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Xenotime at the Nanoscale: U‐Pb Geochronology and Optimisation of Analyses by Atom Probe Tomography
Abstract
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.
... The isotopic composition of U and Pb is measured from a narrow 0.1 Da range on the 206 Pb ++ , 207 Pb ++ , and 238 UO 2 ++ peaks and corrected for background. The background was estimated using a peak-free region (1 Da) adjacent to each peak (constant background estimation method, Joseph et al., 2021). For U-Pb dates, the 206 Pb/ 238 U ratio is calculated using the fractionation correction method, between the ratio of UO 2 ++ / UO ++ and 206 Pb ++ / 238 UO 2 ++ for each analysis (Joseph et al., 2021). ...
... The background was estimated using a peak-free region (1 Da) adjacent to each peak (constant background estimation method, Joseph et al., 2021). For U-Pb dates, the 206 Pb/ 238 U ratio is calculated using the fractionation correction method, between the ratio of UO 2 ++ / UO ++ and 206 Pb ++ / 238 UO 2 ++ for each analysis (Joseph et al., 2021). The 207 Pb/ 206 Pb dates were calculated using a variable background correction method (Joseph et al., 2021). ...
... For U-Pb dates, the 206 Pb/ 238 U ratio is calculated using the fractionation correction method, between the ratio of UO 2 ++ / UO ++ and 206 Pb ++ / 238 UO 2 ++ for each analysis (Joseph et al., 2021). The 207 Pb/ 206 Pb dates were calculated using a variable background correction method (Joseph et al., 2021). ...
... Evidence of mineralogical processes that affect geochronometers and that induce element mobility may be recorded within the crystal. The benefits of examining geochronometers at the nanoscale using transmission electron microscopy (TEM) and atom probe tomography (APT) to address broader issues of element mobility has been demonstrated for zircon (Utsunomiya et al. 2004;Kusiak et al. 2015Kusiak et al. , 2019Valley et al. 2015;Peterman et al. 2016Peterman et al. , 2019Piazolo et al. 2016;Whitehouse et al. 2017), monazite (Seydoux-Guillaume et al. 2003Fougerouse et al. 2018Fougerouse et al. , 2021aBudzyń et al. 2021Budzyń et al. , 2022Turuani et al. 2022), rutile (Verberne et al. 2020), xenotime (Joseph et al. 2021;Budzyń et al. 2023) and baddeleyite (White et al. 2017). ...
... Several processes promoting Pb mobility at the nanoscale, all with the potential to leave behind nanoscale features (cluster, nanophases) operate within monazite. A variety of such features have been recognized (Seydoux-Guillaume et al. 2003;Fougerouse et al. 2018Fougerouse et al. , 2021Turuani et al. 2022) but have not, as yet, been studied systematically in a single sample so that their significance and potential use for nanoscale geochronology can be assed in detail. ...
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.
... Kylander-Clark et al. 2013;Hacker et al. 2015;Volante et al. 2020c;Barrote et al. 2022b). Nanoscale geochronological analysis of monazite and xenotime (Joseph et al. 2021) via atom probe (ATP) is also possible. Analytical development has allowed to investigate diffusion and migration of atoms along monazite grain boundaries and/or crystal defects, and relates e.g. ...
... intracrystalline deformation with age resetting due to radiogenic Pb loss . Great potential to determine ages by using the ATP was shown also on small xenotime crystals (Joseph et al. 2021). In situ studies allow monazite ages to be directly related to mineral textures present in the rock, so the age of deformation can be attributed to metamorphic events in complex terranes (e.g. ...
The study of magmatic and metamorphic processes is challenged by geological complexities like geochemical variations, geochronological uncertainties, and the presence/absence of fluids and/or melts. However, by integrating petrographic and microstructural studies with geochronology, geochemistry, and phase equilibrium diagrams investigations of different key mineral phases, it is possible to reconstruct pressure-temperature-deformation-time histories. Using multiple geochronometers in a rock can provide a detailed temporal account of its evolution, as these geological clocks have different closure temperatures. Given the continuous improvement of existing and new in-situ analytical techniques, this contribution provides an overview of frequently utilised petrochronometers such as garnet, zircon, titanite, allanite, rutile, monazite/xenotime, and apatite, by describing the geological record that each mineral can retain, and explaining how to retrieve this information. These key minerals were chosen as they provide reliable age information in a variety of rock types and, when coupled with their trace element composition, form powerful tools to investigate crustal processes at different scales. This review recommends best applications for each petrochronometer, highlights limitations to be aware of, and discusses future perspectives. Finally, this contribution highlights the importance of integrating information retrieved by multi-petrochronometer studies to gain an in-depth understanding of complex thermal and deformation crustal processes.
... is dominated by Y and heavy rare earth elements (HREE) and, though less abundant in Earth's crust than monazite-(Ce), forms in a wide range of rocks, during diagenesis (Rasmussen, 2005;Vallini et al., 2005;Rasmussen et al., 2010;McNaughton and Rasmussen, 2018), igneous processes and low-to high-grade metamorphism (Hawkins and Bowring, 1999;Spear and Pyle, 2002;Broska et al., 2005;Hetherington et al., 2008;Budzyń et al., 2018Budzyń et al., , 2023aMcNaughton and Rasmussen, 2018) and in hydrothermal veins (Vielreicher et al., 2003;Sarma et al., 2011;Fielding et al., 2017;Jian et al., 2024). Both monazite-(Ce) and xenotime-(Y) are used in U-Th-Pb geochronology (e.g., Harrison et al., 2002;Williams et al., 2007Williams et al., , 2017Hetherington et al., 2008;Budzyń et al., 2018Budzyń et al., , 2021McNaughton and Rasmussen, 2018;Joseph et al., 2021) and as geothermometers (Kingsbury et al., 1993;Gratz and Heinrich, 1997;Heinrich et al., 1997;Andrehs and Heinrich, 1998;Pyle and Spear, 2000;Pyle et al., 2001). Their relevance for geochronology lies (i) in their capability to incorporate significant amounts of Th (particularly in monazite-(Ce)) and U (Förster, 1998;Seydoux-Guillaume et al., 2002b), and (ii) a high closure temperature of ca. ...
This work presents an investigation of naturally and experimentally unaltered and altered monazite-(Ce) and xenotime-(Y), as well as secondary phases and synthetic REE phosphates (YPO4 and LaPO4–LuPO4) with Raman microspectroscopy, using 488 nm, 532 nm, 633 nm and 780 nm excitation lasers, supplemented by electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Monazite-(Ce) spectra of the 532 nm laser from experimental products display a wide range of band positions of the v1(PO4) symmetric stretching band from 962 to 981 cm-1. Secondary fluorcalciobritholite spectra display a prominent band at 962–965 cm-1, a broader band at ca. 860 cm-1 and broad distinct luminescence effects at mid-range. Xenotime-(Y) spectra from experimental products display only minor spectral changes, except for a few spectra near secondary Y-rich fluorcalciobritholite. Spectra of Y-rich fluorcalciobritholite from 532 nm laser display characteristic broad luminescence effects at the range of 1500–3000 cm-1, including a strong luminescence band at ca. 2600 cm-1 (Sm3+ electronic transition), and are accompanied by a broad band at 974 cm-1 (488 nm laser) and 964 cm-1 (633 nm laser). The characteristic changes in monazite-(Ce) spectra near secondary phases are emphasised with hyperspectral maps and EPMA-WDS X-ray compositional maps, which provide spatial context and reveal underlying interferences, important to be noted, when evaluating altered minerals. In all cases, experimental and natural, high values of the full width at half maximum (FWHM) of the v1(PO4) band of unaltered monazite-(Ce) domains represent moderate radiation damage. In contrast, lower FWHM values were observed in altered domains. This work expands the Raman database onto examples of naturally and experimentally altered monazite-(Ce) and xenotime-(Y) enabling the identification of internal structural changes in unaltered and altered domains and differentiation of monazite-(Ce) or xenotime-(Y) from fluorcalciobritholite, Y-rich fluorcalciobritholite and other secondary phases.
