusions and nanogranitoids. In contrast, crystal nucleation is enhanced in more hydrous melts, where H2O reduces melt viscosity and promotes diffusion.
The geometry and composition of deformation-related low-angle boundaries in naturally deformed olivine were characterized by electron backscattered diffraction (EBSD) and atom probe tomography (APT). EBSD data show the presence of discrete low-angle tilt boundaries, which formed by subgrain rotation recrystallisation associated with the (100)[001] slip system during fluid-catalysed metamorphism and deformation. APT analyses of these interfaces show the preferential segregation of olivine-derived trace elements (Ca, Al, Ti, P, Mn, Fe, Na and Co) to the low-angle boundaries. Boundaries with < 2° show marked enrichment associated with the presence of multiple, non-parallel dislocation types. However, at larger disorientation angles (> 2°), the interfaces become more ordered and linear enrichment of trace elements coincides with the orientation of dislocations inferred from the EBSD data. These boundaries show a systematic increase of trace element concentration with disorientation angle. Olivine-derived trace elements segregated to the low-angle boundaries are interpreted to be captured and travel with dislocations as they migrate to the subgrain boundary interfaces. However, the presence of exotic trace elements Cl and H, also enriched in the low-angle boundaries, likely reflect the contribution of an external fluid source during the fluid-present deformation. The observed compositional segregation of trace elements has significant implications for the deformation and transformation of olivine at mantle depth, the interpretation of geophysical data and the redistribution of elements deep in the Earth. The observation that similar features are widely recognised in manufactured materials, indicates that the segregation of trace elements to mineral interfaces is likely to be widespread.
Mining of “invisible gold” associated with sulfides in gold ores represents a significant proportion of gold production worldwide. Gold hosted in sulfide minerals has been proposed to be structurally bound in the crystal lattice as a sulfide-gold alloy and/or to occur as discrete metallic nanoparticles. Using a combination of microstructural quantification and nanoscale geochemical analyses on a pyrite crystal from an orogenic gold deposit, we show that dislocations hosted in a deformation low-angle boundary can be enriched in Ni, Cu, As, Pb, Sb, Bi, and Au. The cumulative trace-element enrichment in the dislocations is 3.2 at% higher compared to the bulk crystal. We propose that trace elements were segregated during the migration of the dislocation following the dislocation-impurity pair model. The gold hosted in nanoscale dislocations represents a new style of invisible gold.
Impact cratering on the Moon and the derived size-frequency distribution functions of lunar impact craters are used to determine the ages of unsampled planetary surfaces across the Solar System. Radiometric dating of lunar samples provides an absolute age baseline, however , crater-chronology functions for the Moon remain poorly constrained for ages beyond 3.9 billion years. Here we present U-Pb geochronology of phosphate minerals within shocked lunar norites of a boulder from the Apollo 17 Station 8. These minerals record an older impact event around 4.2 billion years ago, and a younger disturbance at around 0.5 billion years ago. Based on nanoscale observations using atom probe tomography, lunar cratering records, and impact simulations, we ascribe the older event to the formation of the large Serenitatis Basin and the younger possibly to that of the Dawes crater. This suggests the Serenitatis Basin formed unrelated to or in the early stages of a protracted Late Heavy Bombardment.
The interaction of trace elements, fluids and crystal defects plays a vital role in a crystalline material’s response to an applied stress. Fluid inclusions are typically known to facilitate crystal-plastic deformation in minerals. Herein we discuss a model of fluid hardening whereby dislocations are pinned at fluid inclusions during crystal-plastic deformation, initiating pipe diffusion of trace elements from the fluid inclusions into crystal defects that leads to their stabilization and local hardening. We derive this hypothesis from atom probe tomography data of naturally deformed pyrite, combined with electron backscatter diffraction mapping, electron channelling contrast imaging and scanning transmission electron microscopy. The 2D and 3D micro- to nanoscale structural and chemical data reveal nanoscale fluid inclusions enriched in As, O, Na and K that are linked by As-enriched dislocations. Our efforts advance the understanding of the interplay between nanostructures and impurities during relatively low temperature deformation, which yields insight into the larger scale mass transfer processes on Earth.
Mechanical twins form by the simple shear of the crystal lattice during deformation. In order to test the potential of narrow twins in monazite to record the timing of their formation, we investigated a ca. 1700 Ma monazite grain (from the Sandmata Complex, Rajasthan, India) deformed at ca. 980 Ma, by electron backscattered diffraction (EBSD), transmission electron microscopy (TEM), and atom probe tomography (APT). APT 208Pb/232Th ages indicate that the twin was entirely reset by radiogenic Pb loss during its formation at conditions far below the monazite closure temperature. The results are consistent with a model where Pb is liberated during rupture of rare earth element–oxygen (REE-O) bonds in the large [REE]O9 polyhedra during twinning. Liberated Pb likely migrated along fast diffusion pathways such as crystal defects. The combination of a quantitative microstructural investigation and nano-geochronology provides a new approach for understanding the history of accessory phases.
The elemental and chlorine isotope compositions of calcium-phosphate minerals are key recorders of the volatile inventory of Mars, as well as the planet's endogenous magmatic and hydrothermal history. Most martian meteorites have clear evidence for exogenous impact-generated deformation and metamorphism, yet the effects of these shock metamorphic processes on chlorine isotopic records contained within calcium phosphates have not been evaluated. Here we test the effects of a single shock metamorphic cycle on chlorine isotope systematics in apatite from the highly shocked, enriched shergottite Northwest Africa (NWA) 5298. Detailed nanostructural (EBSD, Raman and TEM) data reveals a wide range of distributed shock features. These are principally the result of intensive plastic deformation, recrystallization and/or impact melting. These shock features are directly linked with chemical heterogeneities, including crosscutting microscale chlorine-enriched features that are associated with shock melt and iron-rich veins. NanoSIMS chlorine isotope measurements of NWA 5298 apatite reveal a range of d 37 Cl values (À3 to 1‰; 2r uncertainties <0.9‰) that is almost as large as all previous measurements of basaltic shergottites, and the measured d 37 Cl values can be readily linked with different nanostructural states of targeted apatite. High spatial resolution atom probe tomography (APT) data reveal that chlorine-enriched and defect-rich nanoscale boundaries have highly negative d 37 Cl values (mean of À15 ± 8‰). Our results show that shock metamorphism can have significant effects on chemical and chlorine isotopic records in calcium phosphates, principally as a result of chlorine mobilization during shock melting and recrystallization. Despite this, low-strain apatite domains have been identified by EBSD, and yield a mean d 37 Cl value of À0.3 ± 0.6‰ that is taken as the best estimate of the primary chlorine isotopic composition of NWA 5298. The combined ScienceDirect Geochimica et Cosmochimica Acta 293 (2021) 422-437 nanostructural, microscale-chemical and nanoscale APT isotopic approach gives the ability to better isolate and identify endogenous volatile-element records of magmatic and near-surface processes as well as exogenous, shock-related effects.
