Calibrating the middle and late Permian palynostratigraphy of Australia to the geologic time-scale via U–Pb zircon CA-IDTIMS dating

Laurie, John R.; Bodorkos, S.; Nicoll, Robert S.; Crowley, James L.; Mantle, Daniel; Mory, Arthur J.; Wood, Geoff; Backhouse, John; Holmes, Erin; Smith, Tegan; Champion, D.C.

doi:10.1080/08120099.2016.1233456

Cited by 76 publications

(97 citation statements)

References 32 publications

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“…Four analytical sessions (110061,110066,110088,110106) were run with standard zircon using the 18 O − 2 primary beam. Two of these contained zircons of unknown age, the geochronological results of which are published in Laurie et al (2016). The reference materials in those runs consisted of the following:…”

Section: Experiments B: Standard Zirconmentioning

confidence: 99%

Author response to referee comments on GI-2017-07

Magee¹

2017

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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 isotopologue-based 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-Th-Sm / 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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Section: Experiments B: Standard Zirconmentioning

confidence: 99%

Author response to referee comments on GI-2017-07

Magee¹

2017

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“…No statistically significant isotopologue disequilibrium trends related to age are observed, but only one session (130058) had OG1 grains analysed, and the extreme age of this reference material (Stern et al, 2009) dominates any age-related statistics. The geochronologic results from unknowns M750 and M736 analysed in session 110088 are given in Laurie et al, (2016) alongside ID-TIMS results for the same samples. The agreement between the SHRIMP ages and the ID-TIMS ages for these samples suggests that the 18 O − 2 primary beam introduces no statistically significant geochronological bias.…”

Section: Experiments B: Standard Zirconsmentioning

confidence: 99%

“…Two of these contained zircons of unknown age, the geochronological results of which are published in Laurie et al (2016). The reference materials in those runs consisted of the following: …”

Section: Experiments B: Standard Zirconmentioning

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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“…26; The climate record for the Permian in eastern Australia (Fielding et al, 2008;Frank et al, 2015) is confused. This is because: (1) new CA-IDTIMS dating has substantially revised the ages of many units in the Bowen Basin formerly placed in the Guadalupian and Lopingian and now considered Lopingian (Metcalfe et al, 2015Laurie et al, 2016);and (2) ACCEPTED MANUSCRIPT…”

Section: Comparison Of East Gondwana Palaeoclimate Trendsmentioning

confidence: 99%

“…No distinct cyclic pattern in sedimentation is recognized, although the basal mudstone beds above the Poole Sandstone were deposited during marine flooding and the transition to sandstones in the overlying Lightjack Formation forms part of a progradational depositional trend. The upper age limit for the Noonkanbah Formation is constrained by the presence of the Roadian ammonoid Daubichites about 60 m above the base of the Lightjack Formation (Glenister et al, 1993) and IDTIMS dates of 268.1-270.1 Ma for ash beds within this formation (Mory et al, 2012;Laurie et al, 2016). The age of the base of the Noonkanbah Formation is…”

Section: Canning Basin: Noonkanbah Formationmentioning

confidence: 99%