Geochronology

Amplifier technology for ID-TIMS U-Pb analysis

The application of 10¹³ ohm resistors for the static collection of all Pb isotopes measured on Faraday cups and ²⁰⁴Pb measured in the axial SEM of a Thermo Scientific Triton Series TIMS instrument permits both sensitive detection and an extended linear range. Both single and multicollector detector systems show excellent agreement, suggesting that a static measurement routine with 10¹³ ohm resistors produces accurate and precise U Pb isotopic data with superior external reproducibility. The amplifier technique has the potential to push the frontiers of high-precision U-Pb geochronology and may represent a crucial advance in the quest towards inter- and intra-laboratory reproducibility at the 0.01% level.

The large dynamic range and high signal-to-noise ratio of our 10¹³ ohm resistors allow static Faraday collection of all relevant UO₂ molecules, including the minor ²⁷²(UO₂) molecule during analyses of typical U-Pb sample loads (which are comparable to single zircons). This allows for online determination of the ¹⁸O/¹⁶O ratio from ²⁷²(UO₂)/ ²⁷⁰(UO₂) and accurate line-by-line isobaric interference correction, eliminating the dominant source of uncertainty in these analytical set-ups.

Either single- or multicollector ICP-MS, combined with laser ablation (LA), offers a powerful technique to obtain unique information on the age and formation of a variety of geological samples. Combining two or even three mass spectrometers has become a very powerful way for studying zircon and monazite petrochronology, for example, because it enables simultaneous analysis of U-Pb, Lu-Hf, and rare Earth elements (REE) within a single crater.

High sensitivity is critical for enabling analysis of samples as small as a few microns. With a high-sensitivity jet pump and the ability to analyze using multiple ion counters, the Neoma MC-ICP-MS and Element XR HR-ICP-MS instruments are able to provide highly-precise in situ U-Pb and Lu-Hf isotope data in zircons, monazites, and other minerals.

With the Neoma MS/MS MC-ICP-MS it is now possible to obtain high precise Rb/Sr age information in a variety of minerals. The unique design of the Neoma MS/MS enable the separation of Rb from Sr, making it a very powerful tool for in-situ laser ablation applications.

The Thermo Scientific iCAP TQ ICP-MS offers a lower-precision LA quadrupole ICP-MS U-Pb dating option. It comes with the advantage of small footprint, easy-to-use Thermo Scientific Qtegra ISDS Software and potential to remove the ²⁰⁴Hg interference on ²⁰⁴Pb (to correct for “common” non-radiogenic lead) by using its collision/reaction cell (CRC) system.

The application of Re-Os mainly evolved around the analysis of molybdenite and sulfides. This dating technique uses the decay of ¹⁸⁷Re to ¹⁸⁷Os and is applied to study the geochemical evolution of the Earth’s interior (mantle), as well as to determine origin and age of ore deposits. The rhenium-osmium chronometer requires highly sensitive techniques to precisely measure isotopic abundances in small osmium samples.

Negative TIMS (N-TIMS) is a powerful ionization technique for a broad range of elements. Many of the transition metals form negatively charged oxide ions and high ion yields can be obtained for those elements that have high electron affinities.

Using N-TIMS, geologists are able to analyze samples with single digit ppm precision. The high Os ion yield also enables the analysis of as little as 1 ng of Os, allowing the chronometer to be applied to very small samples. Successful dating using isotope dilution techniques relies on careful preparation of the mineral and the appropriate choice of spiking concentrations. Multicollector ICP-MS is ideal for in situ Re-Os isotope analysis and analysis of larger samples.

The Hf-W dating technique, based on the decay of ¹⁸²Hf to ¹⁸²W, is used to study the early solar system. Extraterrestrial samples are rare and the elemental concentrations of Hf and W in those samples is limited, so sensitivity critical for obtaining high-precision isotopic information from chondritic grains. Multicollector ICP-MS combines the advantages of magnetic sector instruments with the superior ionization of an ICP source, enabling the measurement elements with ionization potential, such as Hf and W. Applying the Hf-W chronometer to material from meteorites, geologists can study the timing and other details of planetary formation processes.

Isotopes of the noble gases are powerful tools in geochronology, cosmochemistry, and thermochronology. The Ar-Ar dating technique can be used to date any mineral or rock that contains measurable amounts of potassium. These samples include sanidine and micas, but also plagioclase and even pyroxene, which have ages from a few thousand years to as old as the solar system itself. As a result of its broad analysis range, the technique is utilized in all areas of geosciences, including volcanology, weathering processes, tectonics, structural and planetary geology, but also in archaeology, evolution of flora and fauna, provenance studies, ore and petroleum genesis, and climate research.

Ar-Ar technique utilizes the natural decay of ⁴⁰K to ⁴⁰Ar. The samples are irradiated along with known age standards with fast neutrons in a nuclear reactor. The process converts another isotope of potassium (³⁹K) to ³⁹Ar. This allows the simultaneous isotopic noble gas measurement of both the parent (³⁹Ar_K) and the daughter (⁴⁰Ar) in the same aliquot. The technique also gives access to specific elemental composition of the sample (e.g.: Ca/K or Chlorine). We developed the Thermo Scientific Argus IV Static Vacuum Noble Gas Mass Spectrometer especially for high-precision Ar-Ar dating. This instrument enables simultaneous analyses of all five Ar isotopes using different collector configuration Faraday (10¹¹ to 10¹³ ohm amplifier) and ion counting detectors. Its high sensitivity EI source, low volume and patented collector array with wide dynamic range allow the measurement of smaller (likely purer) and younger sample aliquots delivering high analytical precision. In conjunction with this instrument we also offer the high resolution / high resolving power Thermo Scientific Helix MC Plus Multicollector Noble Gas Mass Spectrometer fitted with 5 Faraday detectors (10¹¹ to 10¹³ ohm amplifiers) and 5 ion counting multipliers that is capable of resolving any interference from any of the noble gas isotopes.

Have you ever asked yourself how we can tell when the dinosaurs went extinct? The answers lie in noble gas.