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The geochemical and geochronological implications of nanoscale trace-element clusters in rutile
Abstract
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.
... Atom probe tomography (APT) can be used to directly observe the distribution of individual atoms of elements in a mineral crystal lattice . APT has been applied to baddeleyite in previous studies, mostly using the baddeleyite mineral standard Phalaborwa (Reinhard et al., 2018) and/or crystals that underwent shock metamorphism (White et al., 2018b(White et al., , 2017. One germane finding of these studies is that while major elements are homogeneously distributed at the atomic level in baddeleyite, radiogenic Pb and incompatible trace elements (Si, Mg, Al, Yb, and Fe) can be heterogeneously distributed in nanoscale domains (White et al., 2017(White et al., , 2018a. ...
... APT has been applied to baddeleyite in previous studies, mostly using the baddeleyite mineral standard Phalaborwa (Reinhard et al., 2018) and/or crystals that underwent shock metamorphism (White et al., 2018b(White et al., , 2017. One germane finding of these studies is that while major elements are homogeneously distributed at the atomic level in baddeleyite, radiogenic Pb and incompatible trace elements (Si, Mg, Al, Yb, and Fe) can be heterogeneously distributed in nanoscale domains (White et al., 2017(White et al., , 2018a. The nanoscale U distribution in Phalaborwa baddeleyite crystals was also shown to be variable with localized zonation, and heating baddeleyite to temperatures up to 500°C does not seem to affect that distribution (White et al., 2017). ...
... One germane finding of these studies is that while major elements are homogeneously distributed at the atomic level in baddeleyite, radiogenic Pb and incompatible trace elements (Si, Mg, Al, Yb, and Fe) can be heterogeneously distributed in nanoscale domains (White et al., 2017(White et al., , 2018a. The nanoscale U distribution in Phalaborwa baddeleyite crystals was also shown to be variable with localized zonation, and heating baddeleyite to temperatures up to 500°C does not seem to affect that distribution (White et al., 2017). ...
Atom probe tomography (APT) of ²³⁸U and ²⁰⁶Pb has been applied to baddeleyite crystals from the Hart Dolerite (1791±1 Ma) and the Great Dyke of Mauritania (2732±2 Ma) in an effort to constrain the average nuclear recoil distance of ²³⁸U series daughter nuclei and correct alpha-recoil-induced Pb loss on U–Pb ages from small baddeleyite crystals. The Hart Dolerite sample showed no variations in Pb concentrations near the edge and is interpreted to represent a cleaved surface instead of the original crystal surface. The Great Dyke sample shows U zoning, and the associated ²⁰⁶Pb zoning is affected by alpha recoil, apparently adjacent to a natural grain surface. This sample also shows primary clusters of U atoms at a scale of 10 nm that contain about 40 % of the total U. 207Pb/206Pb nanogeochronology suggests that the clusters are primary in origin; however, they are too small to constrain alpha recoil distance beyond a few nanometres. To constrain alpha recoil distance, a forward-modelling approach is presented where ²⁰⁶Pb redistribution functions were determined for a range of possible recoil distances. Synthetic 206Pb/238U profiles were determined from the convolution of the observed U profile with the redistribution functions for different alpha recoil distances. These were compared to the observed 206Pb/238U profile to determine the recoil distance that gives the best fit. The observed U zoning was extrapolated to account for the full range of possible alpha recoil redistribution effects, which is larger than the 40 by 400 nm size of the APT field of view. Any reasonable extrapolation constrains the average alpha recoil distance to over 70 nm, which is much larger than previous estimates using other methods. This could be because recoil distances can be highly anisotropic within small crystal samples or because laterally non-uniform U zoning was a factor that modified the recoiled Pb distribution. APT is a potentially useful approach for determining average alpha recoil distance but requires sampling of primary smooth crystal faces with demonstrably uniform zones.
... Rutile is ubiquitously found in various types of rocks [14][15][16][17][18][19] and also forms in numerous hydrothermal deposits, such as orogenic gold deposits [20][21][22][23][24][25][26][27]. Due to its unique crystal structure, Ti ions often undergo isomorphic substitution with various trace elements of different valences [28][29][30][31], enriching rutile with trace elements and HFSEs such as W, Sn, Ta, Hf, Sb, Nb, and U [16,[32][33][34][35][36]. The content of these trace elements, especially HFSEs, and their relationship with Ti substitution in rutile can provide insights into geological evolution and alteration-mineralization processes [17,21,28,[37][38][39][40][41]. ...
... The content of these trace elements, especially HFSEs, and their relationship with Ti substitution in rutile can provide insights into geological evolution and alteration-mineralization processes [17,21,28,[37][38][39][40][41]. Furthermore, trace elements such as Mg, Al, Nb, Cr, Ta, U, Pb, Sb, W, V, Sn, Fe, Zr, REEs, and Mn can reveal information about the origin of rutile, chronology, mineralization processes, fluid characteristics, properties of surrounding rocks, and mineralization systems [24,26,30,31,35,36,[42][43][44][45]. ...
Delineating the process of hydrothermal alteration is crucial for effectively enhancing exploration strategies and better understanding the gold mineralization process. Rutile, with its capacity to accommodate a wide range of trace elements including high-field-strength elements and base metals, serves as a reliable fluid tracer in ore systems. As one of the most significant gold ore concentrations globally, Jiaodong boasts a gold reserve exceeding 5500 t. The Xincheng gold deposit is a world-class high-grade mine, boasting a proven gold reserve exceeding 200 t, and stands as one of the largest altered-type gold deposits in the vast gold province of the Jiaodong Peninsula, Eastern China. In this study, rutile (Rt1,2,3) was identified in the K-feldspar alteration, sericitization, and pyrite–sericite–quartz alteration stages of the Xincheng gold deposit in Jiaodong based on petrographic characteristics. Rt1 coexists with hydrothermal K-feldspar and quartz, while Rt2 coexists with minerals such as sericite, quartz, muscovite, and pyrite. Rt3 is widely distributed in hydrothermal veins and is primarily associated with minerals including quartz, pyrite, chalcopyrite, and sericite. Raman spectroscopy, EPMA, and LA-ICP-MS analysis were conducted to investigate the characteristics and evolution of altered hydrothermal fluids. This study indicates that the Zr vs. W and Nb/V vs. W diagrams suggest that Rt1 is of magmatic–hydrothermal origin, while Rt2 and Rt3 are of metamorphic–hydrothermal origin. Notably, the W content in Rt2 and Rt3 is significantly higher than in Rt1 (<100 ppm), suggesting a close relationship between the W content in rutile and mineralization. The three types of rutile exhibit significantly different concentrations of trace elements such as W, V, Nb, Zr, Sn, and Fe, displaying distinct bright spots and elemental zoning characteristics in backscattered electron images and surface scans. These features arise from the isomorphic substitution of Ti⁴⁺. While Rt1 exhibits no significant element exchange with the hydrothermal fluids, Rt2 and Rt3 show a stronger substitution of W, Nb, V, and Fe, indicating a gradual enrichment of F and Cl in the fluids. This process activates and transports these elements into the fluids, leading to their continuous accumulation within the system. From Rt1 to Rt3, the increasing concentrations of Fe and W, along with the negative Eu anomaly, suggest a decrease in fluid temperature and oxygen fugacity during the alteration and mineralization process. The gradual increase in the contents of REEs and high-field-strength elements such as W, V, Nb, and Sn indicates that the hydrothermal fluids are enriched in F and Cl, exhibiting weak acidity. The nature of the fluids during hydrothermal alteration is closely related to the composition of rutile, making it a promising tool for studying hydrothermal alteration and related mineralization processes.
... Atom probe tomography (APT) can be used to directly observe the distribution of individual atoms of a trace element in 70 a crystal lattice (Reddy et al., 2020). APT has been applied to baddeleyite in previous studies, mostly using the baddeleyite mineral standard Phalaborwa ) and/or crystals that underwent shock metamorphism (White et al. 2019;White et al. 2017). ...
... The mechanism by which these clusters were initially formed is unknown, particularly in light of the homogeneous distribution of U in the Hart Dolerite specimen analyzed in the same run, and in previously reported descriptions of APT carried out on Phalaborwa baddeleyite . Several mechanisms have been proposed for the formation of clusters in minerals, including: annealing of radiation damage (Peterman et al., 2021;Verberne et al., 2020); phase exsolution during cooling (Fougerouse et al., 2018); deformation Fougerouse et al., 2019); fluid 255 alteration (Joseph et al., 2023); and during growth (Fougerouse et al., 2016). The samples studied were not subjected to significant metamorphism (above greenschist facies) since crystallization (Ramsay et al., 2019;Tait et al., 2013). ...
Atom probe tomography of 238U and 206Pb has been applied to baddeleyite crystals from the Hart Dolerite (1791 ± 1 Ma) and the Great Dyke of Mauritania (2732 ± 2 Ma) in an effort to constrain the average nuclear recoil distance of U-series daughter nuclei and thereby correct U-Pb ages determined on small baddeleyite crystals for alpha-recoil loss of Pb. Both crystals were thought to expose natural crystal surfaces providing a boundary where maximum recoil loss could be observed, but both surfaces showed no adjacent variations in Pb concentrations. However, the Great Dyke sample shows U zoning and the associated 206Pb zoning is affected by alpha recoil. A forward modelling approach was used where 206Pb redistribution functions were determined for a range of possible alpha recoil distances and synthetic 206Pb/238U profiles were determined from the convolution of the observed U profile with the redistribution functions. These can be compared to the observed 206Pb/238U profile. A complication is that the 400 nm range of sampling is lower than the range of possible alpha recoil redistribution effects. In order to get a realistic match to the observed 206Pb/238U profile, it was necessary to extrapolate the observed zoning as an oscillatory pattern. This gives a best estimate for the average alpha recoil distance of about 40 nm.
