Alpha Recoil Loss of Pb from Baddeleyite Evaluated by High‐Resolution Ion Microprobe (SHRIMP II) Depth Profiling and Numerical Modeling

Davis, W J; Davis, Donald W.

doi:10.1002/9781119227250.ch11

Cited by 10 publications

(5 citation statements)

References 30 publications

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“…U-Pb isotopic data for baddeleyite in unmetamorphosed to low-grade gabbros and dolerites occasionally yield variably discordant data, though the cause of discordance is not always clear. A number of studies have suggested that discordance in baddeleyite analyses in these instances could involve Pb-loss from secondary (late magmatic) zircon overgrowths on baddeleyite crystals, resulting from increasing Si activity in evolved magmas (e.g., Sahin & Hamilton 2019), partial transformation of baddeleyite to zircon during metamorphism (e.g., Söderlund et al 2013) and/or alpha recoil effects due to the wafer-thin, blade-like crystal habit of baddeleyite that yield high surface area:volume ratios, particularly in very small crystals (Romer 2003;Davis & Davis 2017). Most of the baddeleyite U-Pb results presented in this study show variable degrees of discordance -6 of 8 analyses have 206 Pb/ 238 U ages within approximately 1% of (younger than) of the oldest fraction -with the majority of the data clustered on or near concordia, convergent on an age of 596 Ma.…”

Section: Analytical Datamentioning

confidence: 99%

U-Pb baddeleyite age for the Ottfjället Dyke Swarm, central Scandinavian Caledonides: new constraints on Ediacaran opening of the Iapetus Ocean and glaciations on Baltica

Kumpulainen

Hamilton

Nystuen

2021

GFF

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The Ottfjället Dyke Swarm (ODS) is a prominent component of the Ediacaran mafic magmatism associated with opening of the Iapetus ocean, and hosted in the Särv Nappe, Middle Allochthon of the Scandinavian Caledonides. A U-Pb baddeleyite age of 596.3 ± 1.5 Ma for a thick, well preserved, plagioclase-phyric dolerite dyke in Härjedalen, Sweden, dates emplacement of the swarm. The age represents a robust, inheritance-free reference age for variably deformed and metamorphosed tholeiitic dykes in sandstone-dominated sequences of the lower part of the Middle Allochthon, representing the proximal, rifted Baltoscandian margin preceding the opening of Iapetus. The new age is within the narrow time span between 610 and 595 Ma defined by the most reliable age estimates for mafic dykes in structurally higher nappes (upper part of Middle Allochthon), representing the distal margin during the opening of Iapetus. The Ottfjället Dyke Swarm cuts the Tossåsfjället Group succession, which includes sabkha-related carbonate platform and diamictite couples, one of several correlated Neoproterozoic glaciogenic successions in Scandinavia. The intrusion age of ca. 596 Ma therefore sets a minimum age for the glaciogenic successions. It implies that Neoproterozoic glaciations in Scandinavia predate the ca. 580 Ma Gaskiers glaciation event and are probably part of the ca. 635 Ma Marinoan "Snowball-Earth"-type glaciation.

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Section: Analytical Datamentioning

confidence: 99%

U-Pb baddeleyite age for the Ottfjället Dyke Swarm, central Scandinavian Caledonides: new constraints on Ediacaran opening of the Iapetus Ocean and glaciations on Baltica

Kumpulainen

Hamilton

Nystuen

2021

GFF

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“…CC BY 4.0 License. pathway diffusion (e.g., along twin-planes) affects both systems similarly, making 207 Pb/ 206 Pb ages most accurate (Davis and Davis, 2018).…”

Section: Baddeleyite Discordancementioning

confidence: 99%

“…Our ID-TIMS analyses show linear arrays that are typical for varying degrees of recent Pb loss. The portion of radiogenic Pb lost by alpha recoil can be predicted based on crystal shapes (Davis and Davis, 2018) and is generally <0.3%…”

