Two Carboniferous Ages&lt;subtitle&gt;A Comparison of Shrimp Zircon Dating with Conventional Zircon Ages and &lt;sup&gt;40&lt;/sup&gt;Ar/&lt;sup&gt;39&lt;/sup&gt;Ar &lt;italic /&gt;Analysis&lt;/subtitle&gt;

Claoué-Long, Jonathan C.; Compston, W.; Roberts, John C.; Fanning, C. Mark

doi:10.2110/pec.95.04.0003

Cited by 345 publications

(228 citation statements)

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“…Discs were dried in an oven (1 h at 60°C) and the surface was gold coated at 40 μm and processed on the SHRIMP II A (Sensitive High Resolution Ion Microprobe) at Curtin University, Western Australia, following the procedure described by Wingate and Kirkland (2017). Analytical conditions and operational procedures have previously been described (Claoué-Long et al, 1995;Compston et al, 1984;De Laeter and Kennedy, 1998;Kennedy and De Laeter, 1994;Wingate and Kirkland, 2017). Rastering of the O 2-beam was conducted for 120-150 s to reduce common Pb contamination before each analysis, and each spot was focused using a primary ion beam of 1.5-1.9 nA with a diameter of ∼25 μm.…”

Section: Shrimp U-pb Geochronologymentioning

confidence: 99%

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 Geochronologymentioning

confidence: 99%

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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“…This method is commonly used to date Phanerozoic zircons using in situ methods [42,47]. The closure temperature of magmatic zircon is usually higher than 900°C [48,49], in any case higher than 750-800°C [50], while the peak metamorphism temperature in upper HCS in this area is 570-690°C (samples N10 and N12 of Hodges et al [51]), suggesting that the zircon crystallization ages will be preserved.…”

Section: Results and Analysis Of Monazite-zircon U-th/pb Agesmentioning

confidence: 99%

Ductile deformation within Upper Himalaya Crystalline Sequence and geological implications, in Nyalam area, Southern Tibet

Liu

Leloup

et al. 2012

Chin. Sci. Bull.

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The South Tibet Detachment System (STDS) is a flat normal fault that separates the Upper Himalaya Crystalline Sequence (UHCS) below from the Tethyan Sedimentary Sequence (TSS) above. Timing of deformations related to the STDS is critical to understand the mechanism and evolution of the Himalaya collision zone. The Nyalam detachment (ND) (~86°E) locates in the middle portion of STDS (81°-89°E). Dating of deformed leucocratic dykes that are most probably syntectonic at different depth beneath the ND, allow us to constrain the timing of deformation. (1) Dyke T11N37 located ~3500 m structurally below the ND emplaced at 27.4± 0.2 Ma; (2) Dyke T11N32 located ~1400 m structurally below the ND emplaced at 22.0±0.3 Ma; (3) T11N25 located within the top to the north STD shear zone, ~150 m structurally below the ND, emplaced at 17.1±0.2 Ma. Combining ND footwall cooling history and T11N25 deformation temperature, we indicate a probable onset of top to the north deformation at ~16 Ma at this location. These results show an upward younging of the probable timing of onset of the deformation at different structural distance below the ND. We then propose a new model for deformation migration below the ND with deformation starting by pure shear deformation at depth prior to ~27.5 Ma that migrates upward at a rate of ~ 0.3 mm/a until ~18 Ma when deformation switches to top to the north shearing in the South Tibet Detachment shear zone (STDsz). As deformation on the ND stops at 14-13 Ma this would imply that significant top to the North motion would be limited to less than 5 Ma and would jeopardize the importance of lower channel flow.

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“…The current method for doing this is the use of a calibration relating the detected ion ratio of Pb + / U + to the ratio of an oxidized molecular U-bearing ion to a reduced one. The most widely used calibration is a power law relationship between Pb + / U + and UO + / U + (Claoué-Long et al, 1995), but other calibrations using various combinations of U + , UO + , and UO + 2 have been used (Stern and Amelin, 2003). These calibrations apply to both SHRIMP and other SIMS instruments used for geochronology.…”

Section: W Magee Jr Et Al: Isotopologue Disequilibrium In Simsmentioning

confidence: 99%

Extreme isotopologue disequilibrium in molecular SIMS species during SHRIMP geochronology

Magee

Danišík

Mernagh

2017

Geosci. Instrum. Method. Data Syst.

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Abstract. The current limitation in the accuracy and precision of inter-element analysis in secondary ion mass spectrometry (SIMS) is the ability to find measurable quantities that allow relative differences in ionization and transmission efficiency of secondary ions to be normalized. In uraniumthorium-lead geochronology, the ability to make these corrections, or "calibrate" the data, results in an accuracy limit of approximately 1 %. This study looks at the ionization of uranium and thorium oxide species, which are traditionally used in U-Pb calibration, to explore the conditions under which isotopologues, or molecular species whose composition differs only in the isotopic composition of one or more atoms in the molecule, remain in or deviate from equilibrium.Isotopologue deficits of up to 0.2 (200 ‰) below ideal mixing are observed in UO + 2 species during SIMS gechronological analyses using the SHRIMP IIe SIMS instrument. These are identified by bombarding natural U-bearing minerals with an 18 O − 2 primary beam. The large anomalies are associated with repeat analyses down a single SIMS sputtering crater (Compston et al., 1984), analysis of high-uranium, radiation-damaged zircon, and analysis of baddeleyite. Analysis of zircon under routine conditions yield UO + 2 isotopologue anomalies generally within a few percent of equilibrium. The conditions under which the isotopologue anomalies are observed are also conditions in which the UO x -based corrections, or calibration, for relative U vs. Pb ionization efficiencies fail. The existence of these isotopologue anomalies suggest that failure of the various UO x species to equilibrate with each other is the reason that none of them will successfully correct the U / Pb ratio. No simple isotopologuebased correction is apparent. However, isotopologue disequilibrium appears to be a more sensitive tool for detecting high-U calibration breakdowns than Raman spectroscopy, which showed sharper peaks for ∼ 37 Ma high-uranium zircons than for reference zircons OG1 and Temora. U-ThSm / He ages were determined for aliquots of reference zircons OG1 (755±71 Ma) and Temora (323±43 Ma), suggesting that the broader Raman lines for the Temora reference zircons may be due to something other than accumulated radiation damage.Isotopologue abundances for UO + and ThO + and their energy spectra are consistent with most or all molecular species being the product of atomic recombination when the primary beam impact energy is greater than 5.7 keV. This, in addition to the large UO + 2 instrumentally generated isotopologue disequilibria, suggests that any attempts to use SIMS to detect naturally occurring isotopologue deviations could be tricky.

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Two Carboniferous Ages<subtitle>A Comparison of Shrimp Zircon Dating with Conventional Zircon Ages and <sup>40</sup>Ar/<sup>39</sup>Ar <italic />Analysis</subtitle>

Cited by 345 publications

References 0 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

Ductile deformation within Upper Himalaya Crystalline Sequence and geological implications, in Nyalam area, Southern Tibet

Extreme isotopologue disequilibrium in molecular SIMS species during SHRIMP geochronology

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