... The fundamental assumption for the use of minerals as geochronometers is that U, Th and especially their decay product Pb (Pb*) are immobile within the crystal lattice over time, so that an analysed domain represents a volume in which isotopic equilibria are conserved. However, examples of Pb mobility have been reported from mineral geochronometers in polymetamorphic terrains, leading to the formation of nanophases (Seydoux-Guillaume et al. 2003Kusiak et al. 2015;Fougerouse et al. 2018) or nanoscale heterogeneous domains within the crystallographic lattice (Valley et al. 2014(Valley et al. , 2015Peterman et al. 2016;Verberne et al. 2020;Černok et al. 2021). The presence of such nanoscale features implies heterogeneity in the analytical volumes sampled in most microscale dating methods (e.g. ...
... 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). ...
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.
... Rutile is a common accessory mineral in different types of metamorphic and sedimentary rocks and, to a lesser extent, in some magmatic rocks as well as hydrothermal deposits. It contains significant amounts of trace elements (e.g., Zr, Nb, Ta, W, U, and Pb) and can be used to provide geochronologic, provenance, and mineralization information (Meinhold, 2010;Zack and Kooijman, 2017;Verberne et al., 2020;Carocci et al., 2021). In particular, rutile grains often occur in various types of gold deposits, and their U-Pb ages and trace elements have been used to reveal the age and ore-forming process of gold mineralization (Dostal et al., 2009;Pereira et al., 2019;Liu et al., 2021;Zheng et al., 2022). ...
... Previous studies have shown titanium occurring as Ti 4+ in rutile (TiO2), and many other trace elements can occur in rutile as substitutes for Ti 4+ in the rutile structure (e.g., W, Zr, Sb, and Nb) or as nanoscale clusters in crystal defects (e.g., Al, Cr, V, and Pb; Agangi et al., 2019;Majzlan et al., 2021). A recent study has shown that at the >20-μm scale, there is no significant traceelement variation in single rutile grains, and the concordant U-Pb crystallization age results show no evidence of isotopic disturbance (Verberne et al., 2020). In our study, rutile grains without observable inclusions were chosen for analyses with a 35-μm spot size. ...
Rutile grains often occur in different types of gold deposits, and their U-Pb ages have been widely used to determine the formation time of gold mineralization. However, the origin of rutile grains in the gold deposits remains controversial. In this paper, laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of U-Pb ages and trace elements on rutile grains were applied to investigate the metamorphic and hydrothermal processes of the Baiyun gold deposit (70 t Au, avg grade: 3 g/t) in the Liaodong Peninsula in the northeastern part of the North China craton. Rutile grains in the hydrothermal altered gold schist from the Baiyun deposit yielded two group ages of 1924 ± 18 and 237.0 ± 1.8 Ma, respectively. Combined with our systematic U-Pb zircon geochronological results of the ore-hosting schists and post-ore dikes, we suggest these rutile ages record a Paleoproterozoic metamorphic event and a Triassic hydrothermal gold mineralization event, respectively. The metamorphic and hydrothermal rutile grains have no obvious textural differences, but they show distinct trace element contents of Zr, W, Nb, and Ta. Combined with previous published data, we propose that high W (>1,000 ppm) and low Zr (<200 ppm) contents in rutile can be used to distinguish hydrothermal rutile from metamorphic and magmatic rutile. The newly identified ca. 237 Ma hydrothermal event is much older than the ca. 227 to 210 Ma Triassic magmatic rocks in the region, which precludes a temporal and genetic link between the Baiyun gold mineralization and the regional Mesozoic magmatism. Rather, the ca. 237 Ma gold mineralization may be associated with the Triassic orogenic metamorphism, and Baiyun is an orogenic gold deposit. The Triassic gold deposits in the northern margin of the North China craton formed by orogenesis between the Siberian craton and the North China craton. After a hiatus, the large-scale gold deposits formed during the Early Cretaceous in the North China craton due to a westward subduction of the paleo-Pacific plate beneath the craton since the Early Jurassic. Our study highlights that rutile in gold deposits may be inherited from the host rocks and/or formed by hydrothermal fluids. Distinguishing between these two different rutile generations requires a combination of in situ age dating and trace element geochemistry in petrogenetic context.
... Over the last 5 years, this technique has proven to be a powerful tool within the field of Earth sciences Saxey et al., 2018). The combination of APT with analytical techniques employing larger spot sizes (e.g., LA-ICP-MS or SIMS) allows for the correlation of geochronological data (Peterman et al., 2016;Peterman et al., 2019;Seydoux-Guillaume et al., 2019;Valley et al., 2014Valley et al., , 2015Verberne et al., 2020). Furthermore, APT can reveal a structure-chemistry relationship down to the nanoscale when applied in combination with high-resolution structural imaging techniques, such as electron backscatter diffraction (EBSD), transmission Kikuchi diffraction (TKD) or transmission electron microscopy (TEM) Fougerouse et al., 2019;Kirkland et al., 2018;Montalvo et al., 2019;Piazolo et al., 2016;Reddy et al., 2016Reddy et al., , 2020Verberne et al., 2022). ...
... Previous APT studies demonstrated that information on the active diffusion mechanisms could be derived from the APT datasets and correlative analysis. Volume diffusion in combination with U-decay effects results in the formation of Pb enriched clusters (Peterman et al., 2016(Peterman et al., , 2019Valley et al., 2014Valley et al., , 2015Verberne et al., 2020). Fast-diffusion pathways in the form of low-and high-angle and twin boundaries allow for element migration Piazolo et al., 2016); however, these also have been shown to act as trace-element traps (Verberne et al., 2022), and dislocation-impurity pair migration was suggested to lead to trace-element enrichment in titanite and pyrite (Dubosq et al., 2019). ...
The trace‐element composition of rutile is commonly used to constrain P‐T‐t‐conditions for a wide range of metamorphic systems. However, recent studies have demonstrated the redistribution of trace elements in rutile via high‐diffusivity pathways and dislocation‐impurity associations related to the formation and evolution of microstructures. Here we investigate trace‐element migration in low‐angle boundaries formed by dislocation creep in rutile within an omphacite vein of the Lago di Cignana unit (Western Alps, Italy). Zr‐in‐rutile thermometry and inclusions of quartz in rutile and of coesite in omphacite constrain the conditions of rutile deformation to around the prograde boundary from high pressure to ultra‐high pressure (~2.7 GPa) at temperatures of 500–565 °C. Crystal‐plastic deformation of a large rutile grain results in low‐angle boundaries that generate a total misorientation of ~25°. Dislocations constituting one of these low‐angle boundaries are enriched in common and uncommon trace elements, including Fe and Ca, providing evidence for the diffusion and trapping of trace elements along the dislocation cores. The role of dislocation microstructures as fast‐diffusion pathways must be evaluated when applying high‐resolution analytical procedures as compositional disturbances might lead to erroneous interpretations for Ca and Fe. In contrast, our results indicate a trapping mechanism for Zr.
... Although less precise than conventional geochronology methods, APT studies of zircon and baddeleyite also reveal sub-micrometre domains age-reset at the time of impact (White et al., 2017;White et al., 2019;Arcuri et al., 2020), indicating a link between shock metamorphism and nanoscale Pb mobility. In phases such as zircon, monazite, and rutile, Pb diffusion at the 10 s of nanometre scale has also been documented during terrestrial metamorphism (Valley et al., 2014;Valley et al., 2015;Peterman et al., 2016;Piazolo et al., 2016;Fougerouse et al., 2018;Fougerouse et al., 2019;Seydoux-Guillaume et al., 2019;Verberne et al., 2020), and experimental heating (Peterman et al., 2021), however a relationship between shock-induced twinning and nanoscale Pb mobility remains untested. ...
... However, the cluster compositions in the Vredefort specimens differ, and are marked by a stark enrichment in Pb and Si ($7 at.% combined enrichment) compared to a minor enhancement in Ca (0.8 at.% enrichment). The composition of the clusters (7.8 at.% enrichment in Pb, Si and Ca) is most consistent with them representing a separate phase Fougerouse et al., 2018;Seydoux-Guillaume et al., 2019) rather than a domain of the crystal enriched in trace elements (Valley et al., 2014;Peterman et al., 2016;Peterman et al., 2019;Arcuri et al., 2020;Verberne et al., 2020). The composition measured here is most consistent with britholite (REE,Ca,Pb) 5 ((Si, P)O 4 ) 3 OH, with a combined REE + Ca + Pb composition of 25.4 at.% and a Si + P composition of 16.1 at.%. ...
To test the potential of deformation twins to record the age of impact events, micrometre-scale size mechanical twins in shocked monazite grains from three impact structures were analyzed by atom probe tomography (APT). Shocked monazite from Vredefort (South Africa; ∼300 km crater diameter), Araguainha (Brazil; ∼40 km diameter), and Woodleigh (Australia; 60 to 120 km diameter) were studied, all from rocks which experienced pressures of ∼30 GPa or higher, but each with a different post-impact thermal history. The Vredefort sample is a thermally recrystallised foliated felsic gneiss and the Araguainha sample is an impact melt-bearing bedrock. Both Vredefort and Araguainha samples record temperatures > 900 °C, whereas the Woodleigh sample is a paragneiss that experienced lower temperature conditions (350 - 500 °C). A combined ²⁰⁸Pb/²³²Th age for common {12¯2¯} twins and shock-specific (1¯01) twins in Vredefort monazite was defined at 1979 ± 150 Ma, consistent with the accepted impact age of ∼2020 Ma. Irrational η1 [1¯1¯0] shock-specific twins in Araguainha monazite yielded a 260 ± 48 Ma age, also consistent with the accepted 250-260 Ma impact age. However, the age of a common (001) twin in Araguainha monazite is 510 ± 87 Ma, the pre-impact age of igneous crystallisation. These results are explained by the occurrence of common (001) twins in tectonic deformation settings, in contrast to the (1¯01) and irrational η1 [1¯1¯0] twins, which have only been documented in shock-deformed rocks. In Woodleigh monazite, APT age data for all monazite twins [(001), (1¯01), newly identified 102°/<4¯23> twin], and host monazite are within uncertainty at 1048 ± 91 Ma, which is interpreted as a pre-impact age of regional metamorphism. We therefore are able to further constrain the poorly known age of the Woodleigh impact to < 1048 ± 91 Ma. These results provide evidence that Pb is expelled from monazite during shock twin formation at high temperature (Vredefort and Araguainha), and also that Pb is not mobilised during twinning at lower temperature (Woodleigh). Our study suggests that twins formed during shock metamorphism have the potential to record the age of the impact event in target rocks that are sufficiently heated during the cratering process.
