High precision in situ Rb-Sr age dating

Significance of the topic

The rubidium–strontium (Rb–Sr) beta decay system is a foundational tool for geochronology, yet its application to in situ mineral dating has been limited by isobaric overlap between 87Rb and 87Sr. High mass resolving power is required to separate these isotopes, which traditional MC-ICP-MS systems cannot achieve. Introducing a collision/reaction cell coupled with a pre-cell mass filter and static multicollection enhances precision and accuracy for a broad range of geological materials.

Study objectives and overview

This work demonstrates the capabilities of the Thermo Scientific™ Neoma™ MS/MS MC-ICP-MS for high-precision in situ Rb–Sr age dating. The study evaluates performance on reference glasses, granite feldspars, and low-Sr mica to benchmark precision, accuracy, and sensitivity against single collector triple quadrupole ICP-MS and conventional TIMS analyses.

Methodology and instrumentation

The Neoma MS/MS MC-ICP-MS integrates three main components:

• A pre-cell mass filter to remove matrix elements in the 78–94 amu range.
• A hexapole collision/reaction cell pressurized with SF6 to react selectively with Sr+ and generate SrF+ reaction products.
• A magnetic sector for high-sensitivity isotope separation, feeding an 11-cup Faraday array for simultaneous Rb and SrF detection.

Samples were introduced via an ESL imageGEO193 laser ablation system (50–100 µm spot, 3.5 J/cm2, 12 Hz), with data reduction performed in Iolite using a Rb–Sr protocol.

Main results and discussion

• Reference glasses: 87Sr/86Sr reproducibility <1‰ (2 RSD); 87Rb/86Sr precision <4 %; accuracy within 0.7 ‰ and 6 % of reference values.
• Granite feldspars: Plagioclase and K-feldspar in Dartmoor Granite yielded Rb–Sr ages of 283.5 ± 1.6 Ma (0.6 % precision), an order of magnitude improvement over ICP-TQ-MS (284.6 ± 14.7 Ma) and comparable to TIMS benchmarks.
• Micas in garnet-bearing schist: Low-Sr biotite (Ra-D72) achieved a Rb–Sr age of 274.3 ± 10.8 Ma. Single-spot isochrons based on individual 1 s integrations revealed intra-grain age domains and multiple metamorphic events.

Benefits and practical applications

The Neoma MS/MS MC-ICP-MS enables fully static, in situ Rb–Sr dating with:

• Enhanced sensitivity and ion extraction, yielding precision limited by counting statistics rather than instrument noise.
• Simultaneous multicollection of Rb and SrF for improved throughput and reduced matrix effects.
• Capability to resolve isotopic heterogeneity at sub-millimeter scales for complex geological histories.

Future trends and possibilities of utilization

• Integration of multi-chronometer in situ workflows (e.g., U–Pb, Sm–Nd).
• Development of high-resolution isotopic mapping across mineral grains.
• Optimization of reaction gas chemistries for alternative isobaric challenges.
• Extension to exotic isotope systems and non-geological materials in environmental and forensic studies.

Conclusion

The Thermo Scientific Neoma MS/MS MC-ICP-MS represents a significant advance in in situ Rb–Sr geochronology. By combining pre-cell mass filtering, CRC chemistry, and static multicollection, it delivers precision and accuracy previously unattainable in high Rb/Sr minerals and low-Sr phases. This platform opens new possibilities for detailed age mapping and single-spot isochron dating across diverse geological contexts.

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