Geochronology

Geochronology involves understanding time in relation to geological events and processes. Geochronological investigations examine rocks, minerals, fossils and sediments. Absolute and relative dating approaches complement each other. Absolute age determination is performed by radiogenic isotope dating methods such as U-(Th)-Pb, U-series, K-Ar and Ar-Ar methods, as well as Rb-Sr, Sm-Nd and Re-Os dating techniques. Relative age determinations involve paleomagnetism and stable isotope ratio calculations, as well as stratigraphy.

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Accessing Earth’s history using isotopic dating methods

Geoscientists can learn about the absolute timing of geological events as well as rates of geological processes using radioisotopic dating methods. These methods rely on the known rate of natural decay of a radioactive parent nuclide into a radiogenic daughter nuclide. Over time, the daughter nuclide accumulates in certain minerals. By measuring the relative remaining parent/accumulated daughter nuclide amounts using a mass spectrometer, a date can be calculated. Different isotopic systems can be used to date a range of geological materials from a few million to billions of years old. Commonly used dating techniques are the U-(Th)-Pb, U series, K-Ar and Ar-Ar methods.

The U-(Th)-Pb technique measures the amount of accumulated 206Pb, 207Pb and 208Pb relative to the amount of their remaining uranium and thorium parents in a mineral or rock. This technique is commonly applied to minerals from igneous, metamorphic and sedimentary rocks, such as zircons and monazites, and is used to date materials up to 4.5 billion years old.

The U-series technique uses the short half-lives of uranium and thorium isotopes to date geologically young material, such as fossils, speleothems, carbonates and volcanic rocks. This dating technique is applied to samples of just a few years, up to about 700,000 years old.

The K-Ar dating technique is based on measurement of the product of the radioactive decay of an isotope of potassium (K) into argon (Ar) and is used for samples a few thousand years and older such as igneous, volcanic and metamorphic rocks. Ar-Ar dating is similar to K-Ar dating, but provides greater accuracy. In addition to the previously mentioned isotope-based techniques, other isotope systems used for radiometric dating include Re-Os, Rb-Sr, Sm-Nd, Lu-Hf and Hf-W.

Geoscientists can use different mass spectrometric instruments for the dating techniques mentioned above: Thermal Ionization Mass Spectrometry (TIMS), Multicollector High Resolution and Quadrupole Inductively Coupled Plasma Mass Spectrometry (MC-, HR, Q-ICP-MS), and Noble Gas Mass Spectrometry. These instruments, in combination with chemical digestion, micromill and laser ablation sample introduction techniques, provide geochronological data at various ages and levels of spatial resolution.

The innovative mass spectrometer portfolio from Thermo Fisher Scientific helps geoscientists to provide insights into the age of their valuable rocks, minerals, sediments and fossils, enabling them to make exciting, pioneering discoveries.


Popular products

Element Series High Resolution ICP-MS

The Element Series HR-ICP-MS systems, combined with a laser ablation (LA) system, enable high spatial resolution analysis of U-(Th)-Pb isotopes in minerals, which occur as common accessory minerals of rocks such as zircon. The LA-ICP-MS approach is particularly appropriate to capture potential 10s of µm-scale age variability within zircons and other minerals. The high sensitivity of the Element Series HR-ICP-MS in dry plasma allows precise measurements of U/Pb isotopic ratios at high spatial resolution, resulting in final absolute U/Pb age uncertainty of < 2% (2S).

Finally, the fast scanning capabilities and the potential to run analyses in automated mode enable the high throughput required to analyze large numbers of detrital zircons in provenance studies.


U-Pb dating by ID-TIMS

U-Pb geochronology by isotope dilution thermal ionization mass spectrometry (ID-TIMS) 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. 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. Until recently, the Faraday detection system was limited by the noise of the current amplifiers. New amplifier technology, using 1013 Ω resistors in the Faraday cup amplifier feedback loop, enables up to 4-5 times better precision for small samples.