... These microstructural features include precipitates or clusters, solid solutions and/or ordered structures, grain/phase boundaries, dislocations and point defects/vacancies. Numerous reviews detail the applications of APT to characterize, for example, aluminium alloys 161,195 , nickel-based superalloys 196-198 , intermetallics 199 , steel 200,201 , highentropy alloys 202 , nuclear materials [203][204][205] and, beyond metallurgical systems, thermoelectric materials 206 , semiconducting devices for microelectronics [207][208][209] or light emission 210 and geological materials 211,212 . Here, we give some details of what information APT provides in these various contexts. ...
... Geochronology studies reveal that deformational and metamorphic events can allow the diffusion of radiogenic isotopes, leading to local changes in the isotopic ratios. The diffused atoms form nanoscale clusters 212,300,301 , and the characterization of these clusters has the potential to be used to resolve the timing of the cluster-forming geological events. These studies have improved our understanding of the mechanisms for parent-daughter isotopic mobility and yielded important information about the formation and evolution of the Earth's crust 302 . ...
Atom probe tomography (APT) provides three-dimensional compositional mapping with sub-nanometre resolution. The sensitivity of APT is in the range of parts per million for all elements, including light elements such as hydrogen, carbon or lithium, enabling unique insights into the composition of performance-enhancing or lifetime-limiting microstructural features and making APT ideally suited to complement electron-based or X-ray-based microscopies and spectroscopies. Here, we provide an introductory overview of APT ranging from its inception as an evolution of field ion microscopy to the most recent developments in specimen preparation, including for nanomaterials. We touch on data reconstruction, analysis and various applications, including in the geosciences and the burgeoning biological sciences. We review the underpinnings of APT performance and discuss both strengths and limitations of APT, including how the community can improve on current shortcomings. Finally, we look forwards to true atomic-scale tomography with the ability to measure the isotopic identity and spatial coordinates of every atom in an ever wider range of materials through new specimen preparation routes, novel laser pulsing and detector technologies, and full interoperability with complementary microscopy techniques. This Primer on atom probe tomography introduces the fundamentals of the technique and its experimental set-up, describes recent developments in specimen preparation, highlights aspects of data reconstruction and analysis, and showcases various applications of atom probe tomography in the materials sciences, geosciences and biological sciences.
... Rutile hosts a variety of trace elements including Nb, Ta, Zr, Hf, V, Cr, W, Sn, U, and Sb (Meinhold, 2010;Plavsa et al., 2018), that offer the potential for distinguishing between different types of rutile. Recent investigations have demonstrated that coarse-grained individual rutile grains (>20 μm) exhibit insignificant trace element variations, and the consistent U-Pb crystallization ages suggest minimal isotopic disturbance (Verberne et al., 2020). In our analysis, rutile grains ranging from 1 mm to 2 cm in size, devoid of discernible inclusions and composi-tional zoning, were examined. ...
... In APT data of minerals, clusters of trace elements have been interpreted to represent enriched crystal defects(Valley et al. 2014; Peterman et al. 2016Peterman et al. , 2021 La Fontaine et al. 2017;White et al. 2017;Fougerouse et al. 2019;Arcuri et al. 2020;Verberne et al. 2020;Turuani et al. 2023), nanoparticles(Fougerouse et al. 2016(Fougerouse et al. , 2018(Fougerouse et al. , 2021(Fougerouse et al. , 2023Seydoux-Guillaume et al. 2019; Joseph et al. 2023), or fluid inclusions ...
In natural corundum, a strong geochemical correlation is sometimes observed between Be and heavy high field strength elements (HHFSEs) such as Nb, Ta and W, and it has been hypothesized that trace elements are hosted in primary inclusions. However, no known mineral enriched in both Be and HHFSEs stable at these geological conditions can explain this correlation. To understand how Be and HHFSEs are distributed in natural corundum down to the atomic scale, two natural Be-bearing sapphire crystals from Afghanistan and Nigeria are studied using laser ablation inductively coupled plasma and time-of-flights secondary ion mass spectrometry, atom probe tomography and transmission electron microscopy. In addition to common trace elements such as Mg, Ti, and Fe, Be and W are detected in the metamorphic sapphire from Afghanistan, whereas Be, Nb and Ta are detected in the magmatic sapphire from Nigeria. Nanoclustering in both samples shows fractionation of Be and high field strength elements (including Ti) by atomic mass, suggesting a secondary process controlled by solid-state diffusion. The homogeneously distributed W and the secondary nano-precipitates bearing Nb and Ta indicates that HHFSEs can be incorporated into the corundum structure during crystallization, most likely through preferred adsorption on the growth surface. The strong correlation between Be and HHFSEs across the growth zones is probably due to Be being attracted by HHFSEs to partially balance the charge when incorporated into the corundum structure. The enrichment of high field strength elements by growth kinetics may result in supersaturated concentrations during crystallization, allowing them to precipitate out when the host corundum is heated above its formation temperature by basaltic magma. Comparison with previous transmission electron microscope studies suggests the same process for incorporating Be and HHFSEs also applies to other natural corundums from different localities.
... Several branches of Earth Sciences have demonstrated the importance of spatial resolution on a microscopic scale. For example, geochemistry and geochronology applications have benefited from point-wise analyses and compositional maps, allowing significant advances in the understanding of igneous, metamorphic and sedimentary processes (Barnes et al., 2019;Davidson et al., 2007;Verberne et al., 2020). Classical paleomagnetic techniques, on the other hand, consist of analyzing bulk samples, where the magnetic signal of a single specimen is the result of the sum of moments of a large assembly of ferromagnetic grains (Dunlop & Özdemir, 1997). ...
Paleomagnetic data is collected from bulk samples, containing a mixture of stable and unstable magnetic particles. Recently, magnetic microscopy techniques have allowed the examination of individual magnetic grains. However, accurately determining the magnetic moments of these grains is difficult and time‐consuming due to the inherent ambiguity of the data and the large number of grains in each image. Here we introduce a fast and semi‐automated algorithm that estimates the position and magnetization of dipolar sources solely based on the magnetic microscopy data. The algorithm follows a three‐step process: (a) employ image processing techniques to identify and isolate data windows for each magnetic source; (b) use Euler Deconvolution to estimate the position of each source; (c) solve a linear inverse problem to estimate the dipole moment of each source. To validate the algorithm, we conducted synthetic data tests, including varying particle concentrations and non‐dipolarity. The tests show that our method is able to accurately recover the position and dipole moment of particles that are at least 15 μm apart for a source‐sensor separation of 5 μm. For grain concentrations of 6,250 grains/mm³, our method is able to detect over 60% of the particles present in the data. We applied the method to real data of a speleothem sample, where it accurately retrieved the expected directions induced in the sample. The semi‐automated nature of our algorithm, combined with its low processing cost and ability to determine the magnetic moments of numerous particles, represents a significant advancement in facilitating paleomagnetic applications of magnetic microscopy.
... %. Such a composition suggests that the cluster is likely a nanoscale inclusion of a different phase (Fougerouse et al., , 2018(Fougerouse et al., , 2021aSeydoux-Guillaume et al., 2019), rather than a trace element enriched domain of xenotime, as has been observed in zircon and rutile (Valley et al., 2014;Peterman et al., 2016Peterman et al., , 2019Verberne et al., 2020). The chemical composition of the clusters is most consistent with the mineral apatite Ca 5 (PO 4 ) 3 (OH,F,Cl), in which Pb can substitute for Ca (Pan and Fleet, 2002). ...
... Rutile has attracted significant attention in recent years, since it can be used in metamorphic studies to calculate crystallization temperatures (Zack et al. 2004a;Watson et al. 2006;Tomkins et al. 2007;Ewing et al. 2013), in provenance studies to differentiate between mafic and pelitic sources (Zack et al. 2004b;Triebold et al. 2007Triebold et al. , 2012, to deduce the tectonic setting of sedimentary basins (Pereira et al. 2020) and in geochronological studies to determine ages of metamorphic events (Mezger et al. 1989;Zack et al. 2011;Bracciali et al. 2013;Ewing et al. 2015;Verberne et al. 2020). Furthermore, rutile is a common accessory mineral in metamorphic and sedimentary rocks. ...
Low concentrations of Na, Ca, K, Fe, Mg and Al in quartzite commonly prevent the crystallisation of index metamorphic minerals, inhibiting obtaining thermobarometric calculations. Quartzite typically contains quartz, zircon and rutile; therefore, single-element thermometers, such as Zr-in-rutile, may be applied. We investigate changes in trace-element composition of rutile from quartzite through increasing metamorphic conditions. Studied samples derive from a quartzite package (Luminárias Nappe, Minas Gerais, Brazil), where previous thermobarometric constraints on metapelites showed an increasing metamorphic grade southwards, from high-pressure lower amphibolite facies (580 °C; 0.9 GPa) to eclogite facies (630 °C; 1.4 GPa). Rutile from the lower-grade facies samples show a large spread in Zr concentrations, with the highest values corresponding to temperatures estimates higher than metamorphic conditions affecting those units, and thus interpreted as inherited detrital signatures. A narrower spread in Zr concentration is observed in rutile grains from the higher-grade, and estimated Zr-in-rutile temperatures agree with previous thermobarometric constraints. Therefore, we show that at 630 °C, Zr contents in detrital rutile from quartzites re-equilibrate. The comparison between the quartzite- and metapelite-hosting rutile grains from the same area shows that the resetting of the geothermometer in the latter seems to occur at slightly lower temperatures (∼50 ˚C lower).
