Metallic Pb nanospheres in ultra-high temperature metamorphosed zircon from southern India

Whitehouse, Martin J.; Kusiak, Monika A.; Wirth, Richard; Kumar, G. R. Ravindra

doi:10.1007/s00710-017-0523-1

Cited by 27 publications

(29 citation statements)

References 32 publications

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“…These two features limit their ability to be used for site‐specific APT targeting, but provide useful constraints on ‘bulk’ compositions of the host mineral. However, elemental and isotopic mapping can be done by ion imaging techniques, which provide micrometre‐scale spatial resolution (e.g., Bellucci et al , Whitehouse et al ) and such images have provided the context for the discovery of nanoscale Pb nanospheres in radiation damaged zircon (Kusiak et al ), which have the potential to be targeted for atom probe analyses.…”

Section: Sample Selection and Preparationmentioning

confidence: 99%

Atom Probe Tomography: Development and Application to the Geosciences

Reddy

Saxey

Rickard

et al. 2020

Geostandard Geoanalytic Res

109

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Atom probe tomography (APT) is an analytical technique that provides quantitative three‐dimensional elemental and isotopic analyses at sub‐nanometre resolution across the whole periodic table. Although developed and mostly used in the materials science and semiconductor fields, recent years have seen increasing development and application in the geoscience and planetary science disciplines. Atom probe studies demonstrate compositional complexity at the nanoscale and provide fundamental new insights into the atom‐scale mechanisms taking place in minerals over geological time. Here, we provide an overview of APT, including the historical development and technical aspects of the instrumentation, and the fundamentals of data acquisition, data processing and data reconstruction. We also review previous studies and highlight the potential future applications of nanoscale geochemical studies of natural materials.

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Section: Sample Selection and Preparationmentioning

confidence: 99%

Atom Probe Tomography: Development and Application to the Geosciences

Reddy

Saxey

Rickard

et al. 2020

Geostandard Geoanalytic Res

109

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“…The overall issue of Pb mobilization in zircon has been discussed in several papers utilizing various methods 10–21 , with only TEM capable of identifying crystalline metal Pb nanospheres. However, this phenomenon is not limited to zircons from the Napier Complex as metallic Pb nanospheres were also documented in the Kerala Khondalite Belt (KKB) in India 17 . What distinguishes Antarctic zircons from those in India is the presence of Al and Ti concentrations, whereas Pb nanospheres in the KKB only occur individually.…”

Section: Discussionmentioning

confidence: 99%

“…After metamorphism, continued U and Th decay resulted in the renewed accumulation of radiogenic Pb in the zircon between the Pb nanospheres. The migration of radiogenic Pb in zircon during metamorphism has been established by several techniques, including SIMS 6,11–15 , TEM 10,16,17 and atom-probe tomography (APT) 18–21 .…”

Section: Introductionmentioning

confidence: 99%

Pb nanospheres in ancient zircon yield model ages for zircon formation and Pb mobilization

Lyon

Kusiak

Wirth

et al. 2019

Sci Rep

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Nanospheres of lead (Pb) have recently been identified in zircon (ZrSiO4) with the potential to compromise the veracity of U-Pb age determinations. The key assumption that the determined age is robust against the effects of Pb mobility, as long as Pb is not lost from the zircon during subsequent geological events, is now in question. To determine the effect of nanosphere formation on age determination, and whether analysis of nanospheres can yield additional information about the timing of both zircon growth and nanosphere formation, zircons from the Napier Complex in Enderby Land, East Antarctica, were investigated by high-spatial resolution NanoSIMS (Secondary Ion Mass Spectrometry) mapping. Conventional SIMS analyses with >µm resolution potentially mixes Pb from multiple nanospheres with the zircon host, yielding variable average values and therefore unreliable ages. NanoSIMS analyses were obtained of 207Pb/206Pb in nanospheres a few nanometres in diameter that were resolved from 207Pb/206Pb measurements in the zircon host. We demonstrate that analysis for 207Pb/206Pb in multiple individual Pb nanospheres, along with separate analysis of 207Pb/206Pb in the zircon host, can not only accurately yield the age of zircon crystallization, but also the time of nanosphere formation resulting from Pb mobilization during metamorphism. Model ages for both events can be derived that are correlated due to the limited range of possible solutions that can be satisfied by the measured 207Pb/206Pb ratios of nanospheres and zircon host. For the Napier Complex zircons, this yields a model age of ca 3110 Ma for zircon formation and a late Archean model age of 2610 Ma for the metamorphism that produced the nanospheres. The Nanosphere Model Age (NMA) method constrains both the crystallization age and age of the metamorphism to ~±135 Ma, a significant improvement on errors derived from counting statistics.