... The use of U-(Th-) Pb geochronology and its petrological applications have been rapidly increasing in the last two decades due to the development and significant improvement of analytical methods, particularly secondary ion mass spectrometry (SIMS, Schmitt and Vazquez, 2017) and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS, Kylander-Clark, 2017). Recently, xenotime analysis using atom probe tomography has been demonstrated as a powerful geochronological tool to explore geological processes in nanoscale (Joseph et al., 2021(Joseph et al., , 2023. In situ analysis allows to link ages and compositional data with the microtextural context in terms of petrochronological interpretations (Williams et al., 2007(Williams et al., , 2017. ...
Five crystals of pegmatitic xenotime from Ås II feldspar quarry (Evje, S Norway) were studied with comprehensive analytical methods from microscale to nanoscale with respect to fluid-mediated alterations and their geochronological implications. Xenotime crystals are strongly altered during dissolution-reprecipitation processes that resulted in the formation of (Th, U)-depleted xenotime subdomains with numerous microinclusions of (Th, U)-silicate, uraninite and minor galena. Remains of a primary xenotime (Xtm1) yielded LA-ICPMS Usingle bondPb data characterized by a reverse discordance (from −10.5 to −2.6%) with a 208Pb/232Th mean age 988.4 ± 5.9 Ma (95% conf., MSWD = 0.75, n = 71), which is within uncertainty with a 207Pb/235U mean age 979.1 ± 5.0 Ma (95% conf., MSWD = 1.4, n = 22) yielded by data filtered to below ±5% discordance (i.e., from −5.0 to −2.6%). The altered xenotime domains (Xtm2) provided highly scattered dates, including 208Pb/232Th dates from 200 ± 14 to 2135 ± 120 Ma (n = 135). Discordant Usingle bondPb data yielded upper intercept age 909 ± 16 Ma (MSWD = 7.9). EPMA Th-U-total Pb measurements and compositional characteristics of uraninite inclusions indicate three age populations of ca. 852–983, 594–687 and 37–101 Ma.
TEM investigations revealed initial alterations within domains Xtm1, representing a primary xenotime, which progressed along parallel submicron- to nanoscale fractures. These fractures represent partially open grain boundaries, which are empty or are filled with secondary inclusions of (Th, U)-silicates, uraninite or coffinite. The submicron-sized inclusions are accompanied by subdomains of (Th, U)-depleted xenotime. Altered xenotime (Xtm2) is well crystalline in TEM imaging and electron diffraction patterns in contrast to primary unaltered xenotime domains (Xtm1) that demonstrate, in Raman spectra, moderate degree of radiation damage caused by U and Th decay. Secondary inclusions of Th and U phases are nanocrystalline or amorphous, which increases their potential for Pb-loss or accumulation of Pb in excess. Some of them contain nanoinclusions of Pb3O4 or metallic Pb, whereas <100 nm-sized inclusions of (Pb, Sb)-oxide formed in the altered xenotime.
To summarize, this study provides important insights for our understanding of coupled dissolution-reprecipitation processes that affect xenotime and mobilization of released U, Th and Pb. Removal of highly mobile U in a fluid, and the presence of nanoinclusions of Pb3O4, (Pb, Sb)-oxide and metallic Pb have particular importance for xenotime dating and explain disturbance towards older ages. Nevertheless, both removal of U, Th and Pb from altered and recrystallized xenotime as well as presence of submicron- to nanoinclusions can result in age disturbance resulting in spread of dates along a concordia curve.
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.
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.
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.
This review covers advances in the analysis of air, water, plants, soils and geological materials by a range of atomic spectrometric techniques including atomic emission, absorption, fluorescence and mass spectrometry.
Defining analysis parameter space for reliable composition measurement of materials is of significance for atom probe tomography applications. This research carefully explores the influence of specimen temperature and ultraviolet laser energy on measured compositions of precipitates and the matrix in an Al-Mg-Si-Cu alloy with atom probe tomography using voltage pulsing and laser pulsing. Low specimen temperature and high laser energy are beneficial to reduce background noise and improve mass resolution. Under both voltage pulsing and laser pulsing, the detected compositions and Mg/Si ratios of precipitates are highly sensitive to specimen temperature in the range of 20 - 80 K. In contrast, at a fixed temperature of 20 K, laser energy variation from 40 to 80 pJ provides consistent measurements in composition and Mg/Si ratio of precipitates. Related field-induced preferential evaporation and surface migration of certain elements have been discussed.
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.
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.
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 (APT) can theoretically deliver accurate chemical and isotopic analyses at a high level of sensitivity, precision, and spatial resolution. However, empirical APT data often contain significant biases that lead to erroneous chemical concentration and isotopic abundance measurements. The present study explores the accuracy of quantitative isotopic analyses performed via atom probe mass spectrometry. A machine learning-based adaptive peak fitting algorithm was developed to provide a reproducible and mathematically defensible means to determine peak shapes and intensities in the mass spectrum for specific ion species. The isotopic abundance measurements made with the atom probe are compared directly with the known isotopic abundance values for each of the materials. Even in the presence of exceedingly high numbers of multi-hit detection events (up to 80 %), and in the absence of any deadtime corrections, our approach produced isotopic abundance measurements having an accuracy consistent with values limited predominantly by counting statistics.
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.
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 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
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.
Shock microstructures in refractory accessory minerals such as zircon and monazite provide crucial evidence for deciphering impact-related deformation in a wide variety of planetary materials. Here we describe the first occurrence of shock deformation in xeno-time, YPO 4 , from a shocked quartz–bearing shatter cone in granite at the Santa Fe impact structure (New Mexico, USA). Backscattered electron imaging shows that shocked xenotime grains near the surface of a shatter cone contain multiple orientations of closely spaced pla-nar fractures. High-resolution electron backscatter diffraction mapping reveals that some of the planar microstructures in {112} contain deformation twin lamellae that range from 50 nm to 200 nm in width on the polished surface and occur in up to three crystallographic orientations. Other features attributed to impact, such as planar low-angle boundaries and planar deformation bands, record crystal-plastic deformation. Shatter cone formation and co-existing shocked quartz constrain minimum shock pressure experienced by the xenotime grains to 5–10 GPa. An upper limit of 20 GPa is tentatively assigned based on the absence of YPO 4 polymorphs and shock twins in co-existing zircon. We propose that {112} deformation twins in xenotime constitute a diagnostic record of shock metamorphism, similar to {112} twins in zircon; they have not previously been reported in nature and occur in a rock with conspicuous evidence of shock deformation. Documentation of deformation twins in xenotime, a widely applied U-Pb geochronometer, can be used to identify hypervelocity deformation in shocked rocks, detrital grains, and other materials, and may be particularly ideal for recording low-pressure (<20 GPa) impact conditions that do not produce diagnostic shock microstructures in zircon.