Supplementary material at https://doi.org/10.6084/m9.figshare.c.6793887
... Several branches of Earth Sciences have demonstrated the importance of spatial resolution on a microscopic scale. For example, geochemistry and geochronology applications have benefited from point-wise analyses and compositional maps, allowing significant advances in the understanding of igneous, metamorphic and sedimentary processes (Barnes et al., 2019;Davidson et al., 2007;Verberne et al., 2020). Classical paleomagnetic techniques, on the other hand, consist of analyzing bulk samples, where the magnetic signal of a single specimen is the result of the sum of moments of a large assembly of ferromagnetic grains (Dunlop and Özdemir, 1997). ...
Paleomagnetic data are usually obtained from whole cylindric samples, where the signal results from the sum of magnetic moments from hundreds of thousands to millions of magnetic particles within the sample volume. This usually includes both stable and unstable remanence carriers.Recently, magnetic microscopy techniques allowed the investigation of individual grains by directly imaging their magnetic field. However, the determination of the magnetic moments of individual grains is hindered by the intrinsic ambiguity in the inversion of potential field data, as well as by the large number of grains found in any one microscopy image. We present a fast, semi-automated algorithm capable of estimating the position and magnetization of each ferromagnetic (l.s) source using only the magnetic microscopy data. Our algorithm works in three steps: (i) we first apply image processing techniques to identify and isolate data window boundaries for each source; (ii) with these window boundaries, the position of the sources is estimated using Euler deconvolution; and finally (iii) using the position information, the algorithm is able to estimate the magnetic dipole moment direction and intensity for each source through an overdetermined linear inverse problem using a dipolar approximation. The method does not require any type of additional information about the sample or the sources. Sensitivity tests were run to estimate the stability of our routine to the depth of particles, signal-to-noise ratio, and non-dipolarity of the sources. Tests with simple synthetic data show the high effectiveness of the methodology for recovering the position and magnetic information for both dipolar and non-dipolar sources. More complex synthetic data including over 100 different magnetic particles were devised to emulate real rock data. Results obtained on these data also show the feasibility and robustness of the algorithm to semi-automatically estimate the position and magnetic moment of a large number of particles. This is further confirmed through an application to real data in which we are able to retrieve the expected bimodal isothermal remanent directions that were induced in the sample. Given its semi-automatic nature, its low processing cost, and the possibility of simultaneous inversion of the magnetic moment of a great number of magnetic particles, the methodology here proposed is a step forward in enabling paleomagnetic applications of magnetic microscopy.
... Similar to zircon, in rutile it is thought to be transient radiation damaged sites facilitates Pb and other trace element mobility. During high temperature metamorphism the trace elements gets trapped in the core of the damaged site of the crystal lattice which has not undergone annealing (Verberne et al., 2020b). In the case of xenotime, no metamict xenotime has been observed in nature (Harrison et al., 2019). ...
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.
... Rutile is one of three polymorphs, which also include anatase and brookite (Plavsa et al. 2018;Adlakha et al. 2020). In particular, the remobilization of trace elements after the formation of rutile can affect the information on the nature and timing of geological events recorded in rutile (Pe-Piper et al. 2019;Agangi et al. 2020;Verberne et al. 2020). In addition, the TiO 2 mineral polymorphs found in the gold deposits are difficult to distinguish by their geochemical compositions. ...
The Tianshan orogenic belt hosts several world-class gold deposits and is one of the largest gold provinces on Earth. The Katbasu Au-Cu deposit in the Chinese Western Tianshan is hosted in a granite intrusion. Previous researchers have shown that the main gold ores formed much later than the ore-hosting granite. However, the formation age of Cu mineralization and its possible link to Au mineralization remain poorly understood. This paper reports detailed mineralogical studies, combined with zircon U-Pb, in situ hydrothermal monazite as well as rutile U-Pb ages to constrain the timing of Cu mineralization and its possible link to Au mineralization. The two main ore types in the Katbasu deposit include Cu-Au ores with pyrite-chalcopyrite veins that crosscut the granite and Au ores with massive pyrite and quartz as the main minerals. The Cu-Au ores are spatially associated with diorite that intruded the granite, and they are overprinted by massive gold ores. Detailed mineralogical studies show that chalcopyrite is the main Cu-bearing mineral in the Cu-Au ores, and it is closely associated with some native gold, monazite, and rutile. Secondary ion mass spectrometer (SIMS) U-Pb dating of zircon grains from the ore-hosting granite and mafic enclave yielded concordant ages of 354.1 ± 1.6 and 355.8 ± 1.7 Ma, respectively. The diorite that intruded the granite has a zircon U-Pb age of 352.0 ± 3.2 Ma. The trace element compositions of the monazite suggest they were formed by hydrothermal fluids rather than inherited from the ore-hosting granite. Hydrothermal monazite coexisting with chalcopyrite and native gold yielded a concordant age of 348.7 ± 2.3 Ma, and the W-rich hydrothermal rutile grains associated with the chalcopyrite yielded a U-Pb age of 345 ± 27 Ma, indicating an early Cu-Au mineralization event prior to the major Au mineralization (ca. 323-311 Ma). The formation time of early Cu-Au mineralization is consistent with the emplacement age of the diorite and may be of magmatic-hydrothermal origin, whereas the main Au has no genetic associations with magmatic rocks in the ore district and may belong to the orogenic type. Monazite geochronology provided a more reliable age constraint than rutile in the Katbasu Au-Cu deposit, and we suggest hydrothermal monazite has advantages over rutile in dating the complex mineralization ages of gold deposits.
... The composition of rutile is commonly used as a petrogenetic indicator and geochronometer (Zack & Kooijman, 2017;Agangi et al., 2019). APT analyses of rutile revealed the presence of nanoscale clusters enriched in radiogenic Pb with implications for the modeling of trace element diffusion (Verberne et al., 2019;Verberne et al., 2020). A crystal of rutile reference material from the Windmill Hill Quartzite (WHQ; Clark et al., 2000) was prepared in an epoxy mount and polished to expose the surface of the mineral. ...
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.
The causes of U-Pb isotopic discordance documented by Paquette et al. (2004) in monazite grains from the ultra-high temperature (UHT) granulite of the Andriamena unit of Madagascar are re-evaluated in the light of nanoscale crystal-chemical characterization utilising Atom Probe Tomography (APT) and state-of-the-art Scanning Transmission Electron Microscopy (STEM). APT provides isotopic (²⁰⁸Pb/²³²Th) dating and information on the chemical segregation of trace elements (e.g., Pb) in monazite at nanoscale. Latest generation of STEM allows complementary high-resolution chemical and structural characterization at nanoscale. In situ isotopic U–Pb dating with Secondary Ion Mass Spectrometry (SIMS) on 25 monazite grains and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) on zircon have been employed to refine the age spectra. Monazite and zircon grains located in quartz and garnet formed with the peak UHT metamorphic assemblage, which is partially overprinted by retrograde coronitic textures. Zircon grains hosted in garnet and in quartz yield concordant U–Pb ages at 2758 ± 28 Ma and 2609 ± 51 Ma, respectively whereas monazite grains hosted in quartz and garnet show a discordant Pb* loss trend on the Concordia diagram recording disturbance at 1053 ± 246 Ma that is not seen by the zircon, underlining the importance of combining the use of monazite and zircon to understand the history of polymetamorphic rocks. The Pb*-loss trend of monazite is related to petrographic position, with less Pb* lost from monazite hosted in quartz and garnet than monazite hosted in the coronitic reaction texture domains. STEM shows that the garnet- and quartz-hosted monazite grains contain more Pb-bearing nanophases than monazite grains located in the coronitic textures. An inverse correlation between the number of Pb-bearing nanophases and the percentage of Pb*-loss in monazite grains demonstrates that Pb* is retained in the grain in the form of nanophases. The formation of Pb-bearing nanophases limits Pb*-loss at the grain scale and therefore allows the preservation of early events. ²⁰⁸Pb/²³²Th ratios obtained with APT in monazite located in quartz and garnet and excluding Pb*-bearing nanophases indicate a mean age of 1059 ± 129 Ma corresponding to a disturbance event hitherto undetected in the geochronological record of the Andriamena unit. Thus, geochronology with APT allows access to information and the definition of events that may be blurred or obscured when examined at lower spatial resolution.
Atom probe tomography (APT) of 238U and 206Pb has been applied to baddeleyite crystals from the Hart Dolerite (1791 ± 1 Ma) and the Great Dyke of Mauritania (2732 ± 2 Ma) in an effort to map U and Pb concentration at the nanometre scale. The purpose was to constrain the average nuclear recoil distance of 238U-series daughter nuclei in order to correct U-Pb ages determined on small baddeleyite crystals for alpha-recoil loss of Pb. Both crystals were thought to expose natural crystal surfaces providing a boundary where maximum effects of recoil loss could be observed. The Hart Dolerite sample showed no variations in Pb concentrations near the edge. The Great Dyke sample shows U zoning and the associated 206Pb zoning is affected by alpha recoil, apparently adjacent to a natural grain surface. The sample also shows 10 nm-scale apparently primary clusters of U atoms that contain about 40 % of the U. These are too small to constrain alpha recoil distance beyond a few nm but are apparently primary and their formation mechanism poses a dilemma. To constrain alpha recoil distance, a forward modelling approach is presented where 206Pb redistribution functions were determined for a range of possible distances and synthetic 206Pb/238U profiles were determined from the convolution of the observed U profile with the redistribution functions that were compared to the observed 206Pb/238U profile. A complication is the fact that the 40 by 400 nm size of the sample is lower than the range of possible alpha recoil redistribution effects so it was necessary to extrapolate the observed U zoning. An oscillatory pattern gives the best fit to the observed profile but any reasonable extrapolation constrains the average alpha recoil distance to be close to 80–90 nm, which is much larger than previous estimates using other methods. Either recoil distances can be highly anisotropic within small crystal samples or surface roughness was a factor that modified the recoiled Pb distribution. APT is a potentially useful approach to determining average alpha recoil distance but requires sampling of primary, smooth crystal faces.