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“…The lower limit of a useful primary beam diameter is given by the necessity to average away the nm-scale recoil of radiogenic daughter nuclides by natural disintegration of the parent nuclide. Such atom-scale phenomena were documented in zircon by Kusiak et al (2013), Valley et al (2014) and Whitehouse et al (2017) and in monazite by Seydoux-Guillaume et al (2003) and Fougerouse et al (2018), and result in a local disproportionation of parent and daughter nuclides. An unquestioning, contextless application of a single spot age obtained with a < 1 µm primary beam would cause an incorrect age assignment of an entire orogenic cycle.…”

Section: Example 1 Monazite Thermochronometry: the Relevance Of Diffmentioning

confidence: 96%

Petrochronology and hygrochronology of tectono-metamorphic events

Bosse

Villa

2019

Gondwana Research

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U-Th-Pb petrochronology is based on the incontrovertible fact that the diffusion of radiogenic Pb is negligibly small relative to retrograde reaction rates. Multi-element maps demonstrate that patchy textures tightly correspond to (U+Th)-Pb age variations, requiring that fluid-induced dissolution/ reprecipitation is the principal cause of Pb mobility. Attempts to model intracrystalline core-rim Pb zonations as diffusive transport are not legitimate unless genuine bell-shaped diffusion profiles in minerals can be documented, which happens only exceptionally. Monazite and zircon intra-grain age maps confirm that coupled dissolution-reprecipitation and retrogression reactions assisted by fluids control (Th+U)-Pb ages, not temperature. The chemical zonations observed in many (Th+U)-bearing mineral chronometers (e.g. monazite, allanite, xenotime, zircon) provide petrological constraints. Linking petrology with textures and the isotope record allows reconstructing entire segments of the P-T-AX -D-t history of a rock and its geodynamic environment. The dearth of mathematically sound diffusion profiles equally applies to the isotope record of micas and feldspars. The tight link between petrology, microtextures, chemical composition and geochronology also pertains to Rb-Sr and K-Ar. Overdetermined multi-mineral Rb-Sr isochrons with excess scatter, and spatially resolved/stepwise release 39 Ar-40 Ar results, demonstrate ubiquitous correspondence between relict phases and isotopic inheritance. Many rock-forming minerals are highly retentive of Sr and Ar, unless they are obliterated by retrograde reactions. The rates of dissolution in fluid-controlled reactions are several orders of magnitude faster at upper and mid-crustal levels than diffusive reequilibration rates. Thus, as a rule Rb-Sr and K-Ar chronometers date their own formation. Accurately establishing P-T paths of monometamorphic rocks requires assessing petrologic equilibrium using multivariate thermodynamic software. Dating complex parageneses of polymetamorphic, unequilibrated rocks requires labor-intensive disentangling by: (i) qualitative identification of relicts, retrogression reactions, and chemically open systems by imaging techniques (e.g. cathodoluminescence, element maps, etc.); (ii) microchemical analyses at the µm-scale quantifying heterochemical disequilibrium phases and assigning them to a P-T-AX segment; (iii) spatially resolved/stepwise release, relating the chemical signature of the analyzed mineral to its age. K-Ar and Rb-Sr usually provide a different perspective on the P-T evolution of a rock than does (Th+U)-Pb, as K+Rb-rich minerals (phyllosilicates and especially feldspars) mostly form later and react/dissolve faster in the retrograde path than U-rich accessory phases (e.g. Mukai et al., 2014). The present paper reviews these general principles by means of well-understood examples, both successful and insuccessful in matching the independently known external constraints.

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Metallic Pb nanospheres in ultra-high temperature metamorphosed zircon from southern India

Cited by 27 publications

References 32 publications

Atom Probe Tomography: Development and Application to the Geosciences

Atom Probe Tomography: Development and Application to the Geosciences

Pb nanospheres in ancient zircon yield model ages for zircon formation and Pb mobilization

Petrochronology and hygrochronology of tectono-metamorphic events

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