Pb Loss In Zircons Research Articles

The leaky chronometer: Evidence for systematic cryptic Pb loss in laser ablation U‐Pb dating of zircon relative to CA‐TIMS

AbstractU–Pb dating of zircon via laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) has become a primary method for determining the age of crystalline rocks and the provenance and maximum depositional age of (meta)sedimentary rocks. Although chemical abrasion has become a standard approach for mitigating Pb loss in zircon dated via high‐resolution thermal ionization mass spectrometry (CA‐TIMS), laser ablation dating is rarely accompanied by chemical abrasion. We evaluate the accuracy of U–Pb dates acquired via LA‐ICP‐MS relative to CA‐TIMS through analysis of a large database of paired analyses from the same zircon crystals. We show that laser ablation 206Pb/238U dates are systematically younger than their CA‐TIMS counterparts, with a greater shift in detrital zircon (−2.0%) versus igneous or metamorphic zircon (−0.9%). Although systematic biases related to “matrix effects” may be a contributing factor, our analysis suggests that unmitigated cryptic Pb loss is likely widespread in laser ablation U–Pb datasets.

Minimizing the effects of Pb loss in detrital and igneous U–Pb zircon geochronology by CA-LA-ICP-MS

Abstract. Detrital zircon geochronology by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) is a widely used tool for determining maximum depositional ages and sediment provenance, as well as reconstructing sediment routing pathways. Although the accuracy and precision of U–Pb geochronology measurements have improved over the past 2 decades, Pb loss continues to impact the ability to resolve zircon age populations by biasing affected zircon toward younger apparent ages. Chemical abrasion (CA) has been shown to reduce or eliminate the effects of Pb loss in zircon U–Pb geochronology but has yet to be widely applied to large-n detrital zircon analyses. Here, we assess the efficacy of the chemical abrasion treatment on zircon prior to analysis by LA-ICP-MS and discuss the advantages and limitations of this technique in relation to detrital zircon geochronology. We show that (i) CA does not systematically bias LA-ICP-MS U–Pb dates for 13 reference materials that span a wide variety of crystallization dates and U concentrations, (ii) CA-LA-ICP-MS U–Pb zircon geochronology can reduce or eliminate Pb loss in samples that have experienced significant radiation damage, and (iii) bulk CA prior to detrital zircon U–Pb geochronology by LA-ICP-MS improves the resolution of age populations defined by 206Pb/238U dates (Neoproterozoic and younger) and increases the percentage of concordant analyses in age populations defined by 207Pb/206Pb dates (Mesoproterozoic and older). The selective dissolution of zircon that has experienced high degrees of radiation damage suggests that some detrital zircon age populations could be destroyed or have their abundance significantly modified during this process. However, we did not identify this effect in either of the detrital zircon samples that were analyzed as part of this study. We conclude that pre-treatment of detrital zircon by bulk CA may be useful for applications that require increased resolution of detrital zircon populations and increased confidence that 206Pb/238U dates are unaffected by Pb loss.

Dating multiply overprinted granites: The effect of protracted magmatism and fluid flow on dating systems (zircon U-Pb: SHRIMP/SIMS, LA-ICP-MS, CA-ID-TIMS; and Rb–Sr, Ar–Ar) – Granites from the Western Erzgebirge (Bohemian Massif, Germany)