The sphalerite from the Burgstaetter Gangzug, a vein system of the Upper Harz Mountain nearby the town of Clausthal-Zellerfeld, exhibits a very interesting and partly complementary incorporation pattern of Cu, In and Sb, which has not yet been reported for natural sphalerite. A sphalerite specimen was characterized with electron probe micro-analysis (EPMA) and atom probe tomography (APT). Based on the EPMA results and a multilinear regression, a relation expressed as Cu = 0.98In + 1.81Sb + 0.03 can be calculated to describe the correlation between the elements. This indicates, that the incorporation mechanisms of In and Sb in the structure differ substantially. Indium is incorporated with the ratio Cu:In = 1:1 like in roquesite (CuInS2), supporting the coupled substitution mechanism 2Zn²⁺ → Cu⁺ + In³⁺. In contrast, Sb is incorporated with a ratio of Cu:Sb = 1.81:1. APT, which has a much higher spatial resolution indicates a ratio of Cu: Sb = 2.28: 1 in the entire captured volume, which is similar to the ratio calculated by EPMA, yet with inhomogeneities at the nanometer-scale. Analysis of the solute distribution shows two distinct sizes of clusters that are rich in Cu, Sb and Ag.
Many materials science phenomena require joint structural and chemical characterization at the nanometer scale to be understood. This can be achieved by correlating electron microscopy (EM) and atom probe tomography (APT) subsequently on the same specimen. For this approach, specimen yield during APT is of particular importance, as significantly more instrument time per specimen is invested as compared to conventional APT measurements. However, electron microscopy causes hydrocarbon contamination on the surface of atom probe specimens. Also, oxide layers grow during specimen transport between instruments and storage. Both effects lower the chances for long and smooth runs in the ensuing APT experiment. This represents a crucial bottleneck of the method correlative EM/APT. Here, we present a simple and reliable method based on argon ion polishing that is able to remove hydrocarbon contamination and oxide layers, thereby significantly improving APT specimen yield, particularly after EM.
Reconstructions in atom probe tomography (APT) are plagued by image distortions arising from changes in the specimen geometry throughout the experiment. The simplistic and inaccurate geometrical assumptions that underpin the conventional reconstruction approach account for much of this distortion. Here we extend our previous work of modelling APT experiments using level set methods to three dimensions (3D). This model is used to generate and subsequently reconstruct synthetic APT datasets from electron tomography (ET) of an A l - M g - S i multiphase specimen. Finally, we apply our model to the reconstruction of an experimental field-effect transistor (finFET) dataset. This model-driven reconstruction successfully reduces density distortions compared to conventional methods. By combining prior knowledge about the specimen geometry from sources such as ET, such an approach promises new distortion correcting APT reconstruction applicable to complex specimen geometries.
Zircon microcrystals are found in many planetary crustal rocks and nanoscale research on grains from well-characterized impact environments on Earth provide a baseline for reconstructing extreme shock and thermal histories elsewhere. However, using zircon to date large impact events can be challenging given that shock-related isotopic re-setting of U–Pb ratios, when measured at micrometre scale, is often incomplete and difficult to interpret as the underlying Pb migration mechanisms are unclear. To better understand shocked zircon U–Pb systematics, we performed atom probe tomography and electron microscopy with the previous SIMS analyses of two shock metamorphosed Mesoarchean zircon grains from deep (≥ 15 km) beneath the centre of the 2.020 Ga giant Vredefort impact structure. We find evidence of two types of impact-related nanoscale Pb mobility. In one grain, clustering has produced ~ 10 nm diameter bodies of radiogenic Pb, unsupported by U and co-located with Al, with an average 207Pb/206Pb ratio of ~ 0.50 (n = 4); the value extant in the grain at the time of impact. Conversely, nearby nanodomains exhibit randomly distributed radiogenic Pb, U and Al and yield 206Pb/238U dates consistent with 100% loss of pre-impact radiogenic Pb atoms during shock metamorphic processes. Notably, domains with multiple Pb clusters occur within micrometres of domains that experienced 100% Pb loss, precluding a uniform radial pattern of thermally-driven Pb diffusion at the grain scale. These cases of broadly coeval clustering and outward Pb mobility during geologically instantaneous shock metamorphism point to unusually rapid, multi-path diffusion processes within sub-micrometre volumes which, when averaged, yield normally discordant U–Pb dates. The isolation of spatially variable styles of Pb retention and loss at nanoscale amidst classical grain-scale shock microstructures shows promise for recognizing and resolving bombardment histories in planetary crusts using zircon.
Atom probe tomography is an analytical technique that provides quantitative three‐dimensional elemental and isotopic analyses at sub‐nanometre resolution across the whole periodic table. Although developed and mostly used in the materials science and semiconductor fields, recent years have seen increasing development and application in the geoscience and planetary science disciplines. Atom probe studies demonstrate compositional complexity at the nanoscale and provide fundamental new insights into the atom‐scale mechanisms taking place in minerals over geological time. Here we provide an overview of atom probe tomography, including the historical development and technical aspects of the instrumentation, and the fundamentals of data acquisition, data processing and data reconstruction. We also review previous studies and highlight the potential future applications of nanoscale geochemical studies of natural materials.
Several examples of zircon grains from high- to ultrahigh-pressure (UHP) and ultrahigh-temperature (UHT) metapelites exhibit a characteristic, yet atypical, core–rim interface domain < 5-μm wide observed in cathodoluminescence (CL) imaging. The interface domain is located immediately against the magmatic core and is comprised of an irregular, 0–2-μm wide, CL-dark domain that is rimmed by a complex, 0–5-μm wide, CL-bright domain with cuspate margins. The outer margin of the interface domain is rimmed by intermediate-CL zircon with low contrast zoning. To characterize the nature of the interface domain and to identify mechanisms of trace element mobility in metamorphosed zircon, we analyzed several specimens prepared from zircon from the Rhodope Metamorphic Complex (eastern Greece) and the Goshen Dome (western Massachusetts, USA) via atom probe tomography (APT). The data reveal three types of geochemical anomalies, each with a unique morphology. (1) Toroidal clusters with high concentrations of Pb (+ Y, Al) are found exclusively within the core of the Rhodope grain. These clusters are interpreted as decorated dislocation loops that formed during metamorphism and annealing of radiation damage to the lattice. Geochronological and geochemical data support this interpretation. (2) Complex, cross-cutting planar and linear features with anomalous concentrations of Y + P + Yb or U are spatially restricted to the core–rim interface domain; these features do not correlate with inherited geochemical variation (oscillatory zoning) or deformation-induced microstructures. Instead, the planar features likely formed in response to an interface-coupled dissolution–reprecipitation reaction that propagated into the crystal during metamorphism. The observed cross-cutting relationships are the product of either multiple events or complexity of the process that originally formed the domains. (3) Ellipsoidal features with high concentrations of Y + P + Yb (+ Al) are found exclusively within the high-Y + P + Yb planar features. These features are interpreted as the product of spinodal decomposition that occurred during exhumation as the zircon passed the solvus where local equilibria favored nm-scale exsolution to minimize the Gibbs free energy. The presence of multiple types of geochemical features in these examples indicates that trace element mobility in zircon is driven by multiple processes over the course of orogenesis. Given that these atypical domains are apparently restricted to zircon metamorphosed at UHT and (U)HP conditions, their presence may represent a marker of metamorphism at very high-grade conditions.
Carlin-type gold deposits are one of the most important gold mineralization styles in the world. Despite their economic importance and the large volume of work that has been published, there remain crucial questions regarding their metallogenesis. Much of this uncertainty is due to the cryptic nature of the gold occurrence, with gold occurring as dispersed nanoscale inclusions within host pyrite rims that formed on earlier formed barren pyrite cores. The small size of the gold inclusions has made determining their nature within the host sulfides and the mechanisms by which they precipitated from the ore fluids particularly problematic.