Accessory minerals like zircon, rutile and monazite are routinely studied to inform about the timing and nature of geological processes. These studies are underpinned by our understanding of the transfer processes of trace elements and the assumption that the isotopic systems remain undisturbed. However, the presence of microstructures or Pb-bearing phases in minerals can lead to the alteration of the Pb isotopic composition. To gain insight into the relationship between Pb isotopic alterations from inclusions and microstructures, this study focused on inclusions from an ultra-high-temperature metamorphic rutile. The studied inclusions are submicrometer monazites, a common mineral rich in Pb but normally not present in rutile. The sample is sourced from Mt. Hardy, Napier Complex, East Antarctica, an ultra-high-temperature (UHT) metamorphic terrane. By applying correlative analytical techniques, including electron backscatter diffraction mapping, transmission electron microscopy (TEM), and atom probe tomography, it is shown that monazite inclusions are often in contact with low-angle boundaries and yield no preferred orientation. TEM analysis shows the monazite core has a mottled texture due to the presence of radiation damage and nanoclusters associated with the radiation damage defects that are rich in U, Pb, and Ca. Some monazites exhibit a core-rim structure. The rim yields clusters composed of Ca- and Li-phosphate that enclose Pb nanoclusters that are only present in small amounts compared to the core, with Pb likely diffused into the rutile-monazite interface. These textures are the result of two stages of Pb mobility. Initial Pb segregation was driven by volume diffusion during UHT metamorphism (2500 Ma). The second stage is a stress-induced recrystallization during exhumation, leading to recrystallization of the monazite rim and trace element transport. The isotopic signature of Pb trapped within the rutile-monazite interface constrains the timing of Pb mobility to ca. 550 Ma.
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.
The discordance of U–Th–Pb isotopic systems in geochronometers, and how such data are interpreted, are still major issues in the geosciences. To better understand the disturbance of isotopic systems, and how this impacts the derivation of geologically-meaningful ages, previously studied discordant monazite from the ultrahigh temperature paragneiss of the Archean Napier Complex (Antarctica) have been investigated. Monazite grains were characterized from the micro to the nanoscale using an analytical workflow comprising laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), secondary-ion mass spectrometry (SIMS), electron microprobe (EMP), transmission electron microscopy (TEM) and atom probe tomography (APT). Results reveal that the least discordant monazite, hosted in garnet and rutilated quartz, contain a large number of small Pb-bearing nanocrystals (Ø∼ 50 nm) while the most discordant monazite, hosted in the quartzo-feldspathic matrix, contain a smaller number of Pb-bearing nanocrystals bigger in size (Ø∼ 50 to 500 nm). The degree of the discordance, which was previously correlated with textural position is mechanistically related to the partial retention of radiogenic Pb (Pb⁎) in distinct Pb⁎-bearing nanocrystals (e.g. PbS) within the monazite grains. In-situ dating (U–Pb systems with LA-ICP-MS and SIMS), and isotopic information obtained by using APT (²⁰⁷Pb/²⁰⁶Pb isotopic signature of galena and ²⁰⁸Pb/²³²Th ages of the monazite matrix) allow the timing of Pb-disturbance and mobility to be constrained. Results show that monazite grains crystallized at ca. 2.44 Ga and were affected by two episodes of Pb⁎ mobility. The first episode (t1) at ca. 1.05 Ga, led to crystallization of a first generation of Pb⁎-bearing nanocrystals and a complete resetting of the monazite matrix at the nanoscale. The second episode (t2) at ca. 0.55 Ga was associated with the crystallization of a second generation of Pb⁎-bearing nanocrystals with a ²⁰⁷Pb/²⁰⁶Pb signature indicating a mixing of two Pb⁎ components: a component from the monazite matrix and remobilized Pb⁎ from the first generation of Pb⁎-bearing nanocrystals. This second event is characterized by a more localized resetting of the monazite matrix at the nanoscale compared to the t1 event. These results indicate the potential of nanoscale studies of Pb-rich nanocrystals within monazite to yield important details of the themal history of complex metamorphic terranes.
Element mobility is a critical component in all geological processes and understanding the mechanisms responsible for element mobility in minerals is a fundamental requirement for many geochemical and geochronological applications. Volume diffusion of elements is a commonly assumed process. However, linear defects (dislocations) are an essential component of the high-temperature creep of minerals. These defects are commonly inferred to form fast-diffusion pathways along which trace elements can more rapidly migrate. In contrast, dislocations in minerals are also energetically favourable sites of trace element segregation, which counters the notion that they enhance bulk diffusion rates by a pipe diffusion mechanism. In this paper we characterize the trace-element composition of dislocations on twin boundaries in rutile by combining atom probe tomography with transmission electron microscopy. First, morphology and correlative microstructural data are used to demonstrate that the linear compositional features in the atom probe tomography dataset represent dislocations. Assessment of dislocation composition indicates that segregation is trace element specific. The data show that dislocations in rutile act as both, fast-diffusion pathway and trace-element traps which potentially leads to erroneous estimations of the composition.
Xenotime (YPO4) is an accessory phase common in low to high‐temperature geological environments. Xenotime is an established geochronometer, though its small size, low modal abundance, and textural complexity make it more difficult to analyse with traditional techniques but makes a prime candidate for nano‐scale analysis. In this study, we develop an atom probe tomography (APT) protocol to determine the 206Pb/238U and 207Pb/206Pb ages of micro‐scale xenotime crystals with analytical volumes four to six orders of magnitude smaller than typical geochronology techniques. A linear correlation between the 206Pb/238U fractionation and 238UO22+/238UO2+ was used to correct for the atom probe instrument parameters variability between specimens. For 207Pb/206Pb ages we employed two methods of background correction owing to the 206Pb2+ thermal tail contribution to the 207Pb2+ counts: A constant background correction for the younger (~ 1000 Ma) Y1 reference material and a variable correction of background for Archaean age reference material xtc to correct for the thermal tail influence. This contribution also proposes strategies for optimization of xenotime analysis using atom probe tomography and permits us to explore the various geological problems in the nano‐scale realm. This methodology potentially allows determining the age of small xenotime crystals in sedimentary rocks, low metamorphic grade settings, and deformation‐microstructures.
Atom probe tomography (APT) is emerging as a key nanogeochemistry technique for diverse geoscience applications. Estimating stoichiometric mineral compositions is particularly challenging for features at nanoscale. APT provides reliable measurements for metallic systems but the reliability for oxides is problematic, notably due to oxygen deficit. Here, we use laser‐assisted APT to compare results for spinel and garnet crystals. APT compositional results were compared with those by electron microprobe to determine the possible APT analytical inaccuracy. Extensive data processing was accomplished, including correlation histograms, 2D ion distribution maps and 1D elements concentration profiles, to disclose the possible mechanisms leading to mineral stoichiometry biases. Multiple events and neutral molecules formation are probable the main processes responsible for atom deficit. In particular, the amount of the same isotope‐same charge state ion pairs correlated with aluminium and oxygen deficits suggests that the co‐evaporation in a dead space‐dead time window could lead to a significant decrease of detected ions. Also, molecular species dissociation and direct current evaporation could partially account for further atom loss. Overall, better APT compositional estimation was obtained for spinel, which has lesser variation in lattice sites and greater overall lattice symmetry, higher thermal conductivity, and lower band gap compared with garnet.
As a common constituent of metamorphic assemblages, rutile provides constraints on the timing and conditions of rock transformation at high resolution. However, very little is known about the links between trace element mobility and rutile microstructures that result from syn‐metamorphic deformation. To address this issue, here we combine in situ LA‐ICP‐MS and SHRIMP trace element data with EBSD microstructural analyses to investigate the links between rutile lattice distortions and Zr and U–Pb systematics. Furthermore, we apply this integrated approach to constrain further the temperature and timing of amphibolite‐facies metamorphism and deformation in the Bergen Arcs of southwestern Norway. In outcrop, the formation of porphyroblastic rutile in dynamically hydrated leucocratic domains of otherwise rutile‐poor statically‐hydrated amphibolite provides key contextual information on both the ambient conditions of hydration and deformation and the composition of the reactive fluid. Rutile in amphibolite recorded ambient metamorphic temperatures of ~ 590–730°C during static hydration of the granulitic precursor. In contrast, rutile from leucocratic domains in the directly adjacent shear zone indicates that deformation was accompanied by a localized increase in temperature. These higher temperatures are recorded in strain‐free rutile (~600–860°C) and by Zr concentration measurements on low‐angle boundaries and shear bands (620–820°C). In addition, we also observe slight depletions of Zr and U along rutile low‐angle boundaries relative to strain‐free areas in deformed grains from the shear zone. This indicates that crystal‐plastic deformation facilitated the compositional re‐equilibration of rutile upon cooling to slightly below the peak temperature of deformation. Cessation of deformation at mid‐crustal conditions near ~ 600°C is recorded by late stage growth of small (< 150 µm) rutile in the high strain zones. U–Pb age data obtained from the strain‐free and distorted rutile grains cluster in distinct populations of 437.4 ± 2.7 Ma and c. 405–410 Ma, respectively. These different ages are interpreted to reflect the difference in closure for thermally‐induced Pb diffusion between undeformed and deformed rutile during post‐deformation exhumation and cooling. Thus, our results provide a reconstruction of the thermochronological history of the amphibolite‐facies rocks of the Lindås Nappe and highlight the importance of integration of microstructural data during application of thermometers and geochronometers.