Triton Plus Multicollector Thermal Ionization Mass Spectrometer
Triton Plus Multicollector Thermal Ionization Mass Spectrometer

New amplifier technology for ID-TIMS U-Pb analysis

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 Plus 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 new 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.

U-Pb dating by laser ablation ICP-MS

Either single- or multicollector ICP-MS, combined with laser ablation, 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. The 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 Qtegra 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.

Further reading

Re-Os dating

Neptune Plus MC-ICP-MS
Neptune Plus MC-ICP-MS
Triton Plus Multicollector Thermal Ionization Mass Spectrometer
Triton Plus Multicollector Thermal Ionization Mass Spectrometer

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 ppt-level precision. This precision 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.

Hf-W dating

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.

Ar-Ar dating

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 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.

The Ar-Ar dating technique relies on neutron irradiation from a nuclear reactor to convert 39K (stable form of potassium) into 39Ar (radioactive argon). When a standard of known age is co-irradiated with the unknown samples, a single measurement of argon isotopes can be used to calculate the 40K/ 40Ar* ratio (i.e., ratio of radiogenic Ar versus 40Ar from radioactive decay of 40K), and thus to calculate the age of the unknown sample.

Thermo Fisher Scientific developed the Thermo Scientific Argus IV Noble Gas Mass Spectrometer especially for high-precision Ar-Ar dating. This instrument enables simultaneous analysis of all five Ar isotopes on a mixed Faraday-ion counting detection system. For large sample sizes, a Faraday cup array employing standard 1011 Ω amplifiers is the preferred detector. For medium-sized samples, the Faraday cups used to detect the minor isotopes can be equipped with a higher gain 1013 Ω amplifier, which improves the signal-to-noise ratio and results in excellent precision. For smaller samples, the minor isotopes are measured on an ion counting detector.

Further reading

U-Pb dating by ID-TIMS

U-Pb geochronology by isotope dilution thermal ionization mass spectrometry (ID-TIMS) 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. 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. Until recently, the Faraday detection system was limited by the noise of the current amplifiers. New amplifier technology, using 1013 Ω resistors in the Faraday cup amplifier feedback loop, enables up to 4-5 times better precision for small samples.

Triton Plus Multicollector Thermal Ionization Mass Spectrometer
Triton Plus Multicollector Thermal Ionization Mass Spectrometer

New amplifier technology for ID-TIMS U-Pb analysis

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 Plus 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 new 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.

U-Pb dating by laser ablation ICP-MS

Either single- or multicollector ICP-MS, combined with laser ablation, 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. The 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 Qtegra 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.

Further reading

Re-Os dating

Neptune Plus MC-ICP-MS
Neptune Plus MC-ICP-MS
Triton Plus Multicollector Thermal Ionization Mass Spectrometer
Triton Plus Multicollector Thermal Ionization Mass Spectrometer

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 ppt-level precision. This precision 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.

Hf-W dating

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.

Ar-Ar dating

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 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.

The Ar-Ar dating technique relies on neutron irradiation from a nuclear reactor to convert 39K (stable form of potassium) into 39Ar (radioactive argon). When a standard of known age is co-irradiated with the unknown samples, a single measurement of argon isotopes can be used to calculate the 40K/ 40Ar* ratio (i.e., ratio of radiogenic Ar versus 40Ar from radioactive decay of 40K), and thus to calculate the age of the unknown sample.

Thermo Fisher Scientific developed the Thermo Scientific Argus IV Noble Gas Mass Spectrometer especially for high-precision Ar-Ar dating. This instrument enables simultaneous analysis of all five Ar isotopes on a mixed Faraday-ion counting detection system. For large sample sizes, a Faraday cup array employing standard 1011 Ω amplifiers is the preferred detector. For medium-sized samples, the Faraday cups used to detect the minor isotopes can be equipped with a higher gain 1013 Ω amplifier, which improves the signal-to-noise ratio and results in excellent precision. For smaller samples, the minor isotopes are measured on an ion counting detector.

Further reading

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