This study combines high-resolution electron probe microanalysis (EPMA) with atom probe tomography (APT) to constrain whether the gold occurs as nanospheres or is dispersed within the Carlin pyrites. APT offers the unique capability of obtaining major, minor, trace, and isotopic chemical information at near-atomic spatial resolution. We use this capability to investigate the atomic-scale distribution of trace elements within Carlin-type pyrite rims, as well as the relative differences of sulfur isotopes within the rim and core of gold-hosting pyrite.
We show that gold within a sample from the Turquoise Ridge deposit (Nevada) occurs within arsenian pyrite overgrowth (rims) that formed on a pyrite core. Furthermore, this As-rich rim does not contain nanonuggets of gold and instead contains dispersed lattice-bound Au within the pyrite crystal structure. The spatial correlation of gold and arsenic within our samples is consistent with increased local arsenic concentrations that enhanced the ability of arsenian pyrite to host dispersed gold (Kusebauch et al., 2019). We hypothesize that point defects in the lattice induced by the addition of arsenic to the pyrite structure facilitate the dissemination of gold. The lack of gold nanospheres in our study is consistent with previous work showing that dispersed gold in arsenian pyrite can occur in concentrations up to ~1:200 (gold/arsenic). We also report a method for determining the sulfur isotope ratios from atom probe data sets of pyrite (±As) that illustrates a relative change between the pyrite core and its Au and arsenian pyrite rim. This spatial variation confirms that the observed pyrite core-rim structure is due to two-stage growth involving a sedimentary or magmatic-hydrothermal core and hydrothermal rim, as opposed to precipitation from an evolving hydrothermal fluid.
The links between deformation-induced micro- and nanostructures and trace element mobility in sulphide minerals have recently become a popular subject of research in the Earth sciences due to its connections with metallic ore paragenesis. It has been shown that plastic deformation in pyrite creates diffusion pathways in the form of low-angle grain boundaries that act as traps for base- and precious-metals. However, the plastic behavior of pyrite and the physiochemical processes that concentrate these trace elements in deformation-induced micro- and nanostructures remain poorly understood. In this study, we develop strategies for 2D and 3D analysis of naturally deformed sulphides by combining electron backscatter diffraction, electron channeling contrast imaging and atom probe tomography on pyrite in an attempt to better understand the underlying diffusion processes that mobilize trace elements. The combined results reveal structures associated with crystal-plastic deformation in the form of dislocations, stacking faults, and low-angle grain boundaries that are decorated by As and Co. Although our data support a dislocation-impurity pair diffusion model, we have evidence that multiple diffusion mechanisms may have acted simultaneously. In this study, we applied new data processing techniques that allow for orientation measurement of nanostructural crystal defects from atom probe tomography data. Dislocations within our studied sample occur along the (110) planes suggesting glide on {110}.
The chemical widths of grain and phase boundaries in deformed wehrlite (olivine–clinopyroxene; Ol–Cpx) aggregates are characterized by atom probe tomography (APT), a (laser-assisted) field evaporation technique employing time-of-flight mass spectrometry. The wehrlite was deformed to high finite strain in diffusion creep: The effective viscosity measured for the wehrlite is approximately an order of magnitude lower than that of either end-member aggregate; further, phase ordering, in which the spatial density of Ol–Cpx phase boundaries increases with accumulated strain, characterizes the deformation (Zhao et al. in Earth Planet Sci Lett 517:83–94, 2019). The mechanical results imply that the transport properties of the phase boundaries—dictated by their structure and composition—differ from those of grain boundaries. Our APT data show that, indeed, the chemical widths of crystalline Ol–Cpx phase boundaries—3.1–6.6 nm, depending on the element used for their characterization—are up to a factor of two greater than the chemical widths of crystalline Ol–Ol and Cpx–Cpx grain boundaries. Careful statistical analyses of the APT data reveal that the near-boundary compositional profiles of the presented Ol–Cpx phase boundary are consistent with—indeed, evidence for—the rheological model in which diffusion creep is rate-limited by the (mechanism-required) interfacial reactions at the Ol–Cpx phase boundaries. Such an analysis is unavailable by current electron beam/X-ray spectrometry approaches, which have not the requisite spatial precision. APT application to nanometer-scale problems in silicate petrology is challenging, particularly because signal overlap is caused by the evaporation of polyion species. We carefully outline the procedures used to acquire and discriminate the data in order to address the challenges of signal overlap.
Granulitic lunar meteorites offer rare insights into the timing and nature of igneous, metamorphic and impact processes in the lunar crust. Accurately dating the different events recorded by these materials is very challenging, however, due to low trace element abundances (e.g. Sm, Nd, Lu, Hf), rare micrometer- scale U-Th-bearing accessory minerals, and disturbed Ar-Ar systematics following a multi-stage history of shock and thermal metamorphism. Here we report on micro-baddeleyite grains in granulitic mafic breccia NWA 3163 for the first time and show that targeted microstructural analysis (electron backscatter diffraction) and nanoscale geochronology (atom probe tomography) can overcome these barriers to lunar chronology. At winned (w90�/<401>) baddeleyite domain yields a 232Th/208Pb age of 4328 � 309 Ma, which overlaps with a robust secondary ion mass spectrometry (SIMS) 207Pb/206Pb age of 4308 � 18.6 Ma and is interpreted here as the crystallization age for the igneous protolith of NWA 3163. A second microstructural domain, < 2 mm in width, contains patchy overprinting baddeleyite and yields a Th-Pb age of 2175 � 143 Ma, interpreted as dating the last substantial impact event to affect the sample. This finding demonstrates the potential of combining microstructural characterization with nanoscale geochronology when resolving complex P-T-t histories in planetary materials, here yielding the oldest measured crystallization age for components of lunar granulite NWA 3163 and placing further con- straints on the formation and evolution of lunar crust.
Understanding the mechanisms of parent-daughter isotopic mobility at the nanoscale is key to rigorous interpretation of U–Th–Pb data and associated dating. Until now, all nanoscale geochronological studies on geological samples have relied on either Transmission Electron Microscope (TEM) or Atom Probe Microscopy (APM) characterizations alone, thus suffering from the respective weaknesses of each technique. Here we focus on monazite crystals from a ∼1 Ga, ultrahigh temperature granulite from Rogaland (Norway). This sample has recorded concordant U–Pb dates (measured by LA-ICP-MS) that range over 100 My, with the three domains yielding distinct isotopic U–Pb ages of 1034 ± 6 Ma (D1; S-rich core), 1005 ± 7 Ma (D2), and 935 ± 7 Ma (D3), respectively. Combined APM and TEM characterization of these monazite crystals reveal phase separation that led to the isolation of two different radiogenic Pb (Pb*) reservoirs at the nanoscale. The S-rich core of these monazite crystals contains Ca–S-rich clusters, 5–10 nm in size, homogenously distributed within the monazite matrix with a mean inter-particle distance of 40–60 nm. The clusters acted as a sink for radiogenic Pb (Pb*) produced in the monazite matrix, which was reset at the nanoscale via Pb diffusion while the grain remained closed at the micro-scale. Compared to the concordant ages given by conventional micro-scale dating of the grain, the apparent nano-scale age of the monazite matrix in between clusters is about 100 Myr younger, which compares remarkably well to the duration of the metamorphic event. This study highlights the capabilities of combined APM-TEM nano-structural and nano-isotopic characterizations in dating and timing of geological events, allowing the detection of processes untraceable with conventional dating methods. © 2018 China University of Geosciences (Beijing) and Peking University
The feldspar minerals occur in a wide variety of lithologies throughout the Solar System, often containing a variety of chemical and structural features indicative of the crystallization conditions, cooling history, and deformational state of the crystal. Such phenomena are often poorly resolved in micrometre-scale analyses. Here, atom probe tomography (APT) is conducted on Ca-rich (bytownite) and Na-rich (albite) plagioclase reference materials, experimentally exsolved K-feldspar (sanidine), shock-induced maskelynite glass (labradorite-composition), and shocked and recrystallized plagioclase to directly test the application of APT to feldspar and yield new insights into crystallographic features such as amorphisation and exsolution. Undeformed plagioclase reference materials (Amelia albite and Stillwater bytownite) appear chemically homogenous, and yield compositions largely within uncertainty of published data. Within microstructurally complex materials, APT can resolve chemical variations across a ~20nm wide exsolution lamella and define major element (Na, K) diffusion profiles across the lamella boundaries, which appear gradational over a ~10nm length scale in experimentally exsolved K-feldspar NNPP-04b. Maskelynite within the Zagami shergottite shows no heterogeneity in the distribution of major elements, while recrystallization of feldspar during post-shock annealing, such as in poikilitic shergottite NWA 6342, appears to induce a range of chemical nanostructures that locally effect the composition of the material. These findings demonstrate the ability of APT to yield new insights into nanoscale composition and chemical structures of alumniosilicate phases, highlighting an exciting new avenue with which to analyse these key rock forming minerals.