The Variscan granites from the Western Erzgebirge were repeatedly dated by various methods, but no consensus has been reached about their exact intrusion ages. This study presents a multi-dating approach for the four largest intrusions from the Western Erzgebirge (Aue-Schwarzenberg, Bergen, Eibenstock, Kirchberg). We analysed several samples from each pluton/suite with zircon U–Pb CA-ID-TIMS (chemical abrasion-isotope dilution-thermal ionization mass spectrometry) to obtain robust temporal information on their age and tempo of intrusion. These data enable us for the first time to define three intrusive episodes of 1–2 Ma each, separated by quiet periods of several Ma. The Aue-Schwarzenberg suite represents the oldest granites that intruded at ~323–322 Ma followed by the granites from Bergen and Kirchberg 2–4 Ma later. The highly evolved ore-bearing granites from the Eibenstock pluton intruded after a time lag of ~5 Ma at ~315–314 Ma. The new data show that there is a resolvable age difference between the two known granite groups. Granite group 2 (also assigned as younger igneous complex, represented by the Eibenstock pluton) is ≥5 Ma younger than granite group 1 (assigned as older igneous complex, represented by granites from Aue-Schwarzenberg, Bergen and Kirchberg).Protracted magmatism and late-/post-magmatic fluid flow partly reset the U–Pb system of these granites to variable degrees, making a precise and accurate dating of their intrusion ages challenging. Pb loss in zircons is often combined with high common Pb (Pbc). SHRIMP/SIMS (sensitive high mass resolution ion microprobe/secondary ion mass spectrometry) and LA-ICP-MS (laser ablation-inductively coupled plasma-mass spectrometry) on non-CA zircons document that Pb loss and high Pbc is quite variable within zircon grains and may be located in micro-fractures. We demonstrate that chemical abrasion (CA) clearly minimizes or removes both Pb loss and Pbc. Results from prior LA-ICP-MS and SHRIMP dating on non-CA zircons from the same samples considerably helped the interpretation of the CA-ID-TIMS data when Pb loss was not completely erased by CA. In such cases we often had to choose the oldest analyses for mean age calculation in contrast to the common practice of the CA-ID-TIMS community to choose the youngest dates.Rb–Sr and Ar–Ar dating systems revealed age differences between the older group and the younger ore-bearing granites albeit with diverging absolute ages. Most Ar–Ar ages are identical with CA-ID-TIMS ages and would imply rapid cooling. However, samples from the older group have excess Ar that could have led to too old ages. In contrast, Rb–Sr ages for the older granites are 0–7 Ma younger than their intrusions. Fluid induced alteration led to the formation of Li-mica, fluorite and cassiterite (greisenization). For the youngest granite (Eibenstock), Li-mica was used to date the first greisenization. Samples without visible hydrothermal overprint yielded identical Ar–Ar and Rb–Sr ages as severely greisenized samples. This implies re-equilibration due to the hydrothermal overprint for all Ar–Ar and Rb–Sr ages from the Eibenstock pluton. According to Ar–Ar dating, the first ore formation (~315 Ma) is coeval with the CA-ID-TIMS intrusion age of the Eibenstock granite while it is delayed by ~6 (±3) Ma according to Rb–Sr dating (308 ± 3 Ma).

Apollo 12 breccia 12013: Impact-induced partial Pb loss in zircon and its implications for lunar geochronology

Apollo 12 breccia 12013 is composed of two portions, one grey in colour, the other black. The grey portion of the breccia consists mainly of felsite thought to have formed during a single crystallisation event, while the black part is characterized by presence of lithic fragments of noritic rocks and individual plagioclase crystals. In this study, U-Pb analyses of Ca-phosphate and zircon grains were conducted in both portions of the breccia. The zircon grains within the grey portion yielded a large range of ages (4154 ± 7 to 4308 ± 6 Ma, 2σ) and show decreasing U and Th concentrations within the younger grains. Moreover, some grains exhibit recrystallisation features and potentially formation of neoblasts. The latter process requires high temperatures above 1600–1700 °C leading to the decomposition of the primary zircon grain and subsequent formation of new zircon occurring as neoblasts. As a result of the high temperatures, the U-Pb system of the remaining original zircon grains was most likely open for Pb diffusion causing partial resetting and the observed range of 207Pb/206Pb ages. The event that led to the Pb loss in zircon could potentially be dated by the U-Pb system in Ca-phosphates, which have a weighted average 207Pb/206Pb age across both lithologies of 3924 ± 3 Ma (95% conf.). This age is identical within error to the combined average 207Pb/206Pb age of 3926 ± 2 Ma that was previously obtained from Ca-phosphates within Apollo 14 breccias, zircon grains in Apollo 12 impact melt breccias, and the lunar meteorite SaU 169. This age was interpreted to date the Imbrium impact. The zircon grains located within the black portion of the breccia yielded a similar range of ages (4123 ± 13 to 4328 ± 14 Ma, 2σ) to those in the grey portion. Given the brecciated nature of this part of the sample, the interpretation of these ages as representing igneous crystallisation or resetting by impact events remains ambiguous since there is no direct link to their source rocks via textural relationships or crystal chemistry. Similarly, the currently available zircon data set for all lunar samples may be distorted by partial Pb loss, resulting in meaningless and misleading age distribution patterns. Therefore, it is crucial to fully understand and recognize the processes and conditions that may lead to partial resetting of the U-Pb system in zircon in order to better constrain the magmatic and impact history of the Moon.