Section: Baddeleyite Discordancementioning

confidence: 99%

“…Strategies for mitigating this problem are selective dissolution of baddeleyite and not zircon prior to ID-TIMS (isotope dilutionthermal ionization mass spectrometry) analysis (Rioux et al, 2010), or employing high spatial resolution methods (SIMS or LA-ICP-MS) to analyze baddeleyite domains free of metamorphic zircon. Other mechanisms for baddeleyite discordance have been proposed, including alpha recoil (Davis and Sutcliffe, 1985;Davis and Davis, 2018), Pb loss due to fast pathway diffusion (Rioux et al, 2010;Söderlund et al, 2013;Schaltegger and Davies, 2017), and isotopic disequilibrium due to 231 Pa excess (Amelin and Zaitsev, 2002;Crowley and Schmitz, 2009) or 222 Rn loss (Heaman and LeCheminant, 2000). For accurate age interpretation of a given sample, the relevant discordance mechanism(s) must be identified, and therefore a more complete understanding of discordance in baddeleyite is needed.…”

Section: Introductionmentioning

confidence: 99%

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Baddeleyite microtextures and U-Pb discordance: insights from the Spread Eagle Intrusive Complex and Cape St. Mary’s sills, Newfoundland, Canada

Pohlner

Schmitt

Chamberlain

et al. 2020

Preprint

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Abstract. Baddeleyite (ZrO2) is widely used in U-Pb geochronology, but different patterns of discordance often hamper accurate age interpretations. This is also the case for baddeleyite from the Spread Eagle Intrusive Complex (SEIC) and Cape St. Mary’s sills (CSMS) from Newfoundland, which we investigated combining high precision and high spatial resolution methods. Literature data and our own observations suggest that at least seven different types of baddeleyite–zircon intergrowths can be distinguished in nature, among which we describe xenocrystic zircon inclusions in baddeleyite for the first time. Baddeleyite 207Pb/206Pb dates from secondary ionization mass spectrometry (SIMS) and isotope dilution thermal ionization mass spectrometry (ID-TIMS) are in good agreement with each other and with stratigraphic data, but some SIMS sessions of grain mounts show reverse discordance. This suggests that matrix differences between references and unknowns biased the U-Pb relative sensitivity calibration, possibly due to crystal orientation effects, or due to alteration of the baddeleyite crystals, which is indicated by unusually high common Pb contents. ID-TIMS data for SEIC and CSMS single baddeleyite crystals reveal normal discordance as linear arrays with decreasing 206Pb/238U dates, indicating that their discordance is dominated by recent Pb loss due to fast pathway or volume diffusion. Hence, 207Pb/206Pb dates are more reliable than 206Pb/238U dates even for Phanerozoic baddeleyite. Negative lower intercepts of baddeleyite discordias and direct correlations between ID-TIMS 207Pb/206Pb dates and degree of discordance indicate preferential 206Pb loss, possibly due to 222Rn mobilization. In such cases, the most reliable crystallization ages are concordia upper intercept dates or weighted means of the least discordant 207Pb/206Pb dates. We regard the best estimates of the intrusion ages to be 498.7 ± 4.5 Ma (2σ; ID-TIMS upper intercept date for one SEIC dike) and 439.4 ± 0.8 Ma (ID-TIMS weighted mean 207Pb/206Pb date for one sill of CSMS). Sample SL18 of the Freetown Layered Complex, Sierra Leone (associated with the Central Atlantic Magmatic Province) was investigated as an additional reference. For SL18, we report a revised 201.07 ± 0.64 Ma intrusion age, based on a weighted mean 207Pb/206Pb date of previous and new baddeleyite ID-TIMS data, agreeing well with corresponding SIMS data. Increasing discordance with decreasing crystal size in SL18 indicates that Pb loss affected baddeleyite rims more strongly than cores. Employment of SIMS or mechanical abrasion prior to ID-TIMS analysis may therefore produce more concordant baddeleyite data. We emphasize that the combination of high precision and high spatial resolution dating, along with detailed microscale imaging of baddeleyite, is powerful for extracting reliable age information from baddeleyite from rocks with a complex post-magmatic evolution.