The Pb isotopic composition of rocks is widely used to constrain the sources and mobility of melts and hydrothermal fluids in the Earth's crust. In many cases, the Pb isotopic composition appears to represent mixing of multiple Pb reservoirs. However, the nature, scale and mechanisms responsible for isotopic mixing are not well known. Additionally, the trace element composition of sulphide minerals are routinely used in ore deposit research, mineral exploration and environmental studies, though little is known about element mobility in sulphides during metamorphism and deformation. To investigate the mechanisms of trace element mobility in a deformed Witwatersrand pyrite (FeS2), we have combined electron backscatter diffraction (EBSD) and atom probe microscopy (APM). The results indicate that the pyrite microstructural features record widely different Pb isotopic compositions, covering the entire range of previously published sulphide Pb compositions from the Witwatersrand basin. We show that entangled dislocations record enhanced Pb, Sb, Ni, Tl and Cu composition likely due to entrapment and short-circuit diffusion in dislocation cores. These dislocations preserve the Pb isotopic composition of the pyrite at the time of growth (∼3 Ga) and show that dislocation intersections, likely to be common in deforming minerals, limit trace element mobility. In contrast, Pb, As, Ni, Co, Sb and Bi decorate a high-angle grain boundary which formed soon after crystallisation by sub-grain rotation recrystallization. Pb isotopic composition within this boundary indicates the addition of externally-derived Pb and trace elements during greenschist metamorphism at ∼2 Ga. Our results show that discrete Pb reservoirs are nanometric in scale, and illustrate that grain boundaries may remain open systems for trace element mobility over 1 billion years after their formation. © 2018 China University of Geosciences (Beijing) and Peking University
Common Pb, the portion of non-radiogenic Pb within a U bearing mineral, needs to be accurately accounted for in order to subtract its effect on U-Pb isotopic ratios so that meaningful ages can be calculated. The propensity to accommodate common Pb during crystallization, or later, is different across the range of U bearing minerals used for geochronology. Titanite frequently accommodates significant amounts of common Pb. However, the most appropriate method to correct for this requires knowledge on the mechanism and timing of common Pb incorporation; information that is commonly difficult to extract. In this study, the spatial and compositional distribution of trace elements (including Pb) in metamorphic titanites from a Greenland amphibolite is investigated on the grain- to nano-scale. Titanites have an isotopically similar signature for both common and radiogenic-Pb in all grains but significantly different quantities of the non-radiogenic component. Microstructural and compositional examination of these grains reveals undeformed, but high common Pb (F207%) titanites have homogeneous element distributions on the atomic scale suggesting common Pb is incorporated into titanite during its growth and not during later processes. In contrast, deformed titanite comprising low-angle boundaries, formed by subgrain rotation recrystallization, comprise networks of dislocations that are enriched in Mg, Al, K and Fe. Smaller cations may migrate due to elastic strain in the vicinity of the dislocation network, yet the larger K cations more likely reflect the mobility of externally-derived K along the orientation interface. The absence of Pb enrichment along the boundary indicates that either Pb was too large to fit into migrating lattice dislocations or static low-angle boundaries and/or that there was no external Pb available to diffuse along the grain boundary. As the common Pb composition is distinctly different to regional Pb models, the metamorphic titanite grew in a homogeneous Pb reservoir dominated by the break-down of precursor U-bearing phases. The different quantity of common Pb in the titanite grains indicates a mineral-driven element partitioning in an isotopically homogeneous metamorphic reservoir, consistent with low U, low total REE and flat LREE signatures in high F207% analyses. These results have implications for the selection of appropriate common Pb corrections in titanite and other accessory phases.
Resolving the timing of crustal processes and meteorite impact events is central to understanding the formation, evolution and habitability of planetary bodies. However, identifying multi-stage events from complex planetary materials is highly challenging at the length scales of current isotopic techniques. Here we show that accurate U-Pb isotopic analysis of nanoscale domains of baddeleyite can be achieved by atom probe tomography. Within individual crystals of highly shocked baddeleyite from the Sudbury impact structure, three discrete nanostructural domains have been isolated yielding average 206Pb/238U ages of 2,436±94Ma (protolith crystallization) from homogenous-Fe domains, 1,852±45Ma (impact) from clustered-Fe domains and 1,412±56Ma (tectonic metamorphism) from planar and subgrain boundary structures. Baddeleyite is a common phase in terrestrial, Martian, Lunar and asteroidal materials, meaning this atomic-scale approach holds great potential in establishing a more accurate chronology of the formation and evolution of planetary crusts.
The application of atom probe tomography to the study of minerals is a rapidly growing area. Picosecond-pulsed, ultraviolet laser (UV-355 nm) assisted atom probe tomography has been used to analyze trace element mobility within dislocations and low-angle boundaries in plastically deformed specimens of the nonconductive mineral zircon (ZrSiO4), a key material to date the earth's geological events. Here we discuss important experimental aspects inherent in the atom probe tomography investigation of this important mineral, providing insights into the challenges in atom probe tomography characterization of minerals as a whole. We studied the influence of atom probe tomography analysis parameters on features of the mass spectra, such as the thermal tail, as well as the overall data quality. Three zircon samples with different uranium and lead content were analyzed, and particular attention was paid to ion identification in the mass spectra and detection limits of the key trace elements, lead and uranium. We also discuss the correlative use of electron backscattered diffraction in a scanning electron microscope to map the deformation in the zircon grains, and the combined use of transmission Kikuchi diffraction and focused ion beam sample preparation to assist preparation of the final atom probe tip.