We performed experiments with thin film diffusion couples to simultaneously measure diffusion coefficients of Zr, Hf, Nb and Ta parallel to the a- and c-axes of synthetic rutile in a gas mixing furnace at controlled oxygen fugacity at temperatures between 800 and 1100∘C. Depth profiles of the diffusion couples were measured using secondary-ion mass spectrometry. Some of the diffusion profiles show a concentration dependence, which indicates different diffusion mechanisms above and below a particular trace-element concentration level (∼1000μg/g). The diffusion coefficients for the mechanism dominant at high-concentration levels are approximately two orders of magnitude smaller than for the low-concentration mechanism. Below the critical concentration the diffusion coefficient is constant, as consistently shown in all of the experiments. For this diffusion coefficient we have found that DZr∼ DNb> DHf> > DTa, and diffusion is isotropic for the four elements at all investigated T and fO 2 conditions. At 1000∘C for log fO 2< FMQ+1, the diffusion coefficients decrease with increasing oxygen fugacity where D is proportional to fO2n with exponents n≈ - 0.25 for Zr and Hf and n≈ - 0.30 for Nb and Ta. Diffusivites of Nb and Ta strongly differ from each other at all investigated conditions, thus providing the potential to fractionate these geochemical twins, as suggested earlier. The present data and literature data for Zr and Ti self diffusion are interpreted and predicted based on published quantitative point defect models. Two end-member diffusion mechanisms were identified for impurity diffusion of Zr: (i) an interstitialcy mechanism involving Ti 3 + on interstitial sites, which is dominant at approximately log fO 2< FMQ+2; (ii) a vacancy mechanism involving Ti vacancies, which is dominant at approximately log fO 2> FMQ+2. The point defect calculations also explain the observed effects of heterovalent substitutions, such as Nb 5 + for Ti 4 + at high concentration levels for changes in the diffusion mechanism and hence diffusion rates. In the case of rutile, this concentration effect becomes much more sensitive to the substitution level at lower temperature. In natural rutile penta- and hexavalent cations may largely be charge balanced by mono-, di- and trivalent cations, such that the doping effect on diffusion may be reduced or may even be reversed. The Arrhenius relationships established here may therefore not be directly applicable to natural rutile. We obtained the following Arrhenius relationships (with diffusion coefficients D in m ²/ s , fO 2 in Pascal and T in Kelvin), which are only applicable for log fO 2< FMQ+2: logDZr=(-0.40±0.47)+(-0.253±0.019)logfO210-7-414±11kJ/molRTln10logDHf=(-0.08±0.63)+(-0.266±0.023)logfO210-7-428±15kJ/molRTln10logDNb=(-0.19±0.36)+(-0.294±0.014)logfO210-7-421±9kJ/molRTln10logDTa=(0.45±0.73)+(-0.304±0.015)logfO210-7-463±18kJ/molRTln10.
Volume diffusion of Pb occurs over micron length scales in apatite and rutile at temperatures relevant to the evolution of the middle and lower crust. Continuous thermal history information can be resolved from inversion of intracrystalline U-Pb date profiles preserved within individual grains. Recent developments in microbeam analysis permit rapid measurement of these age profiles at sub-micron spatial resolution, thus heralding a new era for U-Pb thermochronology. Here, we review the theoretical, experimental and empirical basis for U-Pb thermochronology and show that rutile, in particular, presents an exceptional opportunity to obtain high-resolution thermal history information from the deep crust. We present a Bayesian procedure that is well suited to the inversion of U-Pb date profile datasets and balances computational efficiency with a full search of thermal history coordinate space. Complications relevant to accurate application of U-Pb thermochronology are discussed i) theoretically and ii) empirically, using a rutile U-Pb dataset from the lower crust of the Grenville orogeny. Purely diffusive date profiles are shown to be the exception to uniform, or step-like, young profiles, suggesting that processes other than thermally-activated volume diffusion may control U-Pb systematics in rutile residing in the lower crust. However, the data obtained from apparent diffusive profiles systematically match cooling histories inferred from other thermochronometers. This result emphasises the importance of integrating microtextural observations, and trace-element concentrations, with U-Pb age data in order to discriminate between diffusive and non-diffusive Pb transport mechanisms in accessory phases and thus minimize the risk of generating spurious thermal histories.
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
Hadean (≥4.0 Ga) zircon grains provide the only direct record of the first half-billion years of Earth's history. Determining accurate and precise crystallization ages of these ancient zircons is a prerequisite for any interpretation of crustal evolution, surface environment, and geodynamics on the early Earth, but this may be compromised by mobilization of radiogenic Pb due to subsequent thermal overprinting. Here we report a detrital zircon from the Jack Hills (Western Australia) with 4486-4425 Ma concordant ion microprobe ages that yield a concordia age of 4463 ± 17 Ma (2σ), the oldest zircon age recorded from Earth. However, scanning ion imaging reveals that this > 4.4 Ga apparent age resulted from incorporation of micrometer-scale patches of unsupported radiogenic Pb with extremely high ²⁰⁷Pb/²⁰⁶Pb ratios and > 4.5 Ga ²⁰⁷Pb/²⁰⁶Pb ages. Isotopic modeling demonstrates that these patches likely resulted from redistribution of radiogenic Pb in a ca. 4.3 Ga zircon during a ca. 3.8 Ga or older event. This highlights that even a concordia age can be spurious and should be carefully evaluated before being interpreted as the crystallization age of ancient zircon.
The geochemistry of rutile (TiO2) has recently found its use in mineral exploration with some studies reporting anomalous concentrations of Fe, W, V, Sn and Sb in rutile associated with mineralized ore systems. However, the use of rutile as a prospecting tool is likely to be complicated by the systematic changes in trace element composition with TiO2 polymorph type (anatase, brookite and rutile).Here we present TiO2 trace element and U-Pb geochronological data from the mineralized and barren portions of the Palaeoproterozoic Moorarie Supersuite and (Capricorn Orogen, Western Australia), with a focus on the Minnie Creek Molybdenum Prospect in the northern part of the Gascoyne Province. The barren samples contain all three TiO2 polymorphs (anatase, brookite and rutile). Textures suggest anatase and brookite may have formed during low-T metamorphism, either through replacement of previous rutile grains or titaniferous minerals. Rutile grains from barren samples yield variable U-Pb ages (ca. 3.0-2.2Ga) as well as variable textures and chemical compositions suggesting detrital origins, thus most likely representing metasedimentary units intruded by the Moorarie Supersuite. Rutile grains from the Minnie Creek prospect yield Palaeoproterozoic (ca. 1.77-1.75Ga) U-Pb cooling ages and Nb+Ta concentrations of up to 17wt% that along with inclusions of manganocolumbite, oscillatory and patchy zonation of Nb and Fe, suggest a magmatic origin.The commonly used pathfinder elements for gold and base-metal mineralisation (Fe, Cr, V, W, Sn and Sb) are shown to be systematically lower in anatase and brookite, thus yielding false negatives if polymorph type is not identified during reconnaissance studies. For this reason, a ternary diagram was constructed based on the systematic changes in chemistry of TiO2 polymorphs to provide a relatively fast and easy chemical discrimination of polymorphs in large volumes of reconnaissance data. Furthermore, it is shown that high Al concentrations are characteristic of brookite and, to a lesser degree, anatase but not rutile. In addition, Sn, Nb, Ta and W concentrations in rutile may be more sensitive to igneous processes and may be used to track processes occurring in strongly fractionated granitic magmas such as pegmatites and associated deposits.
In principle, compositional profiling of the near-rim region of minerals can provide insight into cooling rates, but presumes that loss or gain of material from the crystal rim is not kinetically restricted. Trace element depth profiles collected for Zr, Hf, Ta, Nb, and U in amphibolite-facies rutile grains of the Catalina Schist, southern California, show significant variability within a single rock: Profiles of the same element among different grains can have significantly different slopes, grains with indistinguishable Zr profiles show vastly different Nb profiles, and grains with indistinguishable Nb profiles show different Zr profiles. Textural and kinetic idiosyncrasies within the matrix apparently affect the ability of specific crystals to accept or release trace elements, and impugn the common assumption that mineral surfaces maintain equilibrium at amphibolite-facies conditions. A new model that limits the flux of Zr from rutile grains helps explain commonly observed compositional profiles, and implies that inversion of compositional profiles assuming equilibrium among grain surfaces will invariably overestimate cooling rates. Few grains may record the low closure temperatures that experimentally determined diffusivities imply. Rather, higher temperatures will be retained, depending on the proximity of reactants and products in the matrix. Silicon diffusion does not control Zr reequilibration in rutile, and relative diffusion coefficients (D’s) of trace elements in rutile are DZr ~ DHf ~ 10DNb ~ 20DTa ~ 40DU.
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.
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.
This review summarizes our present geological understanding of the Proterozoic Gascoyne Province in the northwest of Western Australia. The province is located at the western end of the Capricorn Orogen, a major zone of tectonism formed between the Archean Yilgarn and Pilbara Cratons. The Gascoyne Province comprises six fault-bounded zones of granitic and medium- to high-grade metamorphic rocks. Although the province has been commonly regarded as a Paleoproterozoic entity, it has an extended history of reworking and reactivation until the late Neoproterozoic. Basement to the province consists of the Glenburgh Terrane, which comprises granitic rocks of the Halfway Gneiss with igneous crystallization ages typically between 2660 and 2430 Ma, psammitic and pelitic rocks of the Moogie Metamorphics deposited between 2240 and 2125 Ma, and a 2005–1970 Ma Andean- type batholith (Dalgaringa Supersuite). The Glenburgh Terrane is exotic to both its bounding Archean cratons. There are two parts to this Record: the first provides a synthesis of the geological history of the province, along with a summary of the gaps in our knowledge, whereas the second part contains detailed descriptions of all the stratigraphic units within the Gascoyne Province, and the tectonic events recorded in the province.
Under the banner of Western Australia Unearthed, the Geological Survey of Western Australia (GSWA) is progressively publishing a new compilation of Western Australia’s geology.
