Clustering and interfacial segregation of radiogenic Pb in a mineral host – inclusion system. Tracing two-stage Pb and trace element mobility in monazite inclusions in rutile
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Abstract
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 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.
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Grace Shephard and Philip Heron.
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.
Here, we report small randomly-distributed crystalline lead (Pb) nanospheres occurring in detrital zircon grains obtained from a weakly metamorphosed Archean conglomerate at Jack Hills, Western Australia, making this the third known global example of this phenomenon. They form in zircon crystals ranging from Hadean (> 4 billion years—Ga) to Eoarchean (> 3.6 Ga) in age, but are absent from Paleoarchean (~ 3.4 Ga) crystals. Unlike previous discoveries of nanospheres in zircon from Precambrian gneisses in Antarctica and India, detrital zircon from Jack Hills shows no evidence of ever undergoing ultra-high temperature (UHT) metamorphism, either before or after deposition, therefore implying that nanospheres can form at temperatures lower than ca. 900 °C. The nanospheres are composed of radiogenic Pb released by the breakdown of uranium (U) and thorium (Th) and are present in zircon irrespective of its U, Th and water contents, its oxygen isotopic composition, and the degree of discordance due to Pb loss or gain. The nanospheres pre-date annealed cracks in the crystals, showing that, once formed, they effectively ‘freeze’ radiogenic Pb in the zircon structure, precluding any further interaction during subsequent geological processes. Both Pb nanoclusters and nanospheres are now reported from Jack Hills, and it appears likely the former is a precursor stage in the formation of the latter. Although the precise mechanism for this transition remains unresolved, a later thermal event is required, but this likely did not reach UHT conditions at Jack Hills.
A potential record of Earth's magnetic field going back 4.2 billion years (Ga) ago is carried by magnetite inclusions in zircon grains from the Jack Hills. This magnetite may be secondary in nature, however, meaning that the magnetic record is much younger than the zircon crystallization age. Here, we use atom probe tomography to show that Pb-bearing nanoclusters in magnetite-bearing Jack Hills zircons formed during two discrete events at 3.4 and <2 Ga. The older population of clusters contains no detectable Fe, whereas roughly half of the younger population of clusters is Fe bearing. This result shows that the Fe required to form secondary magnetite entered the zircon sometime after 3.4 Ga and that remobilization of Pb and Fe during an annealing event occurred more than 1 Ga after deposition of the Jack Hills sediment at 3 Ga. The ability to date Fe mobility linked to secondary magnetite formation provides new possibilities to improve our knowledge of the Archean geodynamo.
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.
The geometry and composition of deformation-related low-angle boundaries in naturally deformed olivine were characterized by electron backscattered diffraction (EBSD) and atom probe tomography (APT). EBSD data show the presence of discrete low-angle tilt boundaries, which formed by subgrain rotation recrystallisation associated with the (100)[001] slip system during fluid-catalysed metamorphism and deformation. APT analyses of these interfaces show the preferential segregation of olivine-derived trace elements (Ca, Al, Ti, P, Mn, Fe, Na and Co) to the low-angle boundaries. Boundaries with < 2° show marked enrichment associated with the presence of multiple, non-parallel dislocation types. However, at larger disorientation angles (> 2°), the interfaces become more ordered and linear enrichment of trace elements coincides with the orientation of dislocations inferred from the EBSD data. These boundaries show a systematic increase of trace element concentration with disorientation angle. Olivine-derived trace elements segregated to the low-angle boundaries are interpreted to be captured and travel with dislocations as they migrate to the subgrain boundary interfaces. However, the presence of exotic trace elements Cl and H, also enriched in the low-angle boundaries, likely reflect the contribution of an external fluid source during the fluid-present deformation. The observed compositional segregation of trace elements has significant implications for the deformation and transformation of olivine at mantle depth, the interpretation of geophysical data and the redistribution of elements deep in the Earth. The observation that similar features are widely recognised in manufactured materials, indicates that the segregation of trace elements to mineral interfaces is likely to be widespread.
Mining of “invisible gold” associated with sulfides in gold ores represents a significant proportion of gold production worldwide. Gold hosted in sulfide minerals has been proposed to be structurally bound in the crystal lattice as a sulfide-gold alloy and/or to occur as discrete metallic nanoparticles. Using a combination of microstructural quantification and nanoscale geochemical analyses on a pyrite crystal from an orogenic gold deposit, we show that dislocations hosted in a deformation low-angle boundary can be enriched in Ni, Cu, As, Pb, Sb, Bi, and Au. The cumulative trace-element enrichment in the dislocations is 3.2 at% higher compared to the bulk crystal. We propose that trace elements were segregated during the migration of the dislocation following the dislocation-impurity pair model. The gold hosted in nanoscale dislocations represents a new style of invisible gold.
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.
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.
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.