To test the potential of deformation twins to record the age of impact events, micrometre-scale size mechanical twins in shocked monazite grains from three impact structures were analyzed by atom probe tomography (APT). Shocked monazite from Vredefort (South Africa; ∼300 km crater diameter), Araguainha (Brazil; ∼40 km diameter), and Woodleigh (Australia; 60 to 120 km diameter) were studied, all from rocks which experienced pressures of ∼30 GPa or higher, but each with a different post-impact thermal history. The Vredefort sample is a thermally recrystallised foliated felsic gneiss and the Araguainha sample is an impact melt-bearing bedrock. Both Vredefort and Araguainha samples record temperatures > 900 °C, whereas the Woodleigh sample is a paragneiss that experienced lower temperature conditions (350 - 500 °C). A combined ²⁰⁸Pb/²³²Th age for common {12¯2¯} twins and shock-specific (1¯01) twins in Vredefort monazite was defined at 1979 ± 150 Ma, consistent with the accepted impact age of ∼2020 Ma. Irrational η1 [1¯1¯0] shock-specific twins in Araguainha monazite yielded a 260 ± 48 Ma age, also consistent with the accepted 250-260 Ma impact age. However, the age of a common (001) twin in Araguainha monazite is 510 ± 87 Ma, the pre-impact age of igneous crystallisation. These results are explained by the occurrence of common (001) twins in tectonic deformation settings, in contrast to the (1¯01) and irrational η1 [1¯1¯0] twins, which have only been documented in shock-deformed rocks. In Woodleigh monazite, APT age data for all monazite twins [(001), (1¯01), newly identified 102°/<4¯23> twin], and host monazite are within uncertainty at 1048 ± 91 Ma, which is interpreted as a pre-impact age of regional metamorphism. We therefore are able to further constrain the poorly known age of the Woodleigh impact to < 1048 ± 91 Ma. These results provide evidence that Pb is expelled from monazite during shock twin formation at high temperature (Vredefort and Araguainha), and also that Pb is not mobilised during twinning at lower temperature (Woodleigh). Our study suggests that twins formed during shock metamorphism have the potential to record the age of the impact event in target rocks that are sufficiently heated during the cratering process.
Xenotime (YPO4) is an accessory phase common in low to high‐temperature geological environments. Xenotime is an established geochronometer, though its small size, low modal abundance, and textural complexity make it more difficult to analyse with traditional techniques but makes a prime candidate for nano‐scale analysis. In this study, we develop an atom probe tomography (APT) protocol to determine the 206Pb/238U and 207Pb/206Pb ages of micro‐scale xenotime crystals with analytical volumes four to six orders of magnitude smaller than typical geochronology techniques. A linear correlation between the 206Pb/238U fractionation and 238UO22+/238UO2+ was used to correct for the atom probe instrument parameters variability between specimens. For 207Pb/206Pb ages we employed two methods of background correction owing to the 206Pb2+ thermal tail contribution to the 207Pb2+ counts: A constant background correction for the younger (~ 1000 Ma) Y1 reference material and a variable correction of background for Archaean age reference material xtc to correct for the thermal tail influence. This contribution also proposes strategies for optimization of xenotime analysis using atom probe tomography and permits us to explore the various geological problems in the nano‐scale realm. This methodology potentially allows determining the age of small xenotime crystals in sedimentary rocks, low metamorphic grade settings, and deformation‐microstructures.
Atom probe tomography (APT) is emerging as a key nanogeochemistry technique for diverse geoscience applications. Estimating stoichiometric mineral compositions is particularly challenging for features at nanoscale. APT provides reliable measurements for metallic systems but the reliability for oxides is problematic, notably due to oxygen deficit. Here, we use laser‐assisted APT to compare results for spinel and garnet crystals. APT compositional results were compared with those by electron microprobe to determine the possible APT analytical inaccuracy. Extensive data processing was accomplished, including correlation histograms, 2D ion distribution maps and 1D elements concentration profiles, to disclose the possible mechanisms leading to mineral stoichiometry biases. Multiple events and neutral molecules formation are probable the main processes responsible for atom deficit. In particular, the amount of the same isotope‐same charge state ion pairs correlated with aluminium and oxygen deficits suggests that the co‐evaporation in a dead space‐dead time window could lead to a significant decrease of detected ions. Also, molecular species dissociation and direct current evaporation could partially account for further atom loss. Overall, better APT compositional estimation was obtained for spinel, which has lesser variation in lattice sites and greater overall lattice symmetry, higher thermal conductivity, and lower band gap compared with garnet.
Olivine is one of the most important minerals used to reconstruct magmatic processes, yet the rare earth element (REE) systematics of Fe-rich olivine in igneous rocks and ore deposits is poorly understood. As detected by in situ laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) analysis, cumulate fayalite (Fe2SiO4) in the Paleoproterozoic Vergenoeg F-Fe-REE deposit of the Bushveld large igneous province (LIP) in South Africa contains the highest heavy REE (HREE) contents ever recorded for olivine, with HREE enrichment of as much as 6000× chondritic values. Atom probe tomography maps confirm the incorporation of the HREEs into the fayalite crystal lattice, facilitated by lithium acting as a main charge balancer and by high REE contents in the highly fractionated felsic parental melt that is related to the Bushveld LIP. The high HREE concentrations of fayalite in concert with its high modal abundance (>95 vol%) indicate that the fayalite cumulates are the main host for the HREE mineralization of the Vergenoeg deposit. Fayalites of Vergenoeg demonstrate that Fe-rich olivine may fractionate large amounts of HREEs, and we propose fayalite cumulates as potential future targets for HREE exploration.
The geochemical analysis of trace elements in rutile (e.g., Pb, U, and Zr) is routinely used to extract information on the nature and timing of geological events. However, the mobility of trace elements can affect age and temperature determinations, with the controlling mechanisms for mobility still debated. To further this debate, we use laser-ablation–inductively coupled plasma–mass spectrometry and atom probe tomography to characterize the micro- to nanoscale distribution of trace elements in rutile sourced from the Capricorn orogen, Western Australia. At the >20 μm scale, there is no significant trace-element variation in single grains, and a concordant U-Pb crystallization age of 1872 ± 6 Ma (2σ) shows no evidence of isotopic disturbance. At the nanoscale, clusters as much as 20 nm in size and enriched in trace elements (Al, Cr, Pb, and V) are observed. The 207Pb/206Pb ratio of 0.176 ± 0.040 (2σ) obtained from clusters indicates that they formed after crystallization, potentially during regional metamorphism. We interpret the clusters to have formed by the entrapment of mobile trace elements in transient sites of radiation damage during upper amphibolite facies metamorphism. The entrapment would affect the activation energy for volume diffusion of elements present in the cluster. The low number and density of clusters provides constraints on the time over which clusters formed, indicating that peak metamorphic temperatures are short-lived, <10 m.y. events. Our results indicate that the use of trace elements to estimate volume diffusion in rutile is more complex than assuming a homogeneous medium.
The recent application of atom probe tomography (APT) to minerals is becoming a powerful tool to unravel geological information from the nanoscale perspective. Yet, there are still unknown fundamental aspects of this microscopy technique for geological applications and the potential crystallographic orientation effect is a significant one. Here, the influence of the crystallographic orientation on the quality of atom probe tomography geochemical data is investigated for two minerals with the same crystal system and different morphology: spinel (isometric, hexoctahedral, octahedron morphology) and galena (isometric, hexoctahedral, cube morphology). Two separate crystals of barite (orthorhombic, dipyramidal, prism morphology) were also analyzed to test the reproducibility of APT data. Despite the general absence of expected stoichiometry, overall bulk and isotopic chemical composition are not affected by crystallographic orientation. 3D data reconstructions of the specimens showed similar spatial distribution of the ion species for each mineral and 2D density maps showed identical (barite, galena) or specular (spinel) patterns for each pair of planes analyzed. Our findings indicate a negligible effect of the crystallographic orientation in APT geochemical data for standard highly symmetric minerals but also suggest the possible influence of the crystallographic structure and composition on the mineral stoichiometry and elements spatial distribution density.
