Geochronology

Depending on the levels of precisions required, there are several techniques for U-Pb dating of geological samples.

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 Interface  and the ability to analyze using multiple ion counters, Thermo Scientific Neoma MC-ICP-MS and Thermo Scientific Element XR HR-ICP-MS are able to provide highly-precise in situ U-Pb and Lu-Hf isotope data in zircons, monazites, and other minerals.

For higher precision, isotope dilution thermal ionization mass spectrometry (ID-TIMS) can be used. This technique requires precise and accurate determinations of parent-daughter isotope ratios. The small sample size, particularly with respect to radiogenic Pb, demands highly sensitive ion detection systems. Most studies, therefore, employ either secondary electron multipliers (SEMs) or Daly photomultipliers that provide low background noise and high sensitivity. However, they also have limited linear range and require dynamic peak hopping and time-consuming cross-calibration. Such sequential single collector ion counting measurements have several drawbacks, including reduced sample utilization, ion current fluctuations, and mass-dependent detection inefficiency. Also, ion counting measurement requires an accurate deadtime calibration. Multicollection analysis helps optimize sample usage and allow precision to be independent of signal fluctuation. The 10¹³ Ω amplifier technology enables up to 4-5 times better precision for small samples.

The application of 10¹³ Ω 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 Ω 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 Ω 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.

The research of Matthias Willbold's group focuses on the evolution of the early earth and how it transformed into a habitable planet.

Uranium-lead geochronology provides valuable insights into the timing and rates of various geological processes throughout Earth’s history.

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.

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. . Additionally, the patented Emission Suppression Technology (EST) [insert link] eliminates the trade-off between high precision and long equilibration time, enabling prep systems to be miniaturized, dramatically boosting sample sensitivity.  In conjunction with the Argus IV SV-MS 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.

Matt Heizler talks about his geochronology research using the Argus VI Noble Gas Mass Spectrometer.

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

One of the two naturally occurring isotopes of rubidium, ⁸⁷Rb, decays to ⁸⁷Sr with a half-life of 49.23 billion years. Over time, decay of ⁸⁷Rb increases the amount of radiogenic ⁸⁷Sr while the amount of other Sr isotopes remains unchanged.

Different minerals that crystallized from the same silicic melt will all have the same initial ⁸⁷Sr/⁸⁶Sr as the parent melt. However, because Rb substitutes for K in minerals and minerals have different K/Ca ratios, the minerals will have had different starting Rb/Sr ratios. Given sufficient time for significant ingrowth of radiogenic ⁸⁷Sr, the measured ⁸⁷Sr/⁸⁶Sr values will be different in the minerals. Comparison of different minerals in a rock sample thus allows scientists to infer the original ⁸⁷Sr/⁸⁶Sr ratio and determine the age of the rock.

The analytical challenge of rubidium-strontium dating is that ⁸⁷Rb isobarically interferes ⁸⁷Sr. To resolve ⁸⁷Rb from ⁸⁷Sr, a mass resolving power of M/ΔM ~300,000 is required. Such a high mass resolving power is not typically achievable by high precision isotope ratio mass spectrometers. The development and coupling of collision/reaction cells (CRC) to inductively-coupled plasma mass spectrometers permits in situ radiogenic Sr isotopic analysis by using a suitable reaction gas within a collision cell to react with the Sr⁺ ions but not the interfering Rb⁺ ions. The Thermo Scientific Neoma MS/MS MC-ICP-MS is a dedicated collision cell MC-ICP-MS with a magnetic sector pre-cell mass filter, for high precision in situ Rb-Sr dating of a wider range of geological targets. Using Neoma MS/MS MC-ICP-MS, it is possible to achieve TIMS-like precision (0.6%) on Rb/Sr ages on high [Sr] materials - almost an order of magnitude better than traditional TQ-ICP-MS. For heterogeneous materials (e.g. micas in schists), it is possible to generate isochrons from several individual 1s integrations within a single laser spot, thereby revealing multiple distinct age zones within a single spot allowing high resolution mapping of Rb/Sr age across a mineral.