This new understanding of the geological evolution of Western Australia is the distillation of 30 years of geological mapping, complemented by collaborations with university and industry researchers. The emphasis has been on integration with geochronology — mainly U–Pb dating using the sensitive high-resolution ion microprobes (SHRIMPs) in the John de Laeter Centre of Mass Spectrometry at Curtin University in Perth to precisely determine the ages of igneous and metamorphic events. Combined with high-quality geochemistry and isotopic datasets, and new statewide geophysics acquisition, this has given new insights into the geological framework of Western Australia and the associated systems that produced mineral and petroleum deposits.
This volume — The birth of supercontinents and the Proterozoic assembly of Western Australia — is the first to be published, with three more titles to follow — Archean: building the core of the continent; Gondwana: from assembly to break-up; and Australia goes it alone — the emerging island continent (100 Ma to the present). The books are aimed at geologists, particularly newcomers to the State, to enable them to quickly get a feel for the geology and economic potential of various terrains. Each book provides, from a GSWA perspective, current ideas on the geological history of the State, and a list of recommended references is provided for further reading. GSWA has amassed knowledge on the geology of Western Australia over more than 120 years. This book is a prelude to all that information available through our website — the next step is to access the web: <www.dmp.wa.gov.au/GSWA>.
In situ laser ablation ICP-MS U–Pb dating of metamorphic rutile from granulite facies metapelitic rocks of the Archaean Pikwitonei granulite domain (Manitoba, Canada) provides constraints on Pb diffusion and characterizes the closure behavior of rutile. The analysis of transects of 35-μm spots across 15 rutile grains having a size of 120 to 280μm yielded concordant ages with core ages of ca. 2450Ma and core-to-rim younging towards 2280Ma. Age profiles indicate that volume diffusion of Pb occurs in rutile implying that the ages represent cooling ages. To investigate the closure behavior of Pb in rutile closure temperature profiles (Tc(x)) were constructed based on different models combined with experimentally-determined diffusion parameters. The classical Tc(x) model of Dodson (1986; Mat. Sci. Forum 7, 145–154) indicates a rapid decrease of Tc in the rims of grains, providing unrealistic estimates for the cooling rate when combined with U–Pb ages. A new Tc(x) model was constructed based on the analyzed age profiles that are described by an error function. This model shows a more steady decrease in Tc throughout the grain from ca. 640°C in the core (depending on grain size) to a rim intercept (Tc,rim) of 490°C (±7, 2σ), which is interpreted to be the extrapolated theoretical absolute temperature of insignificant Pb diffusion in rutile. The new model provides a better description of the relation between age and Tc for the analyzed grains. However, both Tc(x) models demonstrate that even in small grains the variations of Tc can be significant making it impossible to derive one representative Tc for Pb in rutile. The error function-based Tc(x) model allows the determination of cooling rates, which show a decrease over time from ca. 2.2 to 0.4°C/Ma agreeing well with previous estimates for the Pikwitonei granulite domain. This consistency supports the validity of our model and indicates that cooling rates can be estimated from single grains by LA-ICP-MS U–Pb dating of rutile providing constraints on the cooling history of a metamorphic terrane. The slow cooling rates imply that exhumation was slow (
Diffusion of Pb was measured in natural and synthetic rutile under dry, 1 atmosphere conditions, using mixtures of Pb titanate
or Pb sulfide and TiO2 as the sources of diffusant. Pb depth profiles were then measured with Rutherford Backscattering Spectrometry (RBS). Over
the temperature range 700–1100 °C, the following Arrhenius relation was obtained for the synthetic rutile: D=3.9 × 10−10exp(−250 ± 12 kJ mol−1/RT) m2s−1. Results for diffusion in natural and synthetic rutile were quite similar, despite significant differences in trace element
compositions. Mean closure temperatures calculated from the diffusion parameters are around 600 °C for rutile grains of ∼100 μm
size. This is about 100 °C higher than rutile closure temperature determinations from past field-based studies, suggesting
that rutile is more resistant to Pb loss through volume diffusion than previously thought.
Rutile is the most common naturally occurring titanium dioxide polymorph and is widely distributed as an accessory mineral in metamorphic rocks ranging from greenschist to eclogite and granulite facies but is also present in igneous rocks, mantle xenoliths, lunar rocks and meteorites. It is one of the most stable heavy minerals in the sedimentary cycle, widespread both in ancient and modern clastic sediments.Rutile has a wide range of applications in earth sciences. It is a major host mineral for Nb, Ta and other high field strength elements, which are widely used as a monitor of geochemical processes in the Earth's crust and mantle. Great interest has focused recently on rutile geochemistry because rutile varies not only by bulk composition reflected, for instance, in its Cr and Nb contents but also by the temperature of crystallisation, expressed in the Zr content incorporated into the rutile lattice during crystallisation. Rutile geochemistry and Zr-in-rutile thermometry yield diagnostic data on the lithology and metamorphic facies of sediment source areas even in highly modified sandstones that may have lost significant amounts of provenance information. Rutile may therefore serve as a key mineral in sediment provenance analysis in the future, similar to zircon, which has been widely applied in recent decades. Importantly, rutile from high-grade metamorphic rocks can contain sufficient uranium to allow U–Pb geochronology and (U–Th)/He thermochronology. Furthermore, in situ Lu–Hf isotope analysis of rutile permits insights into the evolution of the Earth's crust and mantle. Besides that, rutile is also of great economic importance because it is one of the favoured natural minerals used in the manufacture of white titanium dioxide pigment, which is a major constituent in various products of our daily life. Heavy mineral sands containing a significant percentage of rutile are therefore the focus of exploration worldwide.This paper aims to provide an overview of the applications of rutile in earth sciences, based on a review of data published in recent years. After giving a summary of various rutile-bearing lithologies, the focus lies on rutile geochemistry, Zr-in-rutile thermometry, O isotope analysis, U–Pb geochronology, (U–Th)/He thermochronology and Lu–Hf isotope analysis. A final outline of the economic importance of rutile highlights the demand for further rutile-related research in earth sciences.
Rutile is an important carrier of high field strength elements (HFSE; Zr, Nb, Mo, Sn, Sb, Hf, Ta, W). Its Zr content is buffered in systems with quartz and zircon as coexisting phases. The effects of temperature (T) and pressure (P) on the Zr content in rutile have been empirically calibrated in this study by analysing rutile-quartz-zircon assemblages of 31 metamorphic rocks spanning a T range from 430 to 1,100°C. Electron microprobe measurements show that Zr concentrations in rutile vary from 30 to 8,400 ppm across this temperature interval, correlating closely with metamorphic grade. The following thermometer has been formulated based on the maximum Zr contents of rutile included in garnet and pyroxene: T(in °C) = 127.8 × In (Zr in ppm) - 10 No pressure dependence was observed. An uncertainty in absolute T of ±50°C is inherited from T estimates of the natural samples used. A close approach to equilibrium of Zr distribution between zircon and rutile is suggested based on the high degree of reproducability of Zr contents in rutiles from different rock types from the same locality. At a given locality, the calculated range in T is mostly ± 10°C, indicating the geological and analytical precision of the rutile thermometer. Possible applications of this new geothermometer are discussed covering the fields of ultrahigh temperature (UHT) granulites, sedimentary provenance studies and metamorphic field gradients.
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.
Since the introduction of laser-assisted atom probe, analysis of nonconductive materials by atom probe tomography (APT) has become more routine. To obtain high-quality data, a number of acquisition variables needs to be optimized for the material of interest, and for the specific question being addressed. Here, the rutile (TiO 2 ) reference material ‘Windmill Hill Quartzite,’ used for secondary ion mass spectrometry U–Pb dating and laser-ablation inductively coupled plasma mass spectrometry, was analyzed by laser-assisted APT to constrain optimal running conditions. Changes in acquisition parameters such as laser energy and detection rate are evaluated in terms of their effect on background noise, ionization state, hit-multiplicity, and thermal tails. Higher laser energy results in the formation of more complex molecular ions and affects the ionization charge state. At lower energies, background noise and hit-multiplicity increase, but thermal tails shorten. There are also correlations between the acquisition voltage and several of these metrics, which remain to be fully understood. The results observed when varying the acquisition parameters will be discussed in detail in the context of utilizing APT analysis of rutile within geology.
In our study we explore the applicability of rutile as a pathfinder for orogenic gold deposits, which are an important source of this metal worldwide. We analysed rutile associated with orogenic Au deposits from three different Precambrian terranes, the Capricorn Orogen, the Barberton Greenstone Belt and the Ashanti Belt, all of which formed under greenschist conditions and share similarities in the style of mineralisation. Microtextural evidence from scanning electron microscopy and electron back-scatter diffraction indicates that rutile formed during the main deformation and alteration stage in these rocks, and is therefore related to mineralisation. We used electron microprobe and laser ablation ICP-MS to investigate the trace element compositions of rutile and we compared our results to other gold deposits. We find that hydrothermal rutile from gold deposits contains certain trace element characteristics, in particular high Sb concentrations (up to ∼1500 ppm in Au deposits of the Capricorn Orogen), that are distinct from rutile from non-mineralised rocks of various petrogenetic origin. Other elements, such as W and Sn, are found to be more enriched in rutile from other rock types, namely felsic magmatic rocks and hydrothermal veins, and are therefore not diagnostic of Au mineralisation in this type of deposits. We also find that the presence of sub-µm-scale inclusions – in particular Zr-(Si, Th)-bearing phases, sulfide minerals and native Au – can severely affect analyses of this type of rutile and compromise the applicability of Zr-in-rutile geothermometry.
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.
