ID-TIMS U–Pb geochronology at the 0.1‰ level using 1013 Ω resistors and simultaneous U and 18O/16O isotope ratio determination for accurate UO2 interference correction

2017 ◽  
Vol 32 (3) ◽  
pp. 579-586 ◽  
Author(s):  
Jörn-Frederik Wotzlaw ◽  
Yannick Buret ◽  
Simon J. E. Large ◽  
Dawid Szymanowski ◽  
Albrecht von Quadt
Keyword(s):  
Mass Spectrometry ◽  
High Precision ◽  
Isotope Dilution ◽  
Isotope Ratio ◽  
Feedback Loop ◽  
Faraday Cup ◽  
Interference Correction ◽  
Ionisation Mass Spectrometry ◽  
Ratio Determination ◽  
Ionisation Mass

We document recent advances in analytical protocols that employ 1013 Ω resistors in the Faraday cup amplifier feedback loop for high-precision U–Pb geochronology by isotope dilution thermal ionisation mass spectrometry (ID-TIMS).

scholarly journals High-precision ID-TIMS cassiterite U–Pb systematics using a low-contamination hydrothermal decomposition: implications for LA-ICP-MS and ore deposit geochronology

Geochronology ◽  
2020 ◽  
Vol 2 (2) ◽  
pp. 425-441
Author(s):  
Simon Tapster ◽  
Joshua W. G. Bright
Keyword(s):  
Mass Spectrometry ◽  
Isotope Dilution ◽  
Hydrothermal Systems ◽  
Geological Time ◽  
Thermal Ionisation Mass Spectrometry ◽  
Low Concentrations ◽  
Ionisation Mass Spectrometry ◽  
Acidic Nature ◽  
Suitable Reference ◽  
Ionisation Mass

Abstract. Cassiterite (SnO2) is the most common ore phase of Sn. Typically containing 1–100 µg g−1 of uranium and relatively low concentrations of common Pb, cassiterite has been increasingly targeted for U–Pb geochronology, principally via microbeam methods, to understand the timing and durations of granite-related magmatic–hydrothermal systems throughout geological time. However, due to the extreme resistance of cassiterite to most forms of acid digestion, there has been no published method permitting the complete, closed-system decomposition of cassiterite under conditions in which the basic necessities of measurement by isotope dilution can be met, leading to a paucity of reference and validation materials. To address this a new low blank (< 1 pg Pb) method for the complete acid decomposition of cassiterite utilising HBr in the presence of a mixed U–Pb tracer, U and Pb purification, and thermal ionisation mass spectrometry (TIMS) analyses has been developed. Decomposition rates have been experimentally evaluated under a range of conditions. A careful balance of time and temperature is required due to competing effects (e.g. HBr oxidation), yet the decomposition of 500 µm diameter fragments of cassiterite is readily achievable over periods comparable to zircon decomposition. Its acid-resistant nature can be turned into an advantage by leaching common Pb-bearing phases (e.g. sulfides, silicates) without disturbing the U–Pb systematics of the cassiterite lattice. The archetypal Sn–W greisen deposit of Cligga Head, SW England, is used to define accuracy relative to chemical abrasion–isotope dilution–thermal ionisation mass spectrometry (CA-ID-TIMS) zircon U–Pb ages and demonstrates the potential of this new method for resolving high-resolution timescales (<0.1 %) of magmatic–hydrothermal systems. However, data also indicate that the isotopic composition of initial common Pb varies significantly, both between crystals and within a single crystal. This is attributed to significant fluid–rock interactions and the highly F-rich acidic nature of the hydrothermal system. At microbeam precision levels, this issue is largely unresolvable and can result in significant inaccuracy in interpreted ages. The ID-TIMS U–Pb method described herein can, for the first time, be used to properly characterise suitable reference materials for microbeam cassiterite U–Pb analyses, thus improving the accuracy of the U–Pb cassiterite chronometer as a whole.

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