The Geochronology Laboratory at the Geophysical Institute of the University of Alaska Fairbanks is a state-of-the-art ⁴⁰Ar/³⁹Ar facility that determines ages of rocks and minerals. The Geochron Lab is located on the third floor of the Reichardt Building (Room 369) on the Troth Yeddha' (Fairbanks) campus. The facility is available for analysis requests. For interest, please contact the Geochron Lab staff. For an introduction to the lab, see this recent Instagram video.

The Geophysical Institute has housed a facility for geochronology since the initial K-Ar facility was established by Don Turner in 1971. This system was converted to ⁴⁰Ar/³⁹Ar by Paul Layer in 1990 and replaced with a VG-3600 mass spectrometer in 1994. A new Isotopx NGX-600 multi-collector mass spectrometer has been taken into operation in June 2023. This system features a CO₂ laser and a low-blank resistance furnace for argon extraction.

How it works

The ⁴⁰Ar/³⁹Ar method

We use the ⁴⁰Ar/³⁹Ar method to measure the ages of rocks and minerals. Most rocks and minerals contain potassium, a small amount of which (about 0.01%) is the radioactive isotope ⁴⁰K. Over time, this isotope decays to ⁴⁰Ar (and ⁴⁰Ca) with a half-life of 1.25 billion years. By measuring the remaining ⁴⁰K and the amount of ⁴⁰Ar produced by radioactive decay, we can calculate the age of a rock.

Measuring both a gas (⁴⁰Ar) and a solid (⁴⁰K) introduces uncertainty into the process. To improve the dating accuracy, we irradiate samples with neutrons in a nuclear reactor which transforms some of the main potassium isotope, ³⁹K, into ³⁹Ar. Since both ³⁹Ar and ⁴⁰Ar are gases, we can measure them at the same time with a single method. This technique, called the ⁴⁰Ar/³⁹Ar method, improves our measurements and enables us to obtain very precise ages.

Mineral separation and age determination

When analyzing rocks, we typically focus on a specific mineral within the rock or, in the case of volcanic rocks, the fine-grained groundmass. The rock samples are first broken up