Multi-stage modification of Paleoarchean crust beneath the Anabar tectonic province (Siberian craton)

According to present views, the crustal terranes of the Anabar province of the Siberian craton were initially independent blocks, separated from the convecting mantle at 3.1 (Daldyn terrane), 2.9 (Magan terrane) and 2.5 Ga (Markha terrane) (Rosen, 2003, 2004; Rosen et al., 1994, 2005, 2009). Previous studies of zircons in a suite of crustal xenoliths from kimberlite pipes of the Markha terrane concluded that the evolution of the crust of the Markha terrane is very similar to that of the Daldyn terrane. To test this conclusion we present results of U-Pb and Hf-isotope studies on zircons in crustal xenoliths from the Zapolyarnaya kimberlite pipe (Upper Muna kimberlite field), located within the Daldyn terrane, and the Botuobinskaya pipe (Nakyn kimberlite field) in the center of the Markha terrane.The data on xenoliths from the Botuobinskaya kimberlite pipe record tectonothermal events at 2.94, 2.8, 2.7 and 2 Ga. The event at 2 Ga caused Pb loss in zircons from a mafic granulite. U-Pb dating of zircons from the Zapolyarnaya pipe gives an age of 2.7 Ga. All zircons from the studied crustal xenoliths have Archean Hf model ages ranging from 3.65 to 3.11 Ga. This relatively narrow range suggests that reworking of the ancient crust beneath the Nakyn and Upper Muna kimberlite fields was minor, compared with the Daldyn and Alakit-Markha fields (Shatsky et al., 2016).This study, when combined with dating of detrital zircons, implies that tectonic-thermal events at 2.9–2.85, 2.75–2.7 and 2.0–1.95 Ga occurred everywhere on the Anabar tectonic province, and could reflect the upwelling of superplumes at 2.9, 2.7 and 2 Ga. The presence of the same tectonic-thermal events in the Daldyn and Markha terranes (Rosen et al., 2006a,b) supports the conclusion that the identification of the Markha terrane as a separate unit is not valid.

<i>In situ</i> U-Pb Zircon Micro-Geochronology of MCT Zone Rocks in the Lesser Himalaya Using LA–MC–ICPMS Technique

A multi-collector (MC) inductively coupled plasma mass spectrometer (ICPMS) was used in combination with an Excimer (193 nm) laser to carry out in situ U-Pb dating of zircons. High performance twovolume sample cell provided unmatched laser ablated aerosol transportation efficiency resulting in reducing laser-related down-hole fractionation. Three wellcharacterized natural zircon reference standards (Harvard zircon 91500, GJ-1 zircon, Plesovice) were repeatedly measured in different sessions to evaluate the analytical figures of merits. Precision of <1% was achieved for spot sizes 20 μm with accuracies well within 2% of the reference values for these standards. Zircons from MCT Zone in the inner Lesser Himalaya reveal a highly discordant Palaeo-proterozoic (1901 ± 11 Ma) magmatic crystallization age inferred from the upper intercept in the concordia plot. The 207 Pb/ 206 Pb ages are also internally consistent with the disconcordia age with a weighted mean of 1900 ± 10 Ma and in turn suggest a major phase of Palaeo-proterozoic magmatic activity along the northern margin of Indian craton, while Early Miocene (~25 Ma) Pb loss in zircon inferred from lower intercept in disordria may be related to tectono-thermal activity along MCT.

