Measurement of SIMS Instrumental Mass Fractionation of Pb Isotopes During Zircon Dating

Stern, Richard A.; Bodorkos, S.; Kamo, Sandra L.; Hickman, Arthur H.; Corfú, Fernando

doi:10.1111/j.1751-908x.2009.00023.x

Cited by 313 publications

(166 citation statements)

References 32 publications

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“…For SHRIMP U-Pb analyses, the individual spot uncertainties for calibrated 206 Pb/ 238 U values are typically about ±2% at 95% confidence (Stern & Amelin 2003, Black et al 2004, Fletcher et al 2010, Stern et al 2009). Our data consistently achieve this level of reproducibility for the standards.…”

Section: Discussionmentioning

confidence: 99%

GRENVILLE SKARN TITANITE: POTENTIAL REFERENCE MATERIAL FOR SIMS U-Th-Pb ANALYSIS

Kennedy¹,

Kamo²,

Nasdala³

et al. 2010

The Canadian Mineralogist

129

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We have investigated the homogeneity, chemical composition, structure, degree of radiation damage, and post-formation evolution of titanite crystals from skarns of the Grenville Province of the Canadian Shield using SHRIMP, TIMS, Raman and PL spectroscopy, EBSD, and EPMA-WDS. These results are used to assess the potential of the titanite as Reference Material (RM) for micro-analytical U-Th-Pb age dating. The SHRIMP data show that these megacrysts (5-31g) have concordant U-Pb isotope systematics, 60 to 500 ppm U, 120 to 1200 ppm Th , 206 Pb/ 204 Pb between 500 and 2500, ages of ~1 Ga, and excellent homogeneity at the scale of the analytical volume of the ion probe. The ID-TIMS titanite data for OLT1, OLT2 and TCB show that these crystals are essentially concordant. Data for OLT1 and OLT2 show slight scatter (i.e., in excess of that expected from the uncertainty in an individual analysis). For OLT1, one of seven analyses shows Pb loss or, possibly, a younger period of growth. Crystals OLT1 and OLT2 have respective TIMS concordia ages of 1014.8 ± 2.0 Ma (2s, n = 6, MSWD = 1.8) and 998.0 ± 4.5 Ma (2s, n = 3, MSWD = 3.3) for domains that have not lost Pb. The TIMS analyses of TCB are tightly clustered and give a concordia age of 1018.1 ± 1.7 Ma (2s, n = 4, MSWD = 0.92). Raman and PL spectra show a low to moderate degree of accumulated radiation-induced damage in the Grenville Skarn Titanite crystals and uniform internal distributions of this damage. The EDSB contrast images indicate little or no crystallographic misorientation. The EMPA-WDS data show that the outer 50-100 mm of the OLT1 and TCB crystals are enriched in Al and F, and depleted in Fe and Nb, when compared with the interior. In spite of the variation in composition and degree of radiation damage amongst samples, there are no identifiable matrix effects in our SHRIMP data. Some Grenville skarn titanite (GST) crystals have potential as RM for micro-analytical U-Th-Pb age dating. Crystal TCB has excellent homogeneity of U-Th-Pb isotopic composition. Crystals OLT1 and OLT2 have minor TIMS age heterogeneity. However, this heterogeneity is smaller than that of the Khan titanite, our current in-house titanite standard. Careful selection of analysis areas during SIMS, and of chips for TIMS analysis, allows high-quality isotopic data to be obtained from these large crystals of titanite.