Monazite U‐Th‐Pb geochronology is widely used for dating geological processes, but current analytical techniques are limited to grains greater than 5 μm in diameter. This limitation precludes the analysis of both micrometre‐scale discrete monazite grains and fine textures within monazite crystals that are commonly found. Here, we analyse reference materials by atom probe tomography and develop a protocol for ²⁰⁸Pb/²³²Th dating of nanoscale domains of monazite (0.0007 μm³ analytical volume). The results indicate that the ²⁰⁸Pb²⁺/²³²ThO²⁺ are higher than the true values. Such fractionation can be corrected using a linear regression between ²⁰⁸Pb²⁺/²³²ThO²⁺ and the M/ΔM10 peak shape parameter, where M is the position of the O2⁺ peak and ΔM10 the full‐width‐tenth‐maximum for the same peak. This correction results in 15 to 20% analytical uncertainty on the corrected ²⁰⁸Pb/²³²Th age. Nonetheless, this approach opens the possibility of obtaining ²⁰⁸Pb/²³²Th ages with sufficient precision to address geological questions on an unprecedented small scale. To illustrate the approach, atom probe geochronology of a small monazite grain from the contact aureole of the Fanad pluton (Ireland) yielded a ²⁰⁸Pb/²³²Th atom probe age of 420 ± 60 Ma (2s ) and is consistent with the known metamorphism in the region.
Atom probe tomography (APT) is used to quantify atomic-scale elemental and isotopic compositional variations within a very small volume of material (typically <0.01 µ m ³ ). The small analytical volume ideally contains specific compositional or microstructural targets that can be placed within the context of the previously characterized surface in order to facilitate a correct interpretation of APT data. In this regard, careful targeting and preparation are paramount to ensure that the desired target, which is often smaller than 100 nm, is optimally located within the APT specimen. Needle-shaped specimens required for atom probe analysis are commonly prepared using a focused ion beam scanning electron microscope (FIB-SEM). Here, we utilize FIB-SEM-based time-of-flight secondary ion mass spectrometry (ToF-SIMS) to illustrate a novel approach to targeting <100 nm compositional and isotopic variations that can be used for targeting regions of interest for subsequent lift-out and APT analysis. We present a new method for high-spatial resolution targeting of small features that involves using FIB-SEM-based electron deposition of platinum “buttons” prior to standard lift-out and sharpening procedures for atom probe specimen manufacture. In combination, FIB-ToF-SIMS analysis and application of the “button” method ensure that even the smallest APT targets can be successfully captured in extracted needles.
Goethite (α-FeOOH) is dispersed throughout Earth’s surface and its propensity to recrystallize in aqueous solutions determines whether this mineral is a source or sink for critical trace elements in the environment. Under reducing conditions, goethite commonly co-exists with aqueous Fe(II) (Fe(II)aq), which accelerates recrystallization by coupled electron transfer and atom exchange. Quantifying the amount of the mineral phase that exchanges its structural Fe(III) atoms with Fe(II)aq is complicated by recrystallization models with untested assumptions of whether, and to what extent, the recrystallized portion of the mineral continues to interact with the solution. Here, we reacted nanoparticulate goethite with 57Fe-enriched Fe(II)aq and used atom probe tomography (APT) to resolve the 3-dimensional distribution of Fe isotopes in goethite at the sub nm scale. We found that the 57Fe tracer isotope is enriched in the bulk structure (tens of nanometers deep), with some samples having 57Fe penetration throughout at a level that is similar to the isotopic composition of Fe(II)aq. This suggests that some particles undergo near-complete recrystallization. In other cases, however, the distribution of 57Fe is more heterogeneous and generally concentrates near the particle periphery. Nanoparticle encapsulation and subsequent APT can hence capture hidden recrystallization mechanisms which are critical to predicting reactivity in aqueous solutions.
We report measurements of experimentally induced diffusion profiles in minerals using local electrode atom probe (LEAP) tomography, and demonstrate that this is a viable technique for use in experimental diffusion studies, with distinct advantages over current state-of-the-art techniques such as secondary ion mass spectrometry and Rutherford backscattering spectroscopy. For this purpose, we have investigated Ca diffusivity in synthetic forsterite, which we measured parallel to [1 0 0] from 750 to 1300 °C, at 1 atm. pressure in air, with silica activity buffered by forsterite-enstatite or forsterite-periclase assemblages. We observe no dependence of Ca diffusion on silica activity, and argue that this may be due the strong preference of Ca for the M2 site, while M-site vacancies in olivine are more favorably located in M1 sites. Taken together, our data yield the following Arrhenius relation for Ca diffusion parallel to [1 0 0] in synthetic forsterite at 1 atm. pressure: logDCa_[100]m2s-1=-8.42±0.36-250±9kJmol-12.303RT. Time series experiments at 750 °C reveal that the shortest profiles measured by secondary ion mass spectrometry depth profiling (using an oxygen primary ion beam) are compromised by ion-beam mixing, while LEAP tomography yields accurate diffusion coefficients from the same samples. Because LEAP tomography does not suffer from the type of analytical artefacts that complicate the measurement of extremely short profiles via secondary ion mass spectrometry depth profiling, and also typically produces an order of magnitude better sensitivity than Rutherford backscattering spectroscopy, this technique offers a valuable new tool for quantifying the diffusivities of slow-diffusing trace elements in crystalline materials.
Spatial Reconstruction of Atom Probe Data from Zircon - Volume 25 Supplement - D.W. Saxey, D. Fougerouse, W.D.A. Rickard, S.M. Reddy
Chemical zoning in minerals records fluid-rock interaction and crystal growth kinetics via texturally complex features, the genesis of which remains a subject of debate. Here, we combined nanoscale secondary ion mass spectrometry (NanoSIMS) and atom probe tomography to better characterize trace-element zoning in a gold (Au)–rich pyrite crystal from the Daqiao epizonal orogenic Au deposit, China. Observations on the micron to atomic scale were used to recognize the multiple processes and mechanisms that created the zoning. Chemically distinct, micron-scale concentric zones of pyrite formed in response to changing fluid composition in a dynamic hydraulic fracturing environment. At a smaller scale, within an Au-rich zone, sector zones of Au, As, and Cu at the micron to sub-micron scale were controlled by the structure of the crystal surface. Micron-scale patchy distribution of Au, As, and Cu and atomic-scale transitions from homogeneous to heterogeneous “island” arsenian pyrite formed as a consequence of heteroepitaxial Stranski-Krastanov growth. Nanoscale Au oscillatory zoning is interpreted as a consequence of diffusion-limited self-organization processes at the crystal-fluid interface. The multiple scales of observation enabled us to see how kinetically driven intrinsic processes interacted with extrinsic factors (e.g., pressure decreases) to produce the complexity in mineral zoning. Nanoscale heterogeneities in Au, As, and Cu present as solid solution in pyrite suggest that interpretation of spikes on microbeam-derived depth-concentration profiles as metallic particles should be treated with caution.