Isotopic discordance is a common feature in zircon that can lead to an erroneous age determination, and it is attributed to the mobilization and escape of radiogenic Pb during its post-crystallization geological evolution. The degree of isotopic discordance measured at analytical scales of ~10 mm often differs among adjacent analysis locations, indicating heterogeneous distributions of Pb at shorter length scales. We use atom probe mi-croscopy to establish the nature of these sites and the mechanisms by which they form. We show that the nanoscale distribution of Pb in a ~2.1 billion year old discordant zircon that was metamorphosed c. 150 million years ago is defined by two distinct Pb reservoirs. Despite overall Pb loss during peak metamorphic conditions, the atom probe data indicate that a component of radiogenic Pb was trapped in 10-nm dislocation loops that formed during the annealing of radiation damage associated with the metamorphic event. A second Pb component, found outside the dislocation loops, represents homogeneous accumulation of radiogenic Pb in the zircon matrix after metamorphism. The 207 Pb/ 206 Pb ratios measured from eight dislocation loops are equivalent within uncertainty and yield an age consistent with the original crystallization age of the zircon, as determined by laser ablation spot analysis. Our results provide a specific mechanism for the trapping and retention of radiogenic Pb during metamorphism and confirm that isotopic discordance in this zircon is characterized by discrete nanoscale reservoirs of Pb that record different isotopic compositions and yield age data consistent with distinct geological events. These data may provide a framework for interpreting discordance in zircon as the heterogeneous distribution of discrete radiogenic Pb populations, each yielding geologically meaningful ages.
Trace elements diffuse negligible distances through the pristine crystal lattice in minerals: this is a fundamental assumption when using them to decipher geological processes. For example, the reliable use of the mineral zircon (ZrSiO4) as a U-Th-Pb geochronometer and trace element monitor requires minimal radiogenic isotope and trace element mobility. Here, using atom probe tomography, we document the effects of crystal-plastic deformation on atomic-scale elemental distributions in zircon revealing sub-micrometre-scale mechanisms of trace element mobility. Dislocations that move through the lattice accumulate U and other trace elements. Pipe diffusion along dislocation arrays connected to a chemical or structural sink results in continuous removal of selected elements (for example, Pb), even after deformation has ceased. However, in disconnected dislocations, trace elements remain locked. Our findings have important implications for the use of zircon as a geochronometer, and highlight the importance of deformation on trace element redistribution in minerals and engineering materials.
Atom-probe tomography (APT) and secondary ion mass spectrometry (SIMS) provide complementary in situ element and isotope data in minerals such as zircon. SIMS measures isotope ratios and trace elements from 1–20 μm spots with excellent accuracy and precision. APT identifies mass/charge and three-dimensional position of individual atoms (±0.3 nm) in 100 nm-scale samples, volumes up to one million times smaller than SIMS. APT data provide unique information for understanding element and isotope distribution; crystallization and thermal history; and mechanisms of mineral reaction and exchange. This atomistic view enables evaluation of the fidelity of geochemical data for zircon because it provides new understanding of radiation damage, and can test for intracrystalline element mobility. Nano-geochronology is one application of APT in which Pb isotope ratios from sub-micrometer domains of zircon provide model ages of crystallization and identify later magmatic and metamorphic reheating.
Based on SEM imaging and SIMS analysis, 11 needle-shaped specimens ~100 nm in diameter were sampled from one Archean and two Hadean zircons by focused ion-beam milling and analyzed with APT. The three-dimensional distribution of Pb and nominally incompatible elements (Y, REEs) differs at the atomic scale in each zircon. Zircon JH4.0 (4.007 Ga, Jack Hills, Western Australia) is homogeneous in Pb, Y, and REEs. In contrast, Pb and Y and REEs are clustered in sub-equant ~10 nm diameter domains, spaced 10–40 nm apart in zircons ARG2.5 (2.542 Ga, Grouse Creek Mountains, Utah) and JH4.4 (4.374 Ga, Jack Hills). Most clusters are flattened parallel to (100) or (010). U and Th are not collocated with Pb in clusters and appear to be homogeneously distributed in all three zircons. The analyzed domains experienced 4 to 8 × 1015 α-decay events/mg due to U and Th decay and yet all zircons yield U-Pb ages by SIMS that are better than 97% concordant, consistent with annealing of most radiation damage. The 207Pb/206Pb ratios for the 100 nm-scale specimens measured by APT average 0.17 for ARG2.5, 0.42 for the JH4.0, and 0.52 for JH4.4. These ratios are less precise (±10–18% 2σ) due to the ultra-small sample size, but in excellent agreement with values measured by SIMS (0.1684, 0.4269, and 0.5472, respectively) and the crystallization ages of the zircons. Thus Pb in these clusters is radiogenic, but unsupported, meaning that the Pb is not spatially associated with its parent isotopes of U and Th. For the domain outside of clusters in JH4.4, the 207Pb/206Pb ratio is 0.3, consistent with the SIMS value of 0.2867 for the zircon overgrowth rim and an age of 3.4 Ga. In ARG2.5, all Pb is concentrated in clusters and there is no detectable Pb remaining outside of the clusters. The Pb-Y-REE-rich clusters and lack of correlation with U in ARG2.5 and JH4.4 are best explained by diffusion of Pb and other elements into ~10 nm amorphous domains formed by α-recoil. Diffusion distances of ~20 nm for these elements in crystalline zircon are consistent with heating at temperatures of 800 °C for ~2 m.y. Such later reheating events are identified and dated by APT from 207Pb/206Pb model ages of clusters in JH4.4 and by the absence of detectable Pb outside of clusters in ARG2.5. SIMS dates for the zircon rims independently confirm reheating of ARG2.5 and JH4.4, which were xenocrysts in younger magmas when rims formed. It is proposed that most domains damaged by α-recoil were annealed at ambient temperatures above 200–300 °C before reheating and only a small number of domains were amorphous and available to concentrate Pb at the time of reheating. The size, shapes and
orientations of clusters were altered by annealing after formation. The absence of enriched clusters in
JH4.0 shows that this zircon was not similarly reheated. Thus APT data provide thermochronologic
information about crustal reworking even for zircons where no overgrowth is recognized. The clusters
in JH4.4 document Pb mobility at the sub-50 nm scale, but show that the much larger 20 μm-scale
domains analyzed by SIMS were closed systems. The reliability of oxygen isotope ratios and other
geochemical data from zircon can be evaluated by these means. These results verify the age of this
zircon and support previous proposals that differentiated crust existed by 4.4 Ga and that the surface
of Earth was relatively cool with habitable oceans before 4.3 Ga. These analytical techniques are of
general applicability to minerals of all ages and open many new research opportunities.
Analysis of the detected Fe ion charge states from laser-assisted field evaporation of magnetite (Fe3O4) reveals unexpected trends as a function of laser pulse energy that break from conventional post-ionization theory for metals. For Fe ions evaporated from magnetite, the effects of post-ionization are partially offset by the increased prevalence of direct evaporation into higher charge states with increasing laser pulse energy. Therefore, the final charge state is related to both the field strength and the laser pulse energy, despite those variables themselves being intertwined when analyzing at a constant detection rate. Comparison of data collected at different base temperatures also shows that the increased prevalence of Fe2+ at higher laser energies is possibly not a direct thermal effect. Conversely, the ratio of 16O+:(16O2
+ + 16O+) is well correlated with field strength and unaffected by laser pulse energy on its own, making it a better overall indicator of the field evaporation conditions. Plotting the normalized field strength versus laser pulse energy also elucidates a non-linear dependence, in agreement with the previous observations on semiconductors, which suggests field-dependent laser absorption efficiency. Together these observations demonstrate that the field evaporation process for laser-pulsed oxides exhibits fundamental differences from metallic specimens that cannot be completely explained by post-ionization theory. Further theoretical studies, combined with detailed analytical observations, are required to understand fully the field evaporation process of non-metallic samples.