Measuring 207Pb/206Pb ratios by atom probe tomography (APT) has provided new insight into the nanoscale behavior of trace components in zircon, and their relationship to time, temperature, and structure. Here we analyze three APT data sets for a 3.77 Ga zircon from the Beartooth Mountains, USA, and apply systematic ranging approaches to understand the spatial and spectral uncertainties inherent in 207Pb/206Pb analysis by APT. This zircon possesses two, 100% concordant U-Pb analyses by secondary ion mass spectrometry (SIMS), indicative of closed U-Pb systematics on the micron scale since crystallization. APT data sets contain sub-spherical Pb-rich (>0.25% atomic) domains with diameter <15 nm. Broadly consistent Pb-rich regions are defined in applying six different permutations of the two most common cluster identification algorithms. Measured 207Pb/206Pb ratios within Pb-rich domains vary between 0.794 ± 0.15 (±2σ) and 0.715 ± 0.052 depending on the ranging approach, cluster definition protocol, and number of clusters interrogated. For the bulk APT data sets, 207Pb/206Pb = 0.353 ± 0.18; this is indistinguishable from the bulk 207Pb/206Pb ratio by SIMS (0.367 ± 0.0037), and statistically distinct from the 207Pb/206Pb ratio within clusters. Bulk and clustered 207Pb/206Pb ratios are consistent with Pb clustering at ~2.8 Ga, during protracted metamorphism and magmatism in the Beartooth Mountains.
The widespread use of zircon in geochemical and geochronological studies of crustal rocks is underpinned by an understanding of the processes that may modify its composition. Deformation during tectonic and impact related strain is known to modify zircon trace element compositions, but the mechanisms by which this occurs remain unresolved. Here we combine electron backscatter diffraction, transmission Kikuchi diffraction and atom probe microscopy to investigate trace element migration associated with a ∼20 nm wide, 2° low-angle subgrain boundary formed in zircon during a single, high-strain rate, deformation associated with a bolide impact. The low-angle boundary shows elevated concentrations of both substitutional (Y) and interstitial (Al, Mg and Be) ions. The observed compositional variations reflect a dynamic process associated with the recovery of shock-induced vacancies and dislocations into lower energy low-angle boundaries. Y segregation is linked to the migration and localisation of oxygen vacancies, whilst the interstitial ions migrate in association with dislocations. These data represent the direct nanoscale observation of geologically-instantaneous, trace element migration associated with crystal plasticity of zircon and provide a framework for further understanding mass transfer processes in zircon.
Granulite-facies metamorphism in the structurally lower part of the Saxon Granulite Massif culminated
Understanding of the thermal and geophysical evolution of the lower continental crust is limited by the resolution of conventional thermochronology. Intracrystalline daughter nuclide distribution profiles preserve a rich and underutilized record of thermal history. Using Laser Ablation Inductively Coupled Plasma Mass Spectrometry, we outline here a method to simultaneously acquire 206Pb/238U age and trace element profiles from U-bearing accessory phases. Inversion of 206Pb/238U age depth profiles yields thermal history information from an extended temperature range compared to inversion of age versus grain size relationships. Thermally-activated volume diffusion of Pb and Zr in rutile is sensitive to the thermal evolution of the mid- to lower-lithosphere. We document the ability of Laser Ablation depth-profiling to simultaneously resolve 206Pb/238U age and Zr diffusion profiles in the outer ∼35 μm of lower-crustal rutile euhedra from the Ivrea Zone, Southern Alps, with <1.2 μm depth resolution. Inversion of the age profiles reveals a continuous cooling history characterized by initially rapid cooling from View the MathML source at ∼180 Ma followed by a period of slower cooling from View the MathML source to View the MathML source. Combined with the topology of Zr diffusion profiles, these data indicate that the Ivrea Zone underwent a brief thermal pulse in the early Jurassic, plausibly associated with hyperextension of the Adriatic margin. Inversion of near-surface 206Pb/238U age distributions can be employed to resolve otherwise inaccessible thermal history information from the lower lithosphere.
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.
We use molecular-dynamics simulations to study processes related to resistance to amorphization by radiation damage. We simulate high-energy radiation events in SiO2, GeO2, TiO2, Al2O3, and MgO, and find that simulation results match the experiments. We discuss the difference between the damage that the structures along this series can support. We find that for the same material, activation barriers for damage recovery can strongly depend on the degree of structural damage. We observe that the effect of resistance to amorphization is primarily governed by the relaxation processes at the time scales of several picoseconds. On this time scale, we observe two distinct relaxation processes, reversible elastic deformation around the radiation cascade and recovery of the in-cascade damage of high topological disorder. Finally, we discuss how resistance to amorphization is related to interatomic interactions and to the nature of the chemical bond.
We examine the defect chemistry of rutile TiO2 using computer simulation techniques. The stable valencies of dopant transition metal ions in TiO2 are calculated to be Nb(V), V(III), Fe(III) and Cr(III). We find that Cr3+, Al3+, Ga3+, V3+ and Fe3+ dissolve preferentially at substitutional sites in TiO2 with charge compensation by oxygen vacancies rather than Ti4+ interstitials, although experimentally both are observed. Nb5+ is calculated to dissolve preferentially at substitutional sites with charge compensation being facilitated via Ti(III) ions or titanium vacancies. The solution enthalpy of Nb5+ is reduced by the introduction of Al3+ into the lattice. The solution enthalpy is further reduced when we consider bound clusters.
The Capricorn Orogen was initiated during Palaeoproterozoic suturing events that brought together the Archaean Yilgarn and Pilbara cratons to form the West Australian Craton. The orogen comprises Palaeoproterozoic plutonic and medium- to high-grade metamorphic rocks of the Gascoyne Complex, a series of Palaeoproterozoic volcano-sedimentary and sedimentary basins, including the Ashburton, Blair, Yerrida, Bryah, Padbury and Earaheedy basins, and the deformed margins of the Pilbara and Yilgarn cratons. Major pulses of deformation and metamorphism took place during the ca. 2200 Ma Ophthalmian Orogeny, the 2000–1960 Ma Glenburgh Orogeny, the 1830–1780 Ma Capricorn Orogeny, and a more localized unnamed event at the end of the Palaeoproterozoic (∼1670–1620 Ma). The orogen has been the site of repeated intracratonic reactivation with renewed basin formation, magmatism and orogeny during the Mesoproterozoic and the Neoproterozoic. The exhumed Palaeoproterozoic components of the orogen are unconformably overlain by the intracratonic Mesoproterozoic Edmund and Collier Basins (Bangemall Supergroup), which have been deformed during the Neoproterozoic Edmundian Orogeny. The Ophthalmian and Glenburgh orogenies affected the northern and southern margins of the orogen, respectively. The former resulted in the development of a foreland basin along the Pilbara Craton margin, whereas the latter is associated with accretion of an allochthonous element, the Glenburgh Terrane of the Gascoyne Complex, to the northwestern margin of the Yilgarn Craton. Effects of the Capricorn Orogeny extended across the entire orogen and provide a younger limit for amalgamation of the Yilgarn and Pilbara cratons. A number of basins developed along both the northern and southern margin of the orogen during this event, and were filled with sediment eroded from the Yilgarn Craton, the Gascoyne Complex, and a presently unexposed early Palaeoproterozoic terrane. The Capricorn Orogeny corresponds with a major phase of continental collision both in Australia and world wide, which appears to mark a major phase of supercontinent assembly. Mineralization within the orogen includes a wide variety of deposit types that can be related to collisional settings, including the world-class iron orebodies of the Hamersley Basin.
Metamorphic rutiles occurring in granulite and upper amphibolite facies metapelitic rocks of the Archean Pikwitonei granulite domain (Manitoba) and the Proterozoic Adirondack terrane (New York) give concordant and near concordant UPb ages. The Pb concentrations in rutile range from 2.85 to 168 ppm, U concentrations range from 10.9 to 390 ppm and the measured 206Pb/204Pb ratios range from 182 to 22,100 corresponding to 238U/204Pb ratios of 398–75,100. The proportions of radiogenic208Pb are very low, ranging from 0.0 to 6.9% of total radiogenic Pb.
Zircon and rutile are common accessory minerals whose essential structural constituents, Zr, Ti, and Si can replace one another to a limited extent. Here we present the combined results of high pressure–temperature experiments and analyses of natural zircons and rutile crystals that reveal systematic changes with temperature in the uptake of Ti in zircon and Zr in rutile. Detailed calibrations of the temperature dependencies are presented as two geothermometers—Ti content of zircon and Zr content of rutile—that may find wide application in crustal petrology. Synthetic zircons were crystallized in the presence of rutile at 1–2 GPa and 1,025–1,450°C from both silicate melts and hydrothermal solutions, and the resulting crystals were analyzed for Ti by electron microprobe (EMP). To augment and extend the experimental results, zircons hosted by five natural rocks of well-constrained but diverse origin (0.7–3 GPa; 580–1,070°C) were analyzed for Ti, in most cases by ion microprobe (IMP). The combined experimental and natural results define a log-linear dependence of equilibrium Ti content (expressed in ppm by weight) upon reciprocal temperature: \log ({\text{Ti}}_{{{\text{zircon}}}}) = (6.01 \pm 0.03) - \frac{{5080 \pm 30}}{{T\;(\hbox{K})}}.
The three-dimensional (3D) atom-probe technique produces a reconstruction of the elemental chemical identities and three-dimensional positions of atoms field evaporated from a sharply pointed metal specimen, with a local radius of curvature of less than 50 nm. The number of atoms collected can be on the order of one million, representing an analysis volume of approximately 20 nm x 20 nm x 200 nm (80,000 nm(3)). This large amount of data allows for the identification of microstructural features in a sample, such as grain or heterophase boundaries, if the feature density is large enough. Correlation of the measured atomic positions with these identified features results in an atom-by-atom description of the chemical environment of crystallographic defects. This article outlines a data compilation technique for the generation of composition profiles in the vicinity of interfaces in a geometrically independent way. This approach is applied to quantitative determination of interfacial segregation of silver at a MgO/Cu(Ag) heterophase interface.
Pressure-temperature studies of anatase, brookite, rutile and TiO2-II
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Capricorn Orogen rutile study: A combined electron backscatter diffraction (EBSD) and laser ablation split stream (LASS) analytical approach: Geological Survey of Western Australia
- D Plavsa
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Petrology and geochronology of rutile: Reviews in Mineralogy and Geochemistry
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Capricorn Orogen rutile study: A combined electron backscatter diffraction (EBSD) and laser ablation split stream (LASS) analytical approach
- Plavsa