We define a measure for the accuracy of tomographic reconstruction in atom probe tomography, named here the spatial error index. We demonstrate that this index can be used to compare rigorously the spatial accuracy of various different approaches to the calculation of tomographic reconstruction. This is useful, for example, to evaluate the performance of alternate tomographic reconstruction approaches, and ensures that the comparisons are independent of individual data quality or other instrumental parameters. We then introduce a new “adaptive reconstruction” formalism that uses a progression of reconstruction parameters based on a per-atom correction from the cube root of the inverse of the voltage, along with linear correction factors linked to the evaporation sequence. We apply the measure for spatial accuracy to this new reconstruction protocol.
Textural and compositional microscale (10-100 μm) and nanoscale (10-100 nm) zoning in a plagioclase phenocryst from a fresh, syn-mineralization diorite porphyry (Black Mountain porphyry Cu-Au deposit, Philippines) was characterized for major and trace elements using electron microprobe, laser ablation-inductively coupled plasma-mass spectrometry, and atom probe tomography. The complex plagioclase crystal (3.0 × 5.4 mm) has a patchy andesine core (An 41-48 mol%), eroded bytownite mantle (An 71-80 mol%), and oscillatory andesine rim (An 39-51 mol%). Microscale variations with a periodic width of 50 to 200 μm were noted for most major and trace elements (Si, Ca, Al, Na, K, Fe, Mg, Ti, Sr, Ba, Pb, La, Ce, and Pr) with a ΔAn amplitude of 4-12 mol% in both the core and rim. The mantle has a distinct elemental composition, indicating the addition of hotter mafic magma to the andesitic magma. Atom probe tomography shows an absence of nanoscale variations in the andesine rim but alternating nanoscale (25-30 nm) Al-rich, Ca-rich, and Si-rich, Na-rich zones with a Ca/(Ca+Na) at% amplitude of ~10 in the bytownite mantle. The restricted variations in physiochemical parameters (H 2 O-rich, T = 865 to 895 °C, P = 5.3 to 6.2 kbar; f O 2 = NNO+0.6 to NNO+1.1 recorded by co-precipitated amphibole) suggest microscale oscillatory zoning was likely controlled by internal crystal growth mechanisms, and not by periodic variations in physiochemical conditions. However, the uniform diffusion timescale for CaAl-NaSi interdiffusion in the mantle is far shorter than the crystallization timescale of the grain from mantle to rim, suggesting nanoscale zonation in the bytownite mantle formed by exsolution after crystallization. The occurrence of micro-scale zoning in plagioclase indicates a minimum cooling rate of 0.0005 °C/yr during crystallization, assuming an initial temperature of 880 °C, the width of 50 μm, and NaSi-CaAl interdiffusion under hydrous conditions. Assuming a formation temperature of ~675 °C for the nanoscale exsolution texture as constrained by zircon crystallization temperatures, the retention of nanoscale zoning (~28 nm) requires a minimum cooling rate of 0.26 °C/yr. Given that this is significantly faster cooling than would occur in a magma chamber, this texture likely records the post-crystallization emplacement history.
Significance
The bioavailability of iron in the environment, and coupled metals, nutrients, and contaminants, depends on the stability of common Fe(III) minerals such as goethite (FeOOH) and hematite (Fe 2 O 3 ). At redox boundaries, iron isotopic tracer studies suggest that interaction with aqueous Fe(II) creates dynamic conditions of atom exchange (AE). However, mechanistic models have not advanced beyond speculation, because of the challenges of mapping AE fronts recorded in isotopic distributions in individual nanoscale crystallites. Here we demonstrate successful use of 3D atom probe tomography for this purpose. The penetration depth, spatial heterogeneity, and ties to mineral defects are visualized, helping constrain mechanistic models and setting a precedent for detailed interrogation of iron redox cycling in the environment.
Since the introduction of laser-assisted atom probe, analysis of nonconductive materials by atom probe tomography (APT) has become more routine. To obtain high-quality data, a number of acquisition variables needs to be optimized for the material of interest, and for the specific question being addressed. Here, the rutile (TiO 2 ) reference material ‘Windmill Hill Quartzite,’ used for secondary ion mass spectrometry U–Pb dating and laser-ablation inductively coupled plasma mass spectrometry, was analyzed by laser-assisted APT to constrain optimal running conditions. Changes in acquisition parameters such as laser energy and detection rate are evaluated in terms of their effect on background noise, ionization state, hit-multiplicity, and thermal tails. Higher laser energy results in the formation of more complex molecular ions and affects the ionization charge state. At lower energies, background noise and hit-multiplicity increase, but thermal tails shorten. There are also correlations between the acquisition voltage and several of these metrics, which remain to be fully understood. The results observed when varying the acquisition parameters will be discussed in detail in the context of utilizing APT analysis of rutile within geology.
In our study we explore the applicability of rutile as a pathfinder for orogenic gold deposits, which are an important source of this metal worldwide. We analysed rutile associated with orogenic Au deposits from three different Precambrian terranes, the Capricorn Orogen, the Barberton Greenstone Belt and the Ashanti Belt, all of which formed under greenschist conditions and share similarities in the style of mineralisation. Microtextural evidence from scanning electron microscopy and electron back-scatter diffraction indicates that rutile formed during the main deformation and alteration stage in these rocks, and is therefore related to mineralisation. We used electron microprobe and laser ablation ICP-MS to investigate the trace element compositions of rutile and we compared our results to other gold deposits. We find that hydrothermal rutile from gold deposits contains certain trace element characteristics, in particular high Sb concentrations (up to ∼1500 ppm in Au deposits of the Capricorn Orogen), that are distinct from rutile from non-mineralised rocks of various petrogenetic origin. Other elements, such as W and Sn, are found to be more enriched in rutile from other rock types, namely felsic magmatic rocks and hydrothermal veins, and are therefore not diagnostic of Au mineralisation in this type of deposits. We also find that the presence of sub-µm-scale inclusions – in particular Zr-(Si, Th)-bearing phases, sulfide minerals and native Au – can severely affect analyses of this type of rutile and compromise the applicability of Zr-in-rutile geothermometry.
In a hypervelocity impact event, the temperatures and pressures generated by the shock waves far exceed the values produced by endogenic processes. The shock-induced processes can modify the distribution of trace elements in zircon grains located in target rocks, potentially affecting the geochemical reliability of zircon, but also providing an opportunity to better understand the mechanisms of shock deformation. The formation of reidite lamellae by the shock-induced phase change of zircon has previously been proposed to be a diffusionless, martensitic transformation, with no associated atomic mobility over nanometre length scales. However, nanoscale characterization of the zircon–reidite interface and a low-angle boundary within the reidite by atom probe tomography, transmission electron microscopy and correlative analytical techniques, shows localised enrichment of particular trace elements (Y, Al, Ca, Be, Mg, Mn, and Ti). These observations indicate the presence of additional short-range diffusional components to explain the local compositional variations observed at the nanoscale for the high-pressure transformation of zircon to reidite lamellae. A new model for this transformation is proposed that consists of two stages: 1) the early stage of the impact event where the shock waves induce defects in the zircon grain and trigger a phase transformation, resulting in trace element segregation by interface migration; and 2) the recovery stage where the trace elements and shock induced defects migrate to areas of lower energy.