Zircon, monazite, and xenotime have proven to be valuable chronometers for various geological processes due to their commonly high-U–Th and low common Pb contents. However, zircons that have crystallized in highly fractionated granites often have such high-U contents that radiation damage can lead to scattered U–Pb ages when measured with secondary ion mass spectrometry (SIMS). In this study, monazite and xenotime were separated from a number of highly fractionated granites at the Xihuashan tungsten mine, Southeast China, for alternative dating methods by SIMS. For monazite analysis, obvious excess 204Pb signal (mainly from interference of 232Th144Nd16O2++) was observed in high-Th (>2 wt%) monazite, which hinders 204Pb-based common Pb corrections. A 207Pb-based common Pb correction method was used instead. By employing power law relationships between Pb+/U+ versus UO2+/U+, Pb+/Th+ versus ThO2+/Th+ and suitable exponentials, monazites with ThO2 contents in the range of ~3–19 % do not exhibit this matrix effect. Independent SIMS U–Pb ages and Th–Pb ages of three phases of Xihuashan granite samples were consistent with each other and yielded dates of 158.7 ± 0.7, 158.0 ± 0.7, and 156.9 ± 0.7 Ma, respectively. Xenotime does show marked matrix effects due to variations of U, Th, and Y [or total rare earth element (REE), referred as ΣREE hereafter] contents. Suitable correction factors require end-member standards with extremely high or low U, Th, and Y (or ΣREE) contents. No excess 204Pb was observed, indicating that the 204Pb-based common Pb correction method is feasible. Independent 207Pb/206Pb ages can be obtained, although multi-collector mode is necessary to improve precision. The main difficulties with dating xenotime are when high-Th (U) mineral inclusions are ablated. We can identify when this occurs, however, by comparing the measured UO2+/U+ and ThO2+/Th+ with those in xenotime standards. Three xenotime samples from the first phase of Xihuashan granite yielded a weighted mean 207Pb/206Pb date of 159.5 ± 4.4 Ma (MSWD = 1.0) and a 206Pb/238U date of 159.4 ± 0.9 Ma (MSWD = 1.6), which are consistent with monazite U–Pb and Th–Pb ages from the same granites. This study demonstrates that monazite and xenotime are better SIMS chronometers for highly fractionated granites than zircon, which can yield doubtful ages due to high-U contents.
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.
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.
High-grade hematite mineralization is widely developed in banded iron formations (BIFs) worldwide. However, in the North China craton where Neoarchean-Paleoproterozoic BIFs are abundant, economic high-grade hematite ores are scarce. High-grade hematite ores hosted in the Paleoproterozoic Yuanjiacun BIFs represent the largest occurrence of this type of ore in the North China craton. The orebodies are fault controlled and show sharp contacts with lower greenschist facies metamorphic BIFs. In situ U-Pb geochronology of monazite and xenotime intergrown with microplaty hematite and martite in high-grade ore established two episodes of metamorphic-hydrothermal monazite/xenotime growth after deposition of the BIFs. The earlier episode at ca.1.94 Ga is interpreted as the timing of lower greenschist-facies metamorphism, and the later episode at 1.41 to 1.34 Ga represents the timing of high-grade hematite mineralization. Petrography and microthermometry of primary fluid inclusion assemblages indicate that the high-grade hematite ore formed from hot (313°–370°C), CO2-rich, and highly saline (~20 wt % NaCl equiv) hydrothermal fluids. These fluids channeled along faults, which concentrated iron through interaction with the BIFs—a process similar to typical hematite mineralization elsewhere. The deposition of hematite was probably related to tectonic extension in the North China craton related to the breakup of the Columbia/Nuna supercontinent. Our results challenge a previously proposed model ascribing the scarcity of high-grade hematite ores in the North China craton to the lack of prolonged weathering conditions. Rather, we argue that the high-grade ore formed in lower metamorphic-grade BIFs at
shallower depths than magnetite mineralization and was largely eroded during later exhumation and uplift of the craton.
High-grade hematite mineralization is widely developed in banded iron formations (BIFs) worldwide. However, in the North China craton where Neoarchean-Paleoproterozoic BIFs are abundant, economic high-grade hematite ores are scarce. High-grade hematite ores hosted in the Paleoproterozoic Yuanjiacun BIFs represent the largest occurrence of this type of ore in the North China craton. The orebodies are fault controlled and show sharp contacts with lower greenschist facies metamorphic BIFs. In situ U-Pb geochronology of monazite and xenotime intergrown with microplaty hematite and martite in high-grade ore established two episodes of metamorphic-hydrothermal monazite/xenotime growth after deposition of the BIFs. The earlier episode at ca.1.94 Ga is interpreted as the timing of lower greenschist-facies metamorphism, and the later episode at 1.41 to 1.34 Ga represents the timing of high-grade hematite mineralization. Petrography and microthermometry of primary fluid inclusion assemblages indicate that the high-grade hematite ore formed from hot (313°–370°C), CO2-rich, and highly saline (~20 wt % NaCl equiv) hydrothermal fluids. These fluids channeled along faults, which concentrated iron through interaction with the BIFs—a process similar to typical hematite mineralization elsewhere. The deposition of hematite was probably related to tectonic extension in the North China craton related to the breakup of the Columbia/Nuna supercontinent. Our results challenge a previously proposed model ascribing the scarcity of high-grade hematite ores in the North China craton to the lack of prolonged weathering conditions. Rather, we argue that the high-grade ore formed in lower metamorphic-grade BIFs at shallower depths than magnetite mineralization and was largely eroded during later exhumation and uplift of the craton.
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 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.
Atom probe microscopy (APM) is a relatively new in situ tool for measuring isotope fractions from nanoscale volumes (< 0.01 μm³). We calculate the theoretical detectable difference of an isotope ratio measurement result from APM using counting statistics of a hypothetical dataset to be ± 4δ or 0.4% (2s). However, challenges associated with APM measurements (e.g., peak ranging, hydride formation and isobaric interferences), result in larger uncertainties if not properly accounted for. We evaluate these factors for Re‐Os isotope ratio measurements by comparing APM and negative thermal ionisation mass spectrometry (N‐TIMS) measurement results of pure Os, pure Re, and two synthetic Re‐Os‐bearing alloys from Schwander et al. (2015) (the original metal alloy (HSE) and alloys produced by heating HSE within silicate liquid (SYN)). From this, we propose a current best practice for APM Re‐Os isotope ratio measurements. Using this refined approach, mean APM and N‐TIMS ¹⁸⁷Os/¹⁸⁹Os measurement results agree within 0.05% and 2s (pure Os), 0.6–2% and 2s (SYN) and 5–10% (HSE). The good agreement of N‐TIMS and APM ¹⁸⁷Os/¹⁸⁹Os measurements confirm that APM can extract robust isotope ratios. Therefore, this approach permits nanoscale isotope measurements of Os‐bearing alloys using the Re‐Os geochronometer that could not be measured by conventional measurement principles.
