Optimizing methodologies for Hf and U-Pb analysis of zircons by MC-ICP-MS using ultrafast laser ablation technology

Importance of the Topic

The precise determination of hafnium (Hf) and uranium–lead (U–Pb) isotope ratios in zircon minerals is fundamental to unraveling geological histories and crustal evolution. Ultrafast laser ablation coupled with multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) offers the potential to reduce sample consumption while improving sensitivity, enabling high‐precision geochronology on complex zircon zones. This approach is particularly valuable for laboratories focusing on geosciences, quality control, and industrial analytics.

Aims and Study Overview

This study evaluates the performance of an ultrafast laser ablation system (Elemental Scientific Lasers DCI) in combination with a Thermo Scientific Neptune XT MC-ICP-MS for separate, sequential Hf and U–Pb analyses on individual zircon spots. By optimizing operational parameters, the goal is to minimize ablated volume while achieving reproducibility and precision comparable to or better than traditional laser ablation split stream (LASS) methods.

Materials and Methods

• Reference materials: NIST SRM 610 and SRM 612 glasses for instrument tuning. Four well‐characterized zircons (91500, Plešovice, GJ-1, Mud Tank), an unknown natural zircon, and the MUN-4 synthetic zircon (for external ytterbium correction).
• Laser system: NWR193 ultrafast laser ablation with a 25 μm spot, 4 J/cm² fluence, 20 Hz repetition rate, and He/N₂ carrier gas flows.
• Mass spectrometer: Thermo Scientific Neptune XT MC-ICP-MS equipped with a dual concentric injector (DCI) and high‐efficiency Jet interface. Faraday cups with 10¹³ Ω amplifiers measured Pb and U isotopes; a central SEM captured Yb, Lu, and Hf isotopes.
• Data reduction: Processed using Iolite v2.5 within Igor Pro v6.37, applying corrections for ¹⁷⁶Yb and drift, and integrating signals over 30 s ablation intervals (262 ms for Hf, 131 ms for U–Pb).

Main Results and Discussion

• Hf isotopes: 20 spot analyses per zircon yielded ¹⁷⁶Hf/¹⁷⁷Hf reproducibility better than 100 ppm (2 RSD) for reference zircons and ~150 ppm for the unknown due to lower Hf content.
• U–Pb ages: 10 spot determinations of ²⁰⁶Pb/²³⁸U ages achieved better than 1 % (2 RSD) precision on reference zircons; the unknown zircon displayed greater scatter but an age consistent with 91500.
• Sensitivity: The ultrafast system produced Hf beam intensities up to 13.4 V on 91500, compared to ~3.8 V in previous LASS studies using a 50 μm spot, resulting in 2–3× lower isotope‐ratio uncertainties for similar shot counts.

Benefits and Practical Applications

Ultrafast laser ablation with MC-ICP-MS enables high‐precision zircon geochronology with significantly reduced ablation volumes. The enhanced signal‐to‐noise ratio is advantageous for studying fine‐scale zoning and minimizing sample destruction. This approach benefits academic research in tectonics and crustal evolution, as well as QA/QC in mining and material sciences.

Future Trends and Opportunities

• Integration of ultrafast ablation into LASS configurations to combine sensitivity gains with simultaneous multi‐chronometer measurements.
• Development of even smaller spot sizes (<20 μm) while maintaining precision for in situ analyses of micro‐domains.
• Expansion of ultrafast techniques to other mineral systems and isotope pairs (e.g., Sm–Nd, Lu–Hf in garnet).

Conclusion

The combination of ultrafast laser ablation (ESI DCI) and the Neptune XT MC-ICP-MS delivers superior sensitivity and reproducibility for sequential Hf and U–Pb zircon analyses. Although simultaneous LASS measurement is sacrificed, the method enhances analytical performance while conserving precious sample material, marking a significant advance in geochronological methodology.

References

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