Baddeleyite formation in zircon by Ca-bearing fluids in silica-saturated systems in nature and experiment: resetting of the U–Pb geochronometer

Lewerentz, Alexander; Harlov, Daniel E.; Scherstén, Anders; Whitehouse, Martin J.

doi:10.1007/s00410-019-1600-8

Cited by 20 publications

(16 citation statements)

References 71 publications

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“…Its stability and lack of common lead has made it a valuable tool in geochronology, especially with the advent of high-spatial-and high-massresolution ion probes (e.g., SHRIMP) and laser ablation ICP-MS techniques (e.g., Harley and Kelly, 2007). Zr, Hf, and O isotope studies are used also to trace magma genesis, metamorphic reactions, and alteration mechanisms (e.g., Liu et al, 2009;Valley et al, 2010;Ibañez-Mejia and Tissot, 2019). Zircon (ZrSiO 4 ), an orthosilicate, conforms to the formula ABO 4 , in which A represents the relatively large Zr ion in eight-fold coordination with O, and B represents the Si ion in tetrahedral coordination with O.…”

Section: Results -Modes Of Occurrence and Compositions Of Zirconmentioning

confidence: 99%

Zirconium-bearing accessory minerals in UK Paleogene granites: textural, compositional, and paragenetic relationships

Belkin

Macdonald

2021

Eur. J. Mineral.

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Abstract. The mineral occurrences, parageneses, textures, and compositions of Zr-bearing accessory minerals in a suite of UK Paleogene granites from Scotland and Northern Ireland are described. Baddeleyite, zirconolite, and zircon, in that sequence, formed in hornblende + biotite granites (type 1) and hedenbergite–fayalite granites (type 2). The peralkaline microgranite (type 3) of Ailsa Craig contains zircon, dalyite, a eudialyte-group mineral, a fibrous phase which is possibly lemoynite, and Zr-bearing aegirine. Hydrothermal zircon is also present in all three granite types and documents the transition from a silicate-melt environment to an incompatible element-rich aqueous-dominated fluid. No textures indicative of inherited zircon were observed. The minerals crystallized in stages from magmatic through late-magmatic to hydrothermal. The zirconolite and eudialyte-group mineral are notably Y+REE-rich (REE signifies rare earth element). The crystallization sequence of the minerals may have been related to the activities of Si and Ca, to melt peralkalinity, and to local disequilibrium.

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Section: Results -Modes Of Occurrence and Compositions Of Zirconmentioning

confidence: 99%

Zirconium-bearing accessory minerals in UK Paleogene granites: textural, compositional, and paragenetic relationships

Belkin

Macdonald

2021

Eur. J. Mineral.

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“…However, the presence of impurities and metamictization cause the appearance of baddeleyite at temperatures below 1600 °C (e.g., 1380–1480; 44 1477; 45 1285; 46 1400; 47 and 1400 °C; 3 see also refs ( 43 ) and ( 48 )). Notably, baddeleyite has also been reported at even lower temperatures by recrystallization of highly metamict zircon (927; 49 852; 50 800–950; 51 600–900; 52 and 900 °C, 53 ).…”

Section: Discussionmentioning

confidence: 99%

Experimental Annealing of Zircon: Influence of Inclusions on Stability, Intracrystalline Melt Migration, Common Lead Leaching, and Permeability to Fluids

Morales

Molina

Cambeses

et al. 2022

ACS Earth Space Chem.

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Zircon derived from crustal rocks can survive dissolution into hot basalts during magma hybridization and rock assimilation if it is shielded as an inclusion phase in early-formed phenocrysts or in minerals from non-disaggregated xenoliths. Under these conditions, zircon can be thermally shocked, triggering recrystallization of metamict domains and reaction with its hosted mineral inclusions. This work simulates this process by performing thermal annealing experiments on zircon grains with variable degrees of metamictization. These were embedded in cristobalite powder under a N 2 atmosphere at 1 bar and 1300 °C. The thermal annealing produces recrystallization of metamict domains, melting of multi-phase mineral inclusions, nanopore formation, and microcrack propagation by thermo-elastic stress. The porosity enhances intracrystalline melt mobility, leaching out trace-element and mineral impurities. Baddeleyite was formed at temperatures below the thermal decomposition of pure zircon by two mechanisms: (i) recrystallization of metamict domains assisted by silica migration from the reaction site and (ii) incongruent zircon dissolution into molten mineral inclusions with a high CaO/SiO 2 ratio. Highly metamict zircons with elevated common Pb and radiogenic Pb loss, which were impossible to date with SHRIMP, lost all their common Pb and some radiogenic Pb upon annealing, producing well-fitted discordias with a significant upper intercept age.

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“…Alteration by fluids with high SiO2 activity causes baddeleyite replacement by zircon, but fluids poor in Si and rich in Ca can induce the opposite effect even in siliceous rocks (Lewerentz et al, 2019). In sample S2C, multiple µm-sized baddeleyite inclusions are hosted in the outer zones of zircon, which shows porous domains (Figure 5a), cracks (Figure 5c), and high contents of common Pb, which are typical alteration features (e.g., Corfu et al, 2003;Rayner et al, 2005;Aranovich et al, 2017).…”

Section: Occurrence Textures and Interrelations Of Accessory Mineralsmentioning

confidence: 99%

“…In sample S2C, multiple µm-sized baddeleyite inclusions are hosted in the outer zones of zircon, which shows porous domains (Figure 5a), cracks (Figure 5c), and high contents of common Pb, which are typical alteration features (e.g., Corfu et al, 2003;Rayner et al, 2005;Aranovich et al, 2017). Whereas other fluid-mediated processes may also be capable of forming secondary baddeleyite inclusions in altered zircon (Lewerentz et al, 2019), the former presence of fluids with high Ca/Si ratio in S2C is likely due to widespread albitization of plagioclase. Previously reported occurrences of secondary baddeleyite inclusions in zircon are from rocks that experienced high temperature (mostly amphibolite facies) alteration (Barth et al, 2002;Aranovich et al, 2013Aranovich et al, , 2017Lewerentz et al, 2019), and experiments reproducing this texture were conducted at 600°C and 900°C (Lewerentz et al, 2019).…”

Section: Occurrence Textures and Interrelations Of Accessory Mineralsmentioning

confidence: 99%

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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Baddeleyite formation in zircon by Ca-bearing fluids in silica-saturated systems in nature and experiment: resetting of the U–Pb geochronometer

Cited by 20 publications

References 71 publications

Zirconium-bearing accessory minerals in UK Paleogene granites: textural, compositional, and paragenetic relationships

Zirconium-bearing accessory minerals in UK Paleogene granites: textural, compositional, and paragenetic relationships

Experimental Annealing of Zircon: Influence of Inclusions on Stability, Intracrystalline Melt Migration, Common Lead Leaching, and Permeability to Fluids

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

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