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The application of zircon and xenotime geochronometers requires knowledge of their potential and limitations related to possible disturbance of the age record. The alteration of the intergrown zircon and xenotime in pegmatite from the Góry Sowie Block (SW Poland) was studied using the electron microprobe analysis, X-ray WDS compositional mapping, micro-Raman analysis, and LA-ICP-MS U-Pb dating of zircon and xenotime, as well as the U-Th-total Pb dating of uraninite. These microanalytical techniques were applied to understand the formation mechanisms of the secondary textures related to post-magmatic processes in the zircon and xenotime intergrowth, and to constrain their timing. Textural and compositional features combined with U-Pb data indicate that the pegmatite-related crystallization of the zircon and xenotime intergrowth occurred ca. 2.09 Ga (2086 ± 35 Ma for zircon and 2093 ± 52 Ma for xenotime), followed by the re-equilibration of zircon and xenotime ca. 370 Ma (373 ± 18 Ma and 368 ± 6 Ma, respectively) during the formation of the younger pegmatite. The zircon and xenotime were most likely derived from Precambrian basement rocks and emplaced in the pegmatite as a restite. The zircon preserved textures related to diffusion-reaction processes that affected its high-U core (up to ca. 9.6 wt% UO2), which underwent further metamictization and amorphization due to self-radiation damage. The zircon rim and xenotime were affected by coupled dissolution-reprecipitation processes that resulted in patchy zoning, age disturbance and sponge-like textures. Xenotime was also partially replaced by fluorapatite or hingganite-(Y) and Y-enriched allanite-(Ce). The termination of the low-temperature alteration was constrained by the U-Th-total Pb age of the uraninite inclusions that crystallized in zircon at 281 ± 2 Ma, which is consistent with the age of 278 ± 15 Ma obtained from the youngest cluster of U-Pb ages in the re-equilibrated high-U zircon domains. This study demonstrates the importance of the careful examination of compositional, microtextural and geochronological data obtained using microanalytical techniques to reconstruct the complex thermal histories recorded by accessory minerals.
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.
Chemical and isotopic signatures recorded by the accessory phase baddeleyite (ZrO2) yield important insights into the formation and evolution of mafic planetary crusts. However, little work has been conducted regarding the effects of structures on the mobilization and diffusion of substitutional and interstitial ions. Coupled nanometer-scale analyses of chemistry and structure in mineral phases are possible using the emerging technique of atom probe tomography (APT). Here we use this technique to describe a range of complex chemical nanostructures within shocked, annealed, and metamorphosed baddeleyite grains sampled in crater floor rocks ~550 m away from the contact with the Sudbury impact melt sheet. This has revealed a wide range of nanostructural phenomena, including domains of clustered incompatible cations (Fe), separated by subgrain boundaries or planar features exhibiting wave-like features decorated with trace amounts of Al, Si, and Fe likely generated by shock metamorphism. In some cases, these nanostructures have facilitated much later, and highly localized, postimpact Pb loss and Si gain ascribed to regional greenschist metamorphism. Characterizing nanoscale heterogeneities within complex, shocked baddeleyite grains using APT for resolution of different deformation pathways and a more confident interpretation of the geologic significance of micron-scale trace element and isotopic analyses.
Advancement in microstructural geochronology using atom probe tomography (APT) depends on baseline measurements and interpretive approaches using geochronology reference materials. Here we present the first APT results for the well-characterized baddeleyite from the 2.060 Ga Phalaborwa intrusion and reference zircon BR266 (0.559 Ga) to explore the applicable run conditions, peak identifications, ranging, and detection sensitivity of trace elements and Pb. Six microtips of monocrystalline baddeleyite, as confirmed by tEBSD, were analyzed in both the annealed (300°C) and un-annealed states. The analyzed stoichiometry of the primary constituents (Zr, O, Hf) yields the correct concentrations but with uncertainty of several percent due to analyst-dependent ranging. The distribution of trace elements is homogeneous, even for the annealed samples. APT analysis was able to detect 5 of the 10 known trace elements above the background level. Zircon BR266 is mostly homogeneous, and the bulk APT analysis of these domains yields the correct 206Pb/238U, and 207Pb/206Pb ages with large uncertainties. Rare nanoclusters of Pb yield similar Pb isotopic ratios to the bulk, whereas signal from the surrounding matrix is too low (~12 ppm atomic) to yield meaningful results. The approaches described here for these two important minerals are a guide to future geoscience applications with APT.
Due to the intrinsic evaporation properties of the material studied, insufficient mass-resolving power and lack of knowledge of the kinetic energy of incident ions, peaks in the atom probe mass-to-charge spectrum can overlap and result in incorrect composition measurements. Contributions to these peak overlaps can be deconvoluted globally, by simply examining adjacent peaks combined with knowledge of natural isotopic abundances. However, this strategy does not account for the fact that the relative contributions to this convoluted signal can often vary significantly in different regions of the analysis volume; e.g., across interfaces and within clusters. Some progress has been made with spatially localized deconvolution in cases where the discrete microstructural regions can be easily identified within the reconstruction, but this means no further point cloud analyses are possible. Hence, we present an ion-by-ion methodology where the identity of each ion, normally obscured by peak overlap, is resolved by examining the isotopic abundance of their immediate surroundings. The resulting peak-deconvoluted data are a point cloud and can be analyzed with any existing tools. We present two detailed case studies and discussion of the limitations of this new technique.
Materials science is an interdisciplinary field devoted to understanding the fundamental origins of materials’ properties to develop and sustain materials technology and engineering. Atom probe microscopy enables the characterisation of many important microstructural features that occur across various length scales in materials. This microscopy can enable new insights into the scientific and engineering aspects of how materials actually work. APM techniques enable the characterisation of the structure and the chemistry of materials, and this is of great significance because both are vital in formulating relationships between microstructure and properties. This short chapter explains how the different methods described previously can be applied to obtain information relevant to the materials scientist.
Over the years a number of atom probe books have been written by various authors
[1–10], so why write another one you might ask. We believe that this book is unique
in specifically targeting atom probe adopters who are new to the technique. The
book introduces new users to the process of performing all of the aspects of a Local
Electrode Atom Probe™ experiment. It includes the fundamentals of preparing
specimens for the microscope from a variety of materials, details of the instrumentation
used in data collection, parameters under which optimal data are collected,
current methods of data reconstruction, and selected methods of data analysis. In
addition, certain topics are explained specifically from a user perspective and
include details that are often learned only through trial and error, allowing users
to succeed more quickly in the challenging areas of specimen preparation and data
collection.
This book is meant to be a useful reference for the “conventional wisdom” type
of information that is not always found in academic books and is usually gained
only through experience. It is not meant to be a comprehensive treatment of atom
probe tomography but rather an everyday reference for data collection on the local
electrode atom probe and for the specimen preparation and data analysis that go
along with such experiments. For the most part, we have dealt with more advanced
topics, such as the details of the spatial reconstruction equations, by including the
information in appendices or by simply referring the reader to other textbooks or
journal articles. In this way we hope to have produced a very usable reference for
both novice users and experienced scientists. The future of atom probe tomography
is bright, and we hope that the path to adoption will be clearer with the availability
of this book.