Calibrating the Guadalupian Series (Middle Permian) of South China

The Guadalupian Epoch represents one of the most critical intervals during the Earth history which is characterized by a series of global geological and biological events. However, it is poorly constrained by radioisotopic ages in comparison with the high-precision dates from the Lopingian and lowest Triassic in South China and the Cisuralian in southern Urals, Russia. In this study, we report combined CA-ID-TIMS and SIMS U-Pb zircon geochronology for three ash beds from the base and lower part of the Kuhfeng Formation in the Chaohu area, southeastern China. Zircons from Ash Beds A, C and D were initially analyzed by SIMS and yielded weighted mean 206Pb/238U dates of 269.0±1.4Ma (MSWD=1.03, n=31), 266.3±2.2Ma (MSWD=2.2, n=29) and 262.2±1.7Ma (MSWD=1.0, n=20) in ascending order. However, high-precion U-Pb geochronology of the same ash beds by the CA-ID-TIMS method point to important inaccuracies in the SIMS results due to Pb-loss in zircons. Ash Beds A and C yielded two TIMS weighted mean 206Pb/238U dates of 272.95±0.11Ma (MSWD=1.9, n=7) and 271.038±0.097Ma (MSWD=2.2, n=8), respectively. The depositional age of 272.95±0.11Ma for Ash Bed A is postulated as the traditional Cisuralian-Guadalupian boundary (CGB) age in South China. Our new geochronology suggests that the Guadalupian Series in South China had a total duration of 13.85±0.52myr given a Guadalupian-Lopingian boundary (GLB) age of 259.1±0.5Ma. The new age calibration of the base of the Guadalupian Series is ca. 0.65 (±0.51) myr older than – and improves upon – the previous estimates for the CGB age and the duration of the Guadalupian Epoch based on the 2016 International Chronostratigraphic Chart (ICC) and the 2012 Geologic Time Scale (GTS).

First Permian–Early Triassic zircon ages for tin‐bearing granites from the Gemeric unit (Western Carpathians, Slovakia): connection to the post‐collisional extension of the Variscan orogen and S‐type granite magmatism

This study presents the first preliminary U–Pb zircon data on tin‐bearing S‐type granites from the Gemeric unit of the Western Carpathians (Slovakia). U–Pb single zircon dating controlled by cathodoluminescence suggests crystallization of the Gemeric granites during Permian to Early Triassic (303–241 Ma) time. Post‐crystallization, low‐temperature metamorphic overprint is reflected by partial Pb loss in zircons. These Gemeric granites are younger than the highly fractionated, S‐type, tin‐ and rare‐element‐bearing leucogranites in the European Variscides. They may have resulted from partial melting, triggered by increased heat flow from the mantle below the continental crust, and most probably intruded during the post‐collisional extension and initial rifting of the Variscan orogenic belt. During Alpine orogeny, the Gemeric granites were affected by a low‐temperature deformation and metamorphism.

