Re-equilibration of Zircon in Aqueous Fluids and Melts

Geisler, Thorsten; Schaltegger, Urs; Tomaschek, Frank

doi:10.2113/gselements.3.1.43

Cited by 710 publications

(420 citation statements)

References 30 publications

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“…Some published illite crystallinity data from the middle Ecca Group support lowgrade metamorphic temperatures of 170-200°C (Hälbich et al, 1983), which is within range of when fluid-mediated lattice damage initiates in zircon (Geisler et al, 2002;Geisler et al, 2007). However, we did not find any clear evidence for reabsorption or alteration of the oscillatory zoning in the zircons (Fig.…”

Section: Shrimp U-pb Geochronologysupporting

confidence: 51%

Refining the chronostratigraphy of the Karoo Basin, South Africa: magnetostratigraphic constraints support an early Permian age for the Ecca Group

Belica¹,

Tohver²,

Poyatos‐Moré

et al. 2017

Geophysical Journal International

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Summary:The Beaufort Group of the Karoo Basin, South Africa provides an important chrono-and biostratigraphic record of vertebrate turnovers that have been attributed to the End-Permian mass extinction events at ca. 252 Ma and ca. 260 Ma. However, an unresolved controversy exists over the age of the Beaufort Group due to a large dataset of published U-Pb SHRIMP zircon results that indicate a ca. 274-250 Ma age range for deposition of the underlying Ecca Group. This age range requires the application of a highly diachronous sedimentation model to the Karoo Basin stratigraphy and is not supported by published paleontologic and palynologic data. This study tested the strength of these U-Pb isotopic datasets using a magnetostratigraphic approach. Here we present a composite ~1500 m section through a large part of the Ecca Group from the Tanqua depocentre, located in the southwestern segment of the Karoo Basin. After the removal of two normal polarity overprints, a likely primary magnetic signal was isolated at temperatures above 450°C. This section is restricted to a reverse polarity, indicating that it formed during the Kiaman Reverse Superchron (ca. 318-265 Ma), a distinctive magnetostratigraphic marker for Early−Middle Permian rocks. The Ecca Group has a corresponding paleomagnetic pole at 40.8°S, 77.4°E (A95 = 5.5°). UPb SHRIMP ages on zircons are presented here for comparison with prior isotopic studies of the Ecca Group. A weighted mean U-Pb age of 269.5 ± 1.2 Ma was determined from a volcanic ash bed located in the uppermost Tierberg Formation sampled from the OR1 research core. The age is interpreted here as a minimum constraint due to a proposed Pb-loss event that has likely influenced a number of published results. A comparison with the Geomagnetic Polarity Time Scale as well as published U-Pb TIMS ages from the overlying Beaufort Group supports a ca. 290-265 Ma age for deposition of the Ecca Group.

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Section: Shrimp U-pb Geochronologysupporting

confidence: 51%

Refining the chronostratigraphy of the Karoo Basin, South Africa: magnetostratigraphic constraints support an early Permian age for the Ecca Group

Belica¹,

Tohver²,

Poyatos‐Moré

et al. 2017

Geophysical Journal International

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“…The detailed study of Geisler et al (2007) inferred a diffusion-limited dissolutionrecrystallization process along a reaction front as a cause for disequilibrium during zircon growth, which strikingly resembles the patchy-zoning texture observed in the MC zircons. This process is dynamic and occurs along inwardmoving interfaces (e.g.…”

Section: Discussionmentioning

confidence: 66%

“…2c, 3a) may represent channels along which dissolution and recrystallization took place. It is inferred that dissolution and recrystallization occurred simultaneously (Geisler et al, 2007), as suggested by evidence of inward replacement (Fig. 2a, e) along porous zircon.…”

Section: Discussionmentioning

confidence: 87%

“…This texture is the result of a diffusionlimited dissolution-recrystallization, which involves progressive replacement of high U-Th-Y zircon, thought to have crystallized from a crustal peraluminous melt in equilibrium with monazite, by low U-Th-Y zircon, interpreted to be a response of mixing between peraluminous and metaluminous melts. As far as the authors are aware, this study provides the first example of zircon re-equilibration in melts through a process of dissolution-recrystallization (Geisler et al 2007). …”

Section: Discussionmentioning

confidence: 89%

“…Micrometer-sized pores, which may be a criterion for such a reaction (Tomaschek et al 2003), result from differences in the molar volume between the parent zircon and the reprecipitated zircon. Such pores enhance exchange of elements during dissolution by creating a pathway from the reaction front (Geisler et al 2007). In the MC zircons, pores tend to occur in the reacted high U-Th-Y zircon phase in close proximity to the contact between low and high U-Th-Y zircons (Figs.…”

Section: Discussionmentioning

confidence: 99%

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Zircon texture and chemical composition as a guide to magmatic processes and mixing in a granitic environment and coeval volcanic system