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3. Bowkett, K.M., Smith, D.A.: Field-Ion Microscopy. North-Holland, Amsterdam (1970)
4. Muller, E.W., Tsong, T.T.: Field Ion Microscopy, Field Ionization and Field Evaporation, vol.
4. Progress in Surface Science (1973)
5. Miller, M.K., Smith, G.D.W.: Atom Probe Microanalysis: Principles and Applications to
Materials Problems. Materials Research Society, Pittsburgh (1989)
6. Sakurai, T., Sakai, A., Pickering, H.W.: Atom probe field ion microscopy and its applications.
Adv. Electron. Electron. Phys. 20, 1–299 (1989)
7. Tsong, T.T.: Atom-Probe Field Ion Microscopy: Field Ion Emission and Surfaces and
Interfaces at Atomic Resolution. Cambridge University Press, Cambridge, Great Britain
(1990)
8. Miller, M.K., Cerezo, A., Hetherington, M.G., Smith, G.D.W.: Atom Probe Field Ion Microscopy.
Oxford University Press, Oxford (1996)
9. Miller, M.K.: Atom Probe Tomography: Analysis at the Atomic Level. Kluwer Academic/
Plenum Publishers, New York (2000)
10. Gault, B., Moody, M.P., Cairney, J.M., Ringer, S.P.: Atom Probe Microscopy. Springer Series
in Materials Science, vol. 160. Springer (2012)
A pod of monazite-xenotime gneiss (MXG) occurs within Mesoproterozoic paragneiss, Hudson Highlands, New York. This outcrop also contains granite of the Crystal Lake pluton, which migmatized the paragneiss. Previously, monazite, xenotime, and zircon from MXG, plus detrital zircon from the paragneiss, and igneous zircon from the granite, were dated using multi-grain thermal ionization mass spectrometry (TIMS). New SEM imagery of dated samples reveals that all minerals contain cores and rims. Thus TIMS analyses comprise mixtures of age components and are geologically meaningless. New spot analyses by sensitive high resolution ion microprobe (SHRIMP) of small homogeneous areas on individual grains allows deconvolution of ages within complexly zoned grains. Xenotime cores from MXG formed during two episodes (1034 +/- 10 and 1014 +/- 3 Ma), whereas three episodes of rim formation are recorded (999 +/- 7, 961 +/- 11, and 874 +/- 11 Ma). Monazite cores from MXG mostly formed at 1004 +/- 4 Ma; rims formed at 994 +/- 4, 913 +/- 7, and 890 +/- 7 Ma. Zircon from MXG is composed of oscillatory-zoned detrital cores (2000-1170 Ma), plus metamorphic rims (1008 +/- 7, 985 +/- 5, and similar to 950 Ma). In addition, MXG contains an unusual zircon population composed of irregularly-zoned elongate cores dated at 1036 +/- 5 Ma, considered to be the time of formation of MXG. The time of granite emplacement is dated by oscillatory-zoned igneous cores at 1058 +/- 4 Ma, which provides a minimum age constraint for the time of deposition of the paragneiss. Selected trace elements, including all REE plus U and Th, provide geochemical evidence for the origin of MXG. MREE-enriched xenotime from MXG are dissimilar from typical HREE-enriched patterns of igneous xenotime. The presence of large negative Eu anomalies and high U and Th in monazite and xenotime are uncharacteristic of typical ore-forming hydrothermal processes. We conclude that MXG is the result of unusual metasomatic processes during high grade metamorphism that was initiated at about 1035 Ma. This rock was then subjected to repeated episodes of dissolution/reprecipitation for about 150 m.y. during regional cooling of the Hudson Highlands.
Xenotime occurs as epitaxial overgrowths on detrital zircons in the Mesoproterozoic Revett Formation (Belt Supergroup) at the Spar Lake red bed-associated Cu-Ag deposit, western Montana. The deposit formed during diagenesis of Revett strata, where oxidizing metal-bearing hydrothermal fluids encountered a reducing zone. Samples for geochronology were collected from several mineral zones. Xenotime overgrowths (1-30 μm wide) were found in polished thin sections from five ore and near-ore zones (chalcocite-chlorite, bornitecalcite, galena-calcite, chalcopyrite-ankerite, and pyrite-calcite), but not in more distant zones across the region. Thirty-two in situ SHRIMP U-Pb analyses on xenotime overgrowths yield a weighted average of 207Pb/ 206Pb ages of 1409 ± 8 Ma, interpreted as the time of mineralization. This age is about 40 to 60 m.y. after deposition of the Revett Formation. Six other xenotime overgrowths formed during a younger event at 1304 ± 19 Ma. Several isolated grains of xenotime have 207Pb/ 206Pb ages in the range of 1.67 to 1.51 Ga, and thus are considered detrital in origin. Trace element data can distinguish Spar Lake xenotimes of different origins. Based on in situ SHRIMP analysis, detrital xenotime has heavy rare earth elements-enriched patterns similar to those of igneous xenotime, whereas xenotime overgrowths of inferred hydrothermal origin have hump-shaped (i.e., middle rare earth elements-enriched) patterns. The two ages of hydrothermal xenotime can be distinguished by slightly different rare earth elements patterns. In addition, 1409 Ma xenotime overgrowths have higher Eu and Gd contents than the 1304 Ma overgrowths. Most xenotime overgrowths from the Spar Lake deposit have elevated As concentrations, further suggesting a genetic relationship between the xenotime formation and Cu-Ag mineralization.
Recent geological and geochronological studies have led to a reevaluation of whether or not the majority of the lode gold deposits of the Yilgarn craton formed nearly synchronously during an Archean craton-wide hydrothermal event, as previously proposed. The majority of reliable data indicate that gold mineralization took place at ca. 2640 to 2625 Ma; however, recent work in the far north of the Eastern Goldfields province provides structural evidence for instances of earlier gold mineralization and geochronological. data for interpreted postore rocks that are considerably older than 2640 Ma. The documentation of xenotime and monazite in association with ore minerals at the Cleo deposit provides a valuable means of determining the absolute age of gold mineralization. At the Cleo deposit, high-grade Western Lodes veins crosscut the Sunrise shear zone at a high angle with only centimeter-scale offset. Given that the stratigraphic sequence indicates at least several hundred meters offset across the shear zone, the minimal offset of the Western Lodes veins indicates that gold mineralization was late in the history of movement in the shear zone. The absolute ages of pre-, syn-, and postmineralization elements in the mine stratigraphic succession have been determined using a variety of geochronological techniques. Fuchsite micas formed in the Sunrise shear zone yield an Ar-40/Ar-39 isochron age of 2667 +/- 19 Ma. Rhyodacite porphyry dikes postdate the formation of the shear zone but predate the main phase of gold mineralization. Dike intrusion is constrained to 2674 +/- 3 Ma by SHRIMP II U-Pb analysis of zircons from these dikes. Rhenium-Os analysis of molybdenite from quartz + chalcopyrite + molybdenite veins that crosscut the porphyry dikes gives an age of formation at 2663 +/- 11 Ma, consistent with their relationship to the porphyry dikes. Both the dikes and these veins are crosscut by gold-bearing Western Lodes veins. Mutually crosscutting relationships with ore minerals indicate that microscopic xenotime and monazite grains were deposited during ore formation in the Western Lodes ore zones. SHRIMP 11 U-Pb analyses of xenotime and monazite grains indicate that Western Lodes mineralization took place at 2654 +/- 8 Ma, at least 9 m.y. after the intrusion of the porphyry dikes (95% confidence). Evidence for substantial temporal separation between intrusive activity and gold mineralization is recorded by other researchers for other deposits associated with granitic intrusions in the Leonora-Laverton area. Phlogopitic micas from a lamprophyre that crosscuts the whole succession, including the Sunrise shear zone, yield an Ar/Ar plateau age of 2080 +/- 4 Ma. This study constrains the timing of multiple events at Cleo, including the main phase of gold mineralization, during a relatively short span of time. However, in the context of the larger debate about the existence of a dominant 2640 to 2625 Ma period of gold mineralization and the extent of a proposed widespread pre-2660 Ma mineralization event in the Yilgarn craton, the data from Cleo are equivocal, the 2654 +/- 8 Ma age for mineralization being significantly older than 2640 Ma but not significantly pre-2660 Ma, even if the maximum possible age is accepted.