Many studies have focussed on the optimization of atom probe tomography data reconstruction. In this article, an enhanced dynamic reconstruction algorithm based on the approach initially developed by Gault et al. in 2011 is proposed. The dynamic reconstruction takes into account the evolution of the reconstruction parameters during the field evaporation of the sample, contrary to the standard reconstruction protocol, for example proposed by Bas et al. in 1995. Here, the evolution of reconstruction parameters is retrieved by field evaporation simulation. This approach allows a complete tabulation of the parameters evolution as a function of the analyzed sample morphology and its initial microstructure. The only inputs for experimental data are the voltage curve and some simple morphological parameters of the sample. This will be demonstrated by applying this algorithm to some experimental cases. Drastic optimization of the spatial accuracy in reconstructed datasets is experimentally and theoretically demonstrated.
Atom probe tomography reconstructions provide valuable information on nanometer-scale compositional variations within materials. As such, the spatial accuracy of the reconstructions is of primary importance for the resulting conclusions to be valid. Here, the use of transmission electron microscopy images before and after atom probe analysis to provide additional information and constraints is examined for a number of different materials. In particular, the consistency between the input reconstruction parameters and the output reconstruction is explored. It is demonstrated that it is possible to generate reconstructions in which the input and known values are completely consistent with the output reconstructions. Yet, it is also found that for all of the datasets examined, a particular power law relationship exists such that, if the image compression factor or detection efficiency is not constrained, a series of similarly spatially accurate reconstructions results. However, if one of these values can be independently assessed, then the other is known as well. Means of incorporating these findings and this general methodology into reconstruction protocols are also discussed.
In recent years, atom probe tomography (APT) has been increasingly used to study minerals, and in particular the mineral zircon. Zircon (ZrSiO4) is ideally suited for geochronology by utilising the U-Th-Pb isotope systems, and trace element compositions are also widely used to constrain petrogenetic processes. However, while standard geoanalytical techniques provide information at micrometer scale lengths, the unique combination of chemical/isotopic sensitivity and spatial resolution of APT allows compositional and textural measurements at the nanoscale. This interlaboratory study aims to define the reproducibility of APT data across research facilities and assess the role of different aspects of the atom probe workflow on reproducibility. This is essential to allow correct evaluation of APT results and full utilization of this emerging technique within the geoscience community. In this study, nine samples from the same homogeneous, GJ-1/87 zircon reference grain were sent to nine APT institutes in Germany, the UK, USA, Canada and Australia. After preparing the sample out of a selectioned slab, each institute conducted three different rounds of APT analyses: using (i) unconstrained analysis parameters, (ii) pre-defined analysis parameters, and (iii) interpreting and quantifying a provided dataset. Data such as the measured elemental composition, acquisition parameters, or mass spectrum peak identifications, were recorded and analyzed. We observe a significant variation in the measured composition across this interlaboratory study as well as the number of trace elements identified. These differences are thought to directly result from the user's choice of atom probe data analysis parameters. The type of instrument does not seem to be a critical factor. Consequently, comparison of absolute trace element concentrations on zircon using APT between laboratories is only valid if the same workflow has been ensured.
The widespread use of monazite (LREEPO4) in U-Pb geochronology is underpinned by the assumption that it incorporates negligible amounts of Pb during initial growth, and that radiogenic Pb remains immobile after formation. We have investigated the nanoscale distribution of Pb in monazite from granulite facies rocks of the Sandmata Metamorphic Complex (Rajasthan, India) by atom probe microscopy to further understand the utility of monazite as a geochronometer. The studied monazite contains distinct 10 nm clusters, enriched in Ca and with a bulk composition consistent with them being apatite (Ca5(PO4)3(OH)), that are also enriched in Si and Pb relative to the monazite host. The ²⁰⁸Pb/²³²Th ratios of the clusters ranged from 1.1 ± 0.1 to 1.4 ± 0.2 (2σ), indicating that the clusters hold unsupported Pb. The ²⁰⁸Pb/²³²Th ratios of the whole specimen (including clusters) and the matrix alone are similar (<6% difference), indicating that the clusters formed shortly after monazite crystallisation by a phase exsolution mechanism that partitioned the initial common Pb and the minor radiogenic Pb into apatite. A volume-dependent analysis of the bulk monazite composition shows that a large variability in the Ca and, by proxy, Pb composition at small volumes (125 to 10,000 nm³) due to its heterogeneous distribution in the clusters, may have detrimental effects on radiometric dating with small analytical volumes. At larger volumes, including those used in EPMA and traditional isotopic dating methods (LA-ICPMS, SIMS), the variability of Pb content is negligible. However, the measured composition may result from the mixing of multiple reservoirs.
As a quantitative nanoscale chemical analysis technique, atom probe microscopy (APM) typically requires careful tuning and optimization of data acquisition parameters in order to obtain the highest quality results. While there is growing interest in the analysis of geological materials, including zircon, by APM, a full understanding of the controls on data quality will take time to develop. Optimization depends not only on the material to be analyzed but also on the particular specimen form, and on the information that is sought from the data. The zircon 91500 reference material has been analyzed by APM in order to explore the dependence of key metrics, such as background noise, composition, hit-multiplicity, and complex ion formation, on a number of acquisition conditions. In general, the best results are obtained under “cold” conditions, corresponding to low laser pulse energies. However, the concentration of the major element species tends to improve at higher energies, and there is also an apparent correlation between several metrics and the acquisition voltage, which remains to be fully understood. Several issues that can arise in the analysis of zircon via APM will be raised and discussed in some detail.
Measuring 207Pb/206Pb ratios by atom probe tomography (APT) has provided new insight into the nanoscale behavior of trace components in zircon, and their relationship to time, temperature, and structure. Here we analyze three APT data sets for a 3.77 Ga zircon from the Beartooth Mountains, USA, and apply systematic ranging approaches to understand the spatial and spectral uncertainties inherent in 207Pb/206Pb analysis by APT. This zircon possesses two, 100% concordant U-Pb analyses by secondary ion mass spectrometry (SIMS), indicative of closed U-Pb systematics on the micron scale since crystallization. APT data sets contain sub-spherical Pb-rich (>0.25% atomic) domains with diameter <15 nm. Broadly consistent Pb-rich regions are defined in applying six different permutations of the two most common cluster identification algorithms. Measured 207Pb/206Pb ratios within Pb-rich domains vary between 0.794 ± 0.15 (±2σ) and 0.715 ± 0.052 depending on the ranging approach, cluster definition protocol, and number of clusters interrogated. For the bulk APT data sets, 207Pb/206Pb = 0.353 ± 0.18; this is indistinguishable from the bulk 207Pb/206Pb ratio by SIMS (0.367 ± 0.0037), and statistically distinct from the 207Pb/206Pb ratio within clusters. Bulk and clustered 207Pb/206Pb ratios are consistent with Pb clustering at ~2.8 Ga, during protracted metamorphism and magmatism in the Beartooth Mountains.
The application of atom probe tomography (APT) within the Earth and planetary sciences has produced novel data sets that answer fundamental questions about the near-atomic scale distribution of elements and isotopes within minerals. It involves the incremental evaporation, detection, and subsequent computer reconstruction of charged particles from a needle-shaped specimen. The range of applications is growing such that protocols for reporting are needed for APT data comparison and quality assessment among natural materials. A particular challenge of APT science relates to documenting the instrumental and analyst-dependent conditions that affect the mass spectral and spatial qualities of the data and their interpretation. This contribution outlines recommended data reporting procedures for publication of ATP data in terms of the sample preparation, data collection, and reconstruction phases as well as the characterization and interpretation of the reconstructed volume. Coordinated reporting of this basic information will promote efficient communication of protocols, and aid in the evaluation of published atom probe data as geologists continue to explore atomic compositions and distributions at nanoscale.