Ultrafast, > 50 Hz LA ‐ICP ‐MS Spot Analysis Applied to U–Pb Dating of Zircon and other U‐Bearing Minerals
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Abstract:
LA ‐ ICP ‐ MS U–Pb detrital zircon studies typically analyse 50–200 grains per sample, with the consequent risk that minor but geologically important age components (e.g., the youngest detrital zircon population) are not detected, and higher abundance age components are misrepresented, rendering quantitative comparisons between samples impossible. This study undertook rapid U–Pb LA ‐ ICP ‐ MS analyses (8 s per 18–47 μm diameter spot including baseline and ablation) of zircon, apatite, rutile and titanite using an aerosol rapid introduction system ( ARIS ). As the ARIS resolves individual single pulses at fast sampling rates, spot analyses require a high repetition rate (> 50 Hz) so the signal does not return to baseline and mass sweep times (> 80 ms) that span several laser pulses (i.e., major undersampling of the signal). All rapid U–Pb spot analyses employed 250–300 pulses, repetition rates of 53–65 Hz (total ablation times of 4.1–5.7 s) and low fluence (1.75–2.5 J cm −2 ), resulting in pit depths of ca . 15 μm. Zircon, apatite, rutile and titanite reference material data yield an accuracy and precision (2 s ) of < 1% for pre‐Cenozoic reference materials and < 2% for younger reference materials. We present a detrital zircon data set from a Neoproterozoic tillite where > 1000 grains were analysed in < 3 h with a precision and accuracy comparable to conventional LA ‐ ICP ‐ MS analytical protocols, demonstrating the rapid acquisition of huge detrital data sets.Keywords:
Titanite
In this study we examined Meso- to Neoarchean granitoids from the North Caribou Terrane within the Western Superior Province, Canada. Petrology, whole-rock geochemistry, zircon and titanite geochronology, and zircon trace element concentrations were analyzed. U–Pb ages from zircon and titanite are between 2·62 and 3·13 Ga. Although most of the granitoids in this study appear to record a complex magmatic history, about a third contain features that we interpret to be a result of hydrothermal alteration. Notable traits in rocks that contain altered zircons include K-feldspar overgrowths on plagioclase and compositional zoning in titanite. The altered zircon material itself occurs as CL-bright resorption shadows showing distinct chemical changes, including lower Th/U values and elevated LREE concentrations. The isotopic ages of the rims on the altered zircons (2835 Ma, 2760–2678 Ma) are similar to coexisting U–Pb titanite ages and regional U–Pb titanite and zircon ages. We propose that during the hydrothermal event, the affected areas of zircon re-equilibrated with fluid, which promoted Pb loss, resetting the isotopic clock. These results suggest that zircon rims might be useful for dating hydrothermal fluid flow episodes in addition to magmatic events and that a multi-element approach is useful for distinguishing ages that are magmatic from those that have been isotopically disturbed owing to alteration.
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Appendix A—Analytical methods; Appendix B—Supplementary tables B1 (summary of zircon characteristics), B2 (zircon U-Pb age deduction summary and notes), B3 (summary of titanite characteristics), and B4 (titanite U-Pb age deduction summary and notes); and Appendix C—zircon and titanite U-Pb data.
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A granitic sample from the Danish island of Christiansø in the Ertholmene island group north of Bornholm is described petrographically and geochemically, and dated using U-Pb in zircon and titanite. Zircon systematics in the sample are complicated by abundant Pb loss and a large population of zircons interpreted as being inherited. Removal of highly disturbed zircons, imprecise analyses, and assumed inherited zircons yield an upper intercept date of 1500 ± 18 Ma (MSWD = 13, n = 58). Removal of zircons with high common Pb from this population yields an identical result of 1500 ± 22 Ma (MSWD = 8, n = 34). Zircons that are ≤3% discordant give a weighted average 206Pb/238U age of 1458 ± 12 Ma (MSWD = 3.0, n = 18), and a weighted average 207Pb/206Pb age of 1495 ± 14 Ma (MSWD = 4.7, n = 19). Titanites from the sample yield a lower intercept age of 1448 ± 15 Ma (MSWD = 6.8, n = 45). The sample contains a significant number of inherited grains indicative of ages around 1.7–1.8 Ga. The relatively large MSWDs for these age determinations indicate geological complexity, likely reflecting Pb loss, and the possible presence of inherited zircons which suffered major Pb loss during incorporation in the granitic magma. The zircon and titanite dates agree reasonably well with previous age determinations on felsic lithologies from the Bornholm mainland, as well as from the Blekinge Province of southern Sweden. Petrographically and geochemically, the Christiansø granite is indistinguishable from, and can be correlated with, the A-type granites and gneisses which occur on Bornholm. The high abundance of disturbed and inherited zircons (c. 1.7–1.8 Ga) may indicate that the granite was intruded into and assimilated a nearby region of unexposed Transscandinavian Igneous Belt rocks. The somewhat altered nature of the rock, and overall disturbance of U-Pb zircon systematics, suggest alteration associated with fluid-flow along nearby faults defining the northern margin of the Sorgenfrei–Tornquist Zone.
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Appendix A—Analytical methods; Appendix B—Supplementary tables B1 (summary of zircon characteristics), B2 (zircon U-Pb age deduction summary and notes), B3 (summary of titanite characteristics), and B4 (titanite U-Pb age deduction summary and notes); and Appendix C—zircon and titanite U-Pb data.
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