Precise electron microprobe analysis makes it possible to determine the Th, U and Pb concentrations in an area less than 5 μm across in a single grain of monazite, zircon or xenotime; the detection limit of PbO at 2σ confidence level is 0.005–0.008 wt.% and the relative error is 3% for 0.2 wt.% concentration level. The subgrain analyses of monazite are plotted on the coordinates of PbO and ThO2* (ThO2 plus the equivalent of UO2), and those of zircon and xenotime on the coordinates of PbO and UO2* (UO2 plus the equivalent of ThO2). Data points are arrayed linearly and enable us to define an isochron which passes through the origin. The chemical Th-U-total Pb isochron ages coincide well with mineral and whole-rock ages isotopically determined for the same samples. The chemical Th-U-total Pb isochron method was applied to the age determination of monazite, zircon and xenotime from the Tsubonosawa paragneiss and the host Hikami granite in the South Kitakami terrane of Northeast Japan. The chemical ages show that (1) the sedimentation of the gneiss-protolith occurred soon after the emplacement of 500 Ma granitoids in the source region, where the 500 Ma granitoids were widespread together with Precambrian rocks possibly dated back to as old as Archean (3080 ± 180 Ma), (2) the gneiss-protolith was metamorphosed to the amphibolite facies grade about 430 ± 10 Ma ago, and (3) the gneiss was affected by multiple thermal events of 350, 260, 180 and 100 Ma. The 350 Ma event corresponds to the intrusion of the Hikami granite, and the 100 Ma one to the intrusion of the Cretaceous Kesengawa granite. The 260 and 180 Ma events may correspond in age to the main metamorphic event and subsequent igneous activity in the Hida terrane, central Japan. The chemical Th-U-total Pb isochron age in terms of the precise microprobe analysis of low-level Th, U and Pb will open a new vista on the Paleozoic-Mesozoic tectonics of the Japanese Islands.
The effects of laser wavelength (355nm and 532nm) and laser pulse energy on the quantitative analysis of LiFePO4 by atom probe tomography are considered. A systematic investigation of ultraviolet (UV, 355nm) and green (532nm) laser assisted field evaporation has revealed distinctly different behaviors. With the use of a UV laser, the major issue was identified as the preferential loss of oxygen (up to 10at%) while other elements (Li, Fe and P) were observed to be close to nominal ratios. Lowering the laser energy per pulse to 1pJ/pulse from 50pJ/pulse increased the observed oxygen concentration to nearer its correct stoichiometry, which was also well correlated with systematically higher concentrations of (16)O2(+) ions. Green laser assisted field evaporation led to the selective loss of Li (~33% deficiency) and a relatively minor O deficiency. The loss of Li is likely a result of selective dc evaporation of Li between or after laser pulses. Comparison of the UV and green laser data suggests that the green wavelength energy was absorbed less efficiently than the UV wavelength because of differences in absorption at 355 and 532nm for LiFePO4. Plotting of multihit events on Saxey plots also revealed a strong neutral O2 loss from molecular dissociation, but quantification of this loss was insufficient to account for the observed oxygen deficiency.
Atom probe tomography (APT) is a powerful materials characterization technique capable of ppm chemical resolution and near atomic scale spatial resolution. However, owing to a number of factors, the technique has not been widely applied to insulating materials and even less to complex oxides. In this study, we outline the methodology necessary to obtain high-quality results on a technologically relevant complex oxide Pb(Zr,Ti)O3 (or PZT) using laser-assisted APT on both bulk and thin film specimens. We show how, with optimized and well-controlled conditions, APT complements conventional techniques such as STEM-EDS. The correlative information can be used to obtain the nanoscale 3-D chemical information and investigate the nanoscale distribution of cations. Using nearest-neighbor cluster analysis routines, 5–10 nm segregation of B-site cations was detected in bulk sintered PZT 53/47 from chemically prepared powders. No statistically significant segregation of B-site cations was observed in thin film specimens. This work opens new avenues toward understanding the process-structure properties in complex materials at length scales heretofore unachievable.
Diffusion of Pb and the rare earth elements Sm, Dy and Yb have been characterized in synthetic xenotime under dry conditions. The synthetic xenotime was grown via a Na2CO3–MoO3 flux method. The sources of diffusant for the rare earth diffusion experiments were REE phosphate powders, with experiments run using sources containing a single REE. For Pb, the source consisted a mixture of YPO4 and PbTiO3. Experiments were performed by placing source and xenotime in Pt capsules, and annealing capsules in 1 atm furnaces for times ranging from 30 min to several weeks, at temperatures from 1000 to 1500 °C. The REE and Pb distributions in the xenotime were profiled by Rutherford Backscattering Spectrometry (RBS).
The only physical evidence from the earliest phases of Earth's evolution comes from zircons, ancient mineral grains that can be dated using the U-Th-Pb geochronometer. Oxygen isotope ratios from such zircons have been used to infer when the hydrosphere and conditions habitable to life were established. Chemical homogenization of Earth's crust and the existence of a magma ocean have not been dated directly, but must have occurred earlier. However, the accuracy of the U-Pb zircon ages can plausibly be biased by poorly understood processes of intracrystalline Pb mobility. Here we use atom-probe tomography to identify and map individual atoms in the oldest concordant grain from Earth, a 4.4-Gyr-old Hadean zircon with a high-temperature overgrowth that formed about 1 Gyr after the mineral's core. Isolated nanoclusters, measuring about 10 nm and spaced 10-50 nm apart, are enriched in incompatible elements including radiogenic Pb with unusually high 207Pb/206Pb ratios. We demonstrate that the length scales of these clusters make U-Pb age biasing impossible, and that they formed during the later reheating event. Our tomography data thereby confirm that any mixing event of the silicate Earth must have occurred before 4.4 Gyr ago, consistent with magma ocean formation by an early moon-forming impact about 4.5 Gyr ago.
Cerium oxide (CeO2) is an ideal surrogate material for trans-uranic elements and fission products found in nuclear fuels due to similarities in their thermal properties; therefore, cerium oxide was used to determine the best run condition for atom probe tomography (APT) of nuclear fuels. Laser-assisted APT is a technique that allows for spatial resolution in the nm scale and isotopic/elemental chemical identification. A systematic study of the impact of laser pulse energy and specimen base temperature on the mass resolution, measurement of stoichiometry, multiple detector hits, and evaporation mechanisms are reported in this paper. It was demonstrated that using laser-assisted APT stoichiometric field evaporation of cerium oxide was achieved at 1 pJ laser pulse energy and 20 K specimen base temperature.
