Changes in zircon chemistry during Archean UHT metamorphism in the Napier Complex, Antarctica

Kusiak, Monika A.; Whitehouse, Martin J.; Wilde, Simon A.; Dunkley, Daniel J.; Menneken, Martina; Nemchin, A. A.; Clark, Chris

doi:10.2475/09.2013.05

Cited by 56 publications

(44 citation statements)

References 63 publications

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“…3 and 4. Furthermore, as noted in previous studies (2,3,12), if the redistribution of Pb occurred in geologically ancient times, it would have a higher 207 Pb/ 206 Pb ratio than any radiogenic lead that accumulated from the time of annealing to the present day. This would inevitably produce reverse discordance of U-Pb and Pb-Pb ages, with spuriously old 207 Pb/ 206 Pb age estimates obtained from domains where mobilized radiogenic lead was preferentially concentrated.…”

Section: Discussionmentioning

confidence: 64%

“…The occurrence of metallic Pb nanospheres in zircon affected by UHT metamorphism can explain the unusual U-Pb behavior of such grains during SIMS U-Pb analysis (2,3,12). Their formation is related to an annealing process under high-grade conditions, and once formed, the transformation of the enclosing metamict domains into newly crystallized zircon effectively arrests Pb loss, explaining why zircon that has experienced such extreme conditions is not completely reset to its metamorphic age.…”

Section: Discussionmentioning

confidence: 99%

“…This incompatibility of Pb raises questions about how radiogenic Pb is retained in zircon, especially through geological events where elevated temperature, fluid activity, and deformation can enhance element mobility. Furthermore it has been established that an irregular redistribution of lead in metamorphic zircon can degrade the precision and accuracy of U-Pb isotopic data, in extreme cases leading to spurious ages (2,3).…”

mentioning

confidence: 99%

“…This is important because 207 Pb/ 206 Pb ages are generally considered to be more robust than U-Pb ages for older zircons. However, if radiogenic Pb has been decoupled from its parent U and not locally incorporated into the crystal lattice during an ancient geological event, when radiogenic Pb age estimates may result (2,3,18).…”

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confidence: 99%

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Metallic lead nanospheres discovered in ancient zircons

Kusiak

Dunkley

Wirth

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

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Zircon (ZrSiO 4 ) is the most commonly used geochronometer, preserving age and geochemical information through a wide range of geological processes. However, zircon U-Pb geochronology can be affected by redistribution of radiogenic Pb, which is incompatible in the crystal structure. This phenomenon is particularly common in zircon that has experienced ultra-high temperature metamorphism, where ion imaging has revealed submicrometer domains that are sufficiently heterogeneously distributed to severely perturb ages, in some cases yielding apparent Hadean (>4 Ga) ages from younger zircons. Documenting the composition and mineralogy of these Pb-enriched domains is essential for understanding the processes of Pb redistribution in zircon and its effects on geochronology. Using high-resolution scanning transmission electron microscopy, we show that Pb-rich domains previously identified in zircons from East Antarctic granulites are 5-30 nm nanospheres of metallic Pb. They are randomly distributed with respect to zircon crystallinity, and their association with a Ti-and Al-rich silica melt suggests that they represent melt inclusions generated during ultra-high temperature metamorphism. Metallic Pb is exceedingly rare in nature and previously has not been reported in association with high-grade metamorphism. Formation of these metallic nanospheres within annealed zircon effectively halts the loss of radiogenic Pb from zircon. Both the redistribution and phase separation of radiogenic Pb in this manner can compromise the precision and accuracy of U-Pb ages obtained by high spatial resolution methods.metallic Pb | nanospheres | zircon | Antarctica | early Earth Z ircon is the mineral of choice for precisely determining the timing of both magmatism and metamorphism in a wide range of geological samples as well as providing constraints on the source and time of deposition of clastic sedimentary rocks. Accurate zircon geochronometry is facilitated by zircon having a lattice structure that is stable over a wide range of temperatures and pressures (1), together with the fact that whatever Pb is present derives almost entirely from radioactive decay of U and Th. During its growth, zircon incorporates small amounts of nonformula elements, including Hf, Ti, and Y, and rare earth elements, U and Th, but generally excludes Pb. This incompatibility of Pb raises questions about how radiogenic Pb is retained in zircon, especially through geological events where elevated temperature, fluid activity, and deformation can enhance element mobility. Furthermore it has been established that an irregular redistribution of lead in metamorphic zircon can degrade the precision and accuracy of U-Pb isotopic data, in extreme cases leading to spurious ages (2, 3).Experimental and natural studies have revealed that element mobility in zircon is strongly dependent on the accumulation of α-recoil damage (4, 5). Multiple α-decay events along the U and Th decay chains leading to stable daughter Pb isotopes destroy the crystal lattice, creating amorphous ...

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Section: Discussionmentioning

confidence: 64%

Section: Discussionmentioning

confidence: 99%

mentioning

confidence: 99%

mentioning

confidence: 99%

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Metallic lead nanospheres discovered in ancient zircons

Kusiak

Dunkley

Wirth

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

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“…Fortunately for geochemists, radiation damage has been partially healed in many zircons and primary compositions are reliably preserved. However, uncertainty about post-crystallization modification has fueled challenges to the ages and the fidelity of geochemical data, especially for Archean and Hadean zircons (Ashwal et al 1999;Nelson et al 2000;Whitehouse and Kamber 2002;Parrish and Noble 2003;Hoskin and Schaltegger 2003;Hoskin 2005;Nemchin et al 2006;Holden et al 2009;Kusiak et al 2013aKusiak et al , 2013b.…”

Section: Introductionmentioning

confidence: 99%

Nano- and micro-geochronology in Hadean and Archean zircons by atom-probe tomography and SIMS: New tools for old minerals

Valley

Reinhard

Cavosie

et al. 2015

American Mineralogist

114

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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 × 10 15 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 207 Pb/ 206 Pb 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 207 Pb/ 206 Pb 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 heatin...

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Crystallization and Disturbance Histories of Single Zircon Crystals From Hadean‐Eoarchean Acasta Gneisses Examined by LA‐ICP‐MS U‐Pb Traverses

Guitreau

Mora

Paquette

2018

Geochem Geophys Geosyst

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Zircon U‐Pb geochronology is a very robust dating method but the accuracy of determined ages can sometimes be compromised. This is because, as commonly observed, the U‐Pb isotope system can be reopened during the post‐crystallization evolution of a zircon. The present manuscript investigates the capability of traverses by LA‐ICP‐MS to identify zones of well‐preserved U‐Pb isotope systematics within zircon crystals. The data for the different zones can be used to construct internal Discordia lines that provide age information about crystallization and metamorphism of single zircon crystals. To test our approach, we analyzed zircons from three ca. 3.96 Ga Acasta gneisses. Results demonstrate that such a method allows retrieval of original crystallization age in most of the studied zircons, even in those that experienced ancient U‐Pb disturbances resulting in discordant U‐Pb data. It was also possible to estimate disturbance ages, though not as precisely as for the igneous crystallization, in many of the studied crystals.

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Changes in zircon chemistry during Archean UHT metamorphism in the Napier Complex, Antarctica

Cited by 56 publications

References 63 publications

Metallic lead nanospheres discovered in ancient zircons

Metallic lead nanospheres discovered in ancient zircons

Nano- and micro-geochronology in Hadean and Archean zircons by atom-probe tomography and SIMS: New tools for old minerals

Crystallization and Disturbance Histories of Single Zircon Crystals From Hadean‐Eoarchean Acasta Gneisses Examined by LA‐ICP‐MS U‐Pb Traverses

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