Gagnevin

Daly

Kronz

2009

Contrib Mineral Petrol

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This study documents the chemical and textural responses of zircon in the Elba igneous complex, with particular reference to the 7-to 7.8-Ma-old Monte Capanne pluton in relation to its coeval volcanic counterpart (Capraia), using BSE imaging and quantitative electron microprobe analyses. The Monte Capanne pluton displays multiple field and geochemical evidence for magma mixing. The samples we have investigated (including monzogranitic, mafic enclave and dyke samples) display similar zircon textures and are associated with an extremely large range of trace and minor element (Hf, Y, HREE, Th, U) compositions, which contrast with relatively simple textures and zoning patterns in zircons from a Capraia dacite. We have used a relatively simple textural classification (patchy zoning, homogenous cores, oscillatory zoning and unzoned zircon) as the basis for discussing the chemical composition and chemical variation within zircons from the Monte Capanne pluton. Based on these data and other works in Lithos 78:101-118, 2004 , it is inferred that mixing between metaluminous and peraluminous melts occurred early in the evolution of the Monte Capanne magma chamber. In particular, mixing was responsible for the development of the patchy-zoning texture in the zircon cores, which was associated with reactions between other accessory phases (including monazite, apatite, allanite), which we infer to have significantly affected the Th distribution in zircon. Zircons from the MC pluton displaying ''homogeneous cores'' have chemical affinities with zircons in the coeval Capraia volcanic system, consistent with the participation of a Capraia-like mantle end-member during mixing. Further zircon growth in the MC pluton produced the oscillatory zoning texture, which records both long-term (crystal fractionation) and transient (recharge with both silicic and mafic magmas) events in a hybrid magma chamber. It is inferred that Hf and the Th/U ratio cannot be used alone to infer magmatic processes due to their dependency on temperature, nor are they a diagnostic feature of xenocrystic grains. This study shows that zircon chemistry coupled with detailed textural analysis can provide a powerful tool to elucidate the complex evolution of a magma system.

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U‐Pb dating of interspersed gabbroic magmatism and hydrothermal metamorphism during lower crustal accretion, Vema lithospheric section, Mid‐Atlantic Ridge

et al. 2015

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New U/Pb analyses of zircon and xenotime constrain the timing of magmatism, magmatic assimilation, and hydrothermal metamorphism during formation of the lower crust at the Mid-Atlantic Ridge. The studied sample is an altered gabbro from the Vema lithospheric section (11°N). Primary gabbroic minerals have been almost completely replaced by multiple hydrothermal overprints: cummingtonitic amphibole and albite formed during high-temperature hydration reactions and are overgrown first by kerolite and then prehnite and chlorite. In a previous study, clear inclusion-free zircons from the sample yielded Th-corrected 206 Pb/ 238 U dates of 13.528 ± 0.101 to 13.353 ± 0.057 Ma. Ti concentrations, reported here, zoning patterns and calculated Th/U of the dated grains are consistent with these zircons having grown during igneous crystallization. To determine the timing of hydrothermal metamorphism, we dated a second population of zircons, with ubiquitous <1-20 μm chlorite inclusions, and xenotimes that postdate formation of metamorphic albite. The textures and inclusions of the inclusion-rich zircons suggest that they formed by coupled dissolution-reprecipitation of metastable igneous zircon during or following hydrothermal metamorphism. Th-corrected 206 Pb/ 238 U dates for the inclusion-rich zircons range from 13.598 ± 0.012 to 13.503 ± 0.018 Ma and predate crystallization of all but one of the inclusion-free zircons, suggesting that the inclusion-rich zircons were assimilated from older hydrothermally altered wall rocks. The xenotime dates are sensitive to the Th correction applied, but even using a maximum correction, 206 Pb/ 238 U dates range from 13.341 ± 0.162 to 12.993 ± 0.055 Ma and postdate crystallization of both the inclusion-rich zircons and inclusion-free igneous zircons, reflecting a second hydrothermal event. The data provide evidence for alternating magmatism and hydrothermal metamorphism at or near the ridge axis during accretion of the lower crust at a ridge-transform intersection and suggest that hydrothermally altered crust was assimilated into younger gabbroic magmas. The results of this study show that high-precision U-Pb dating is a powerful method for studying the timing of magmatic and hydrothermal processes at mid-ocean ridges.

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Re-equilibration of Zircon in Aqueous Fluids and Melts

Cited by 710 publications

References 30 publications

Refining the chronostratigraphy of the Karoo Basin, South Africa: magnetostratigraphic constraints support an early Permian age for the Ecca Group

Refining the chronostratigraphy of the Karoo Basin, South Africa: magnetostratigraphic constraints support an early Permian age for the Ecca Group

Zircon texture and chemical composition as a guide to magmatic processes and mixing in a granitic environment and coeval volcanic system

U‐Pb dating of interspersed gabbroic magmatism and hydrothermal metamorphism during lower crustal accretion, Vema lithospheric section, Mid‐Atlantic Ridge

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