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“…At the smallest grain sizes, ejection of daughter Pb atoms from crystal surfaces through direct alpha recoil ejection can result in discordant U-Pb ages from the outermost 24 nm (± 7 nm) of the baddeleyite crystal (Davis and Davis, 2017). This would only become an issue when sampling small grains (< 15 µm thick) in their entirety, as the large surface-area to volume ratio would potentially lead to slightly discordant U-Pb ages following extensive ejection of daughter isotopes (Romer, 2003), requiring a simple linear correction on the order of 0.1 -0.5% (Davis and Davis, 2017). Subsampling internal domains of larger grains will circumvent this issue, allowing the targeted extraction of centralised regions which are unlikely to have lost Pb during an alpha recoil event.…”

Section: Minimum Sample Sizes Accessible By Fib-timsmentioning

confidence: 99%

Highly accurate dating of micrometre-scale baddeleyite domains through combined focused ion beam extraction and U-Pb thermal ionisation mass spectrometry (FIB-TIMS)

White¹,

Tait²,

Kamo³

et al. 2019

Preprint

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Abstract. Baddeleyite is a powerful chronometer of mafic magmatic and meteorite impact processes. High precision and accuracy U-Pb ages can be measured from single grains by isotope dilution thermal ionisation mass spectrometry (ID-TIMS), but this requires destruction of the host rock for highly challenging grain isolation and dissolution. As a result, the technique is rarely applied to precious samples with very limited availability (such as lunar, Martian and asteroidal meteorites and returned samples) or samples containing small baddeleyite grains that cannot readily be isolated by conventional mineral separation techniques. Here, we use focused ion beam (FIB) techniques, utilising both Xe+ plasma and Ga+ ion sources, to liberate baddeleyite subdomains in-situ, allowing their extraction for ID-TIMS dating. We have analysed the U-Pb isotope systematics of domains ranging between 200 um and 10 um in length and 5 ug to 0.1 ug in mass. In total, seven domains of Phalaborwa baddeleyite extracted using a Xe+-pFIB yield a weighted mean 207 Pb/206 Pb age of 2060.1 ± 2.4 Ma (0.12 %; all uncertainties 2 sigma), within uncertainty of reference values. The smallest extracted domain (ca. 10 × 15 times; 10 um) yields an internal 207 Pb/206 Pb uncertainty of ±0.15 %. Comparable levels of precision are achieved using a Ga+-source FIB instrument (±0.20 %), though the slower cutting speed limits potential application to larger grains. While the U-Pb data are between 0.5 and 13.6 % discordant, the results generate a precise upper intercept age in U-Pb concordia space of 2061.1 × 7.4 Ma; (0.72 %). Importantly, the extent of discordance does not correlate with the ratio of material to ion-milled surface area, showing that the FIB extraction does not induce disturbance of U-Pb systematics even the smallest extracted domains. Instead, we confirm the natural U-Pb variation and discordance within the Phalaborwa baddeleyite population observed with other geochronological techniques. Our results demonstrate the FIB-TIMS technique to be a powerful tool for high-accuracy in-situ U-Pb dating, which makes a wide range of targets and processes newly accessible to geochronology.

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Alpha Recoil Loss of Pb from Baddeleyite Evaluated by High‐Resolution Ion Microprobe (SHRIMP II) Depth Profiling and Numerical Modeling

Cited by 10 publications

References 30 publications

U-Pb baddeleyite age for the Ottfjället Dyke Swarm, central Scandinavian Caledonides: new constraints on Ediacaran opening of the Iapetus Ocean and glaciations on Baltica

U-Pb baddeleyite age for the Ottfjället Dyke Swarm, central Scandinavian Caledonides: new constraints on Ediacaran opening of the Iapetus Ocean and glaciations on Baltica

Baddeleyite microtextures and U-Pb discordance: insights from the Spread Eagle Intrusive Complex and Cape St. Mary’s sills, Newfoundland, Canada

Highly accurate dating of micrometre-scale baddeleyite domains through combined focused ion beam extraction and U-Pb thermal ionisation mass spectrometry (FIB-TIMS)

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