The southern margin of the Pilbara Craton in northwestern Australia underwent regional heating, folding, and thrusting as well as extensive fluid flow during collision in the Paleoproterozoic. However, the precise timing of this event is uncertain. Previous geochronologic studies of Archean basement rocks have yielded a wide range of younger radiometric dates (from ca. 2.4 Ga to ca. 2.0 Ga), interpreted to record multiple hydrothermal alteration events. In situ U-Pb geochronology of authigenic monazite and xenotime in very low grade Archean metasedimentary rocks from across the craton suggests that the Pilbara was affected by at least two cryptic thermotectonic events, the first between ca. 2430 Ma and ca. 2400 Ma, and the second between ca. 2215 Ma and ca. 2145 Ma. The older event is restricted to three localities in the west, and its cause is unknown. The younger event affected most of the craton (>100,000 km2), spanning a 70 m.y. period from ca. 2215 Ma nearest the collisional margin in the south, to ca. 2145 Ma toward the craton interior in the north. The widespread geographic and stratigraphic distribution of ca. 2.2 Ga phosphates suggests that fluid flow was pervasive. This event was probably driven by the northward-advancing Ophthalmian fold-and-thrust belt that developed during protracted collision. The associated low-grade metamorphic front migrated over ˜350 km at an average rate of ˜5.0 mm/yr, placing important constraints on the rate and duration of deformation and metamorphism in orogenic settings.
The depositional age of nonfossiliferous, metamorphosed sedimentary rocks is commonly bracketed between the age of the youngest detrital mineral and the age of the oldest metamorphic mineral. The technique of dating diagenetic xenotime by ion microprobe can provide robust minimum ages for sediment deposition. However, in most cases, xenotime is only a few microns (mum) in size and rarely exceeds 10 mum, the minimum size for in situ ion microprobe analysis. Phosphatic sandstone in the greenschist facies Mount Barren Group, in southwestern Australia, contains unusually abundant xenotime occurring as exceptionally coarse (200 mum) pore-filling cement that nucleated on detrital zircon grains. The optimum environmental site for the formation of the cement was sand beds within a black shale condensed section. Analysis of xenotime by sensitive high-resolution ion microprobe yields two age populations, 1696 ± 7 Ma for cement adjacent to detrital zircon grains, and 1646 ± 8 Ma for outer zones. Preserved textures show that initial xenotime growth was early diagenetic, establishing the ca. 1700 Ma age as a proxy for the depositional age of the Mount Barren Group. The younger age (ca. 1650 Ma) is regarded as burial related. The xenotime data reduce considerably the previous limits on the age of the succession, i.e., between ca. 1850 Ma (youngest zircon population) and ca. 1200 Ma (peak metamorphism). The significant achievement of our results is establishing that early diagenetic xenotime retains its physical form and U-Pb isotopic age despite greenschist-facies metamorphism and penetrative deformation.
The advent of SHRIMP, the Sensitive High Mass-Resolution Ion Microprobe, defines a milestone in Australian geochronology. SHRIMP was the first ion microprobe dedicated to geological isotopic analysis and opened up zircon geochronology to in situ analysis where single domains could be directly targeted. The ease and simplicity of the SHRIMP procedures facilitated rapid analyses of zircon populations. In Archean quartzites of Western Australia Hadean (>4 Ga), zircons were discovered as one of the first scientific reports from SHRIMP. The Hadean zircons gave access to the early history of the Earth and represent a unique resource for determining processes operating during this period. SHRIMP has often been regarded as an instrument solely for U–Pb geochronology, but applications in stable-isotope analysis, cosmochemistry, and trace-element abundance measurements were all parts of the early development. Advances in SHRIMP design have proceeded to enable multiple collection, stable-isotope analysis through negative ion measurement, and construction of different versions of SHRIMP for specific applications. The reverse geometry SHRIMP RG design allows ultra-high mass resolution, whereas the SHRIMP SI will allow a dedicated stable-isotope instrument for light elements.
After a decade of studies and development, it is now accepted that reliable U–Th–total Pb isochron ages can be calculated for monazite using an electron microprobe at μm scale, either directly on thin sections or on separated grains mounted in polished section. The potential for determining U–Th–Pb chemical ages from other U- and Th-enriched phases has been investigated compared to chemical monazite-dating results for which individual spot-age precisions of 20 to 100 Ma can be achieved from individual spot analyses. Using isochron plots for monazite, the age homogeneity of a given population of data can be assessed and, depending upon the number of analyses (n∼50), a precision of 5 to 10 Ma can be obtained. The U content in xenotime widely varies from less than 0.1 wt.% up to 3 wt.%, but Th rarely exceeds 1 wt.%. As a consequence, the amount of radiogenic Pb produced during a given period remains significantly lower for xenotime than for monazite, leading to a lower precision (±20 Ma) on the mean ages. Xenotime, however, appears to remain as a closed system, but common Pb must be carefully checked. Furthermore, the electron-microprobe technique (EPMA) allows controlling any age discrepancy on xenotime grains as small as 10–20 μm that cannot be dated by other isotopic methods. Such xenotime ages can be useful when studying the monazite–xenotime equilibrium. The electron microprobe is not the most reliable method for dating zircon since U and Th concentrations are generally low and common Pb is not negligible. Nevertheless, the spatial resolution of EPMA coupled with isotope methods allows conclusive in situ studies about radiogenic Pb mobility and metamictization. Thorite does not seem suitable for dating with either isotope methods or EPMA because of continuous radiogenic Pb loss. Conversely, the oxide phases, thorianite and baddeleyite are robust minerals with closed systems. They are rather rare and seem to incorporate negligible common Pb, making EPMA a method of choice for dating them. For thorianite, the precision on the mean age can be similar as that obtained for monazite, or even better, while the precision for baddeleyite cannot be significantly better than 20 to 50 Ma due to the limited amount of U (∼0.1%) and the lack of Th.
Absolute ages for hydrothermal mineralization and fluid flow are critical for understanding the geological processes that concentrate metals in the Earth's crust, yet many ore deposits remain undated because suitable mineral chronometers have not been found. The origin of giant hematite ore deposits, which are hosted in Precambrian banded-iron formations (BIFs), remains contentious. Several models have been formulated based on different sources and timing for the mineralizing fluids; supergene-metamorphic, syn-orogenic, late-orogenic extensional collapse and syn-extensional. Precise geochronology of the ore offers a means of discriminating between these models. In this study, two U–Pb chronometers, xenotime and monazite, have been identified in high-grade hematite ore bodies from the Mount Tom Price mine in the Hamersley Province, northwestern Australia. Both phosphate minerals occur as inclusions within the hematite ore and as coarser crystals intergrown with martite (hematite pseudomorph after magnetite) and microplaty hematite, indicating that the xenotime and monazite precipitated during mineralization. In situ U–Pb dating by ion microprobe indicates that both phosphate minerals grew during multiple discrete events. Our results suggest that ore genesis may have commenced as early as ∼2.15 Ga, with subsequent hydrothermal remobilization and/or mineralization at ∼2.05 Ga, ∼1.84 Ga, ∼1.67 Ga, ∼1.59 Ga, ∼1.54 Ga, ∼1.48 Ga and ∼0.85 Ga. The location of the ore bodies along ancient fault systems, and the coincidence of at least some of the U–Pb phosphate dates with episodes of tectonothermal activity in the adjacent Proterozoic Capricorn Orogen, implies that fluids were channelled through major structures in the southern Pilbara Craton during discrete phases of tectonic compression and extension. Our results show that the hematite ore bodies formed at sites of repeated focussed hydrothermal fluid flow. In contrast to the aforementioned models, our results imply that iron-ore formation was probably a long-lived, multi-stage process spanning more than one billion years.