Pb diffusion in zircon

Diffusion of Pb was characterized in natural and synthetic zircon under a range of conditions. In most experiments, mixtures of Pb sulfate and ground zircon were used as the sources of diffusant, with Pb depth profiles measured with Rutherford Backscattering Spectrometry (RBS). As complement to these “in-diffusion” experiments, “out-diffusion” experiments were run on both synthetic Pb-doped and natural zircon with relatively high Pb concentrations, and analyzed with either electron microprobe or RBS. Over the temperature range 1000–1500°C, the following Arrhenius relation was obtained: D=1.1×10 −1 exp(−550±30 kJ mol −1/RT) m 2 s −1 Results for diffusion in natural and synthetic zircon were quite similar, as are those for in- and out-diffusion. Pb diffusion does not appear to be strongly influenced by pressure, crystallographic orientation, or the presence of water. The slow diffusion rate for Pb indicates that Pb isotope ratios will not be altered by volume diffusion in crystalline zircon under most geologic conditions, a finding consistent with the frequent observation of inheritance and preservation of multistage histories in zircon. Most Pb loss in zircon, then, is likely a consequence of recrystallization or Pb transport in zircons with severe radiation damage (although the latter can be effective only for vast annealing times at very low temperatures). Mean closure temperatures, calculated from the diffusion parameters, are in excess of 900°C for zircons of typical size, a result in consonance with field-based estimates of closure temperatures.

Slow cooling of deep crustal granulites and Pb-loss in zircon

We attempted to obtain the magmatic crystallization age of a metamorphosed (granulite facies, T = 800 ± 90°C; P = 8 ± 1 kbar) massif-type anorthosite from the Ankafotia body of southwest Madagascar. The sample studied is a coarse-grained leuconorite with good preservation of igneous texture and mineralogy, although plagioclase, which contains abundant rutile and zircon inclusions, has been slightly recrystallized. Thirty three isotope dilution U-Pb analyses of zircons representing single-grain fragments (29 analyses) and multi-grain fractions (4 analyses) yield a spectrum of concordant ages from 631 to 549 Ma, a time span of more than 80 myr. Back-scattered electron and cathodoluminescence images show that most grains are either homogeneous, structureless fragments (35%), or are permeated to a variable degree by anastamosing cracks occupied by relatively U- and/or Th-enriched zircon (45%); a smaller percentage of grains show relict magmatic zoning (20%). Thin, U- and/or Th-rich overgrowths occur on about 25% of grains. Raman spot analyses demonstrate that all fragments are highly crystalline and non-metamict. There are marked correlations between zircon grain size and internal features, such that the oldest grains are larger, and show relict magmatic zoning; the youngest grains are small fragments containing high-U crack networks. Ion microprobe spot analyses show that each zircon grain preserves a distinct trace element signature; rare earth element patterns show heavy REE-enrichment, with negative Eu anomalies and positive Ce anomalies. We suggest that the ca. 80 myr spread in concordant U-Pb ages in this sample is indicative of high-temperature Pb-loss during one or more protracted periods of granulite facies metamorphism, with only minor episodic or continuous metamorphic zircon growth. Volume diffusion and/or fracture-assisted diffusion seems to be the dominant mechanism of Pb-loss. Cooling curves, calculated using recently-measured Pb diffusion parameters, conform to the age-size relationship, and imply very slow cooling rates (1–2°C/myr or less), as might be expected for a terrane in which granulite conditions were maintained for an extended period of time. Our results, therefore, suggest a note of caution for interpretation of concordant zircon ages in meta-igneous rocks affected by high-grade metamorphism of long duration.

Crustal evolution and terrane correlation in the eastern Arabian Shield, Yemen: geochronological constraints

Ion-microprobe U–Pb zircon and 40 Ar– 39 Ar geochronology is presented for two gneiss terranes and an island arc terrane in the Arabian Shield of Yemen. These data confirm the presence of late Archaean ( c. 2.95–2.55 Ga) crust in the Al-Mahfid (eastern) terrane, previously inferred from Nd model age determinations; a minor late Archaean component in the Abas (western) terrane is revealed by a single c. 2.6 Ga zircon core. No ages have been obtained corresponding to documented c. 1.8–1.4 Ga events in neighbouring Somalia and Saudi Arabia, but both gneiss terranes are strongly affected by c. 760 Ma Pan-African reworking which is recorded by Pb loss in zircons of the Al-Mahfid terrane and zircon crystallization ages of granitic gneisses and sheets in both the Al-Mahfid and Abas gneiss terranes. 40 Ar– 39 Ar ages obtained from intrusive rocks in the intervening Al-Bayda arc terrane provide a lower age limit of 615 Ma upon terrane assembly. Correlation of pre-Pan-African events within the Arabian–Nubian Shield remains problematic because of the still limited sampling and available geochronology.

Dating lower crust and upper mantle events: an ion microprobe study of xenoliths from kimberlitic pipes, South Australia

Zircons separated from five xenoliths (three quartzofeldspathic granulites, one mafic granulite and one eclogite) of lower crustal and uppermost mantle derivation were dated using the SHRIMP ion microprobe. The zircon data indicate that different types of xenoliths had different origins and formed at different times. The xenoliths are from the Calcutteroo kimberlitic pipes (167–174 Ma) that outcrop on the eastern margin of the Australian craton. The combined data-set of all zircon UPb ages from all the xenoliths records four major tectonic events associated with zircon formation, recrystallization or Pb loss. These events at about 1600-1500, 780, 620 or earlier and 330 Ma, correlate with large-scale tectonic events recorded in adjacent crustal and mantle rocks: the ∼ 1600 magmatism and tectonism of the exposed Willyama Complex and Gawler Block (in New South Wales and South Australia), the 850-650 Ma rifting and faulting of the Adelaidean Fold Belt, the 650-575 Ma extension, uplift and collision episodes at the Tasman suture line and the 500-300 Ma metasomatic events in the mantle in western Victoria. However, more detail is provided by the data from individual xenolith types. A considerable number of zircons from the quartzofeldspathic granulite xenoliths grew during the granulite facies metamorphism which occurred at some time between 1600 and 1500 Ma. A small number of zircons with ages up to 1800 Ma reflect the xenoliths' prior history and other zircons grew at 610 Ma or earlier. Zircons from the mafic granulite xenolith record an event at ∼ 780 Ma, interpreted as the age of the granulite facies metamorphism of the protolith, probably shortly after the underplating and differentiation of the mantle-derived mafic magma. Zircons from the eclogite xenolith have very low U (4–13 ppm) and Th (<3 ppm) contents, similar to those observed in zircons from some exposed eclogites of N-type MORB magma derivation. Eclogite facies metamorphism not later than 620±46 Ma is inferred from the oldest 206Pb 238 U zircon age obtained from the xenolith. An event at ∼ 330 Ma caused Pb loss in zircons from all three types of xenoliths. The coincidence of the zircon age clusters with large-scale crustal or mantle events indicates the coupling of major mantle and crustal tectonic episodes in this region. The identification of multiple zircon-forming events and the inferred different times of formation of different xenolith assemblages show that SmNd isochron ages based on an assumption of a cogenetic relationship are invalid.

Recent pb loss in zircon: a natural or laboratory-induced phenomenon?

Recent pb loss in zircon: a natural or laboratory-induced phenomenon?

Recent Pb loss in zircon: A natural or laboratory-induced phenomenon?

Monazite and zircon from a Proterozoic terrain in Antarctica had different isotopic responses to temporally discrete and geologically dissimilar Phanerozoic events. The monazite lost much of its Pb during upper-greenschist-facies metamorphism at ∼ 540 Ma, whereas the zircon did not respond to this event. In contrast, although potentially more susceptible to Pb loss, the monazite was not at all affected by a recent event which caused significant isotopic resetting of coexisting zircon. The different responses to the recent event are not a consequence of chemical processing during sample preparation and are unlikely to be related to mineral separation. A semi-quantitative correlation with the degree of alteration of feldspar to clay minerals indicates that the loss of Pb from zircon was induced by incipient weathering. Though recent Pb loss is easily identifiable in zircons with simple histories it is mostly very difficult to recognise in zircons with more complex histories. As the latter can generate geologically meaningless concordia ages, it is important to assess the extent of recent Pb loss in the interpretation of all UPb zircon discordia.