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Section: Discussionmentioning

confidence: 99%

GRENVILLE SKARN TITANITE: POTENTIAL REFERENCE MATERIAL FOR SIMS U-Th-Pb ANALYSIS

Kennedy¹,

Kamo²,

Nasdala³

et al. 2010

The Canadian Mineralogist

129

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“…Isotopic intensities were measured using an Agilent 7700 s quadrupole ICPMS, with high-purity Ar as the plasma gas (flow rate 0.98 L min −1 ). The dwell time for all masses was 0.01 s. The primary reference material used for zircon U-Pb dating in this study was the OG-1 zircon standard (3465.4 ± 0.6 Ma; (Stern et al, 2009)) with the following secondary zircon standards: Plešovice (337.13 ± 0.37 Ma; Sláma et al, 2008), R33 (419.26 ± 0.39 Ma;Black et al, 2004), and GJ-1(601.7 ± 1.4 Ma; Jackson et al, 2004). 206 Pb/ 238 U ages calculated for all zircon age standards, treated as unknowns, were within 3% of the accepted values.…”

Section: U-pb Geochronologymentioning

confidence: 99%

Apatite: a U-Pb thermochronometer or geochronometer?

Kirkland

Yakymchuk

Szilas

et al. 2018

Lithos

116

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Apatite is an accessory mineral that is frequently found in both igneous and clastic sedimentary rocks. It is conventionally considered to be characterized by a closure temperature range between 375 and 600°C and hence has been employed to address mid-temperature thermochronology questions relevant to the reconstruction of thermal events in the middle to lower crust. However, questions remain as to whether apatite faithfully records thermally-activated volume diffusion profiles, or rather is influenced by recrystallization and new growth processes. We present a case study of two apatite samples from the Akia Terrane in Greenland that help chart some of the post magmatic history of this region. Apatite in a tonalitic gneiss has distinct U-enriched rims and its U-Pb apparent ages correlate with Mn chemistry, with a high Mn group yielding an age of c. 2813 Ma. The U-Pb and trace element chemistry and morphology support an interpretation in which these apatite crystals are originally igneous and record cooling after metamorphism, with subsequent generation of discrete new rims. Epidote observed in the sample implies a b600°C fluid infiltration event associated with apatite rims. The second sample, from a granitic leucosome, contains apparently homogeneous apatite, however U-Pb analyses define two distinct discordia arrays with different common Pb components. An older, c. 2490 Ma, component is associated with elevated Sr, whereas a younger, c. 1800 Ma, component has lower Sr concentration. A depth profile reveals an older core with progressively younger ages towards a compositionally discrete late Paleoproterozoic rim. The chemical and age profiles do not directly correspond, implying different diffusion rates between trace elements and U and Pb. The variation in core ages is interpreted to reflect radiogenic-Pb loss from a metamorphic population during new rim growth. The younger, c. 1800 Ma U-Pb age is interpreted to date new apatite growth from a compositionally distinct reservoir driven by tectonothermal and fluid activity, consistent with regional mica Ar-Ar ages. Results from these two samples show that recrystallization, dissolution and regrowth processes likely formed the younger rim overgrowths, and at temperatures below those often considered to be closure temperatures for Pb diffusion in apatite. The results from these samples imply many apatite grains may not record simple thermally activated Pb diffusion profiles and cautions against inversion of apatite U-Pb data to thermal histories without detailed knowledge of the grain growth/alteration processes.

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“…2. 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.…”

Section: Experiments B: Standard Zirconsmentioning

confidence: 99%

“…(Black et al, 2004) zircon were analysed, along with 22 grains of R-33 (Black et al, 2004) and 8 grains of OG1 (Stern et al, 2009). In 110057, an oriented Phalabowra baddeleyite mount from the original orientation work (Wingate and Compston, 2000) was analysed, with a dozen analyses on each of the four oriented megacrysts.…”

Section: Methodsmentioning

confidence: 99%

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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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Measurement of SIMS Instrumental Mass Fractionation of Pb Isotopes During Zircon Dating

Cited by 313 publications

References 32 publications

GRENVILLE SKARN TITANITE: POTENTIAL REFERENCE MATERIAL FOR SIMS U-Th-Pb ANALYSIS

GRENVILLE SKARN TITANITE: POTENTIAL REFERENCE MATERIAL FOR SIMS U-Th-Pb ANALYSIS

Apatite: a U-Pb thermochronometer or geochronometer?

Extreme isotopologue disequilibrium in molecular SIMS species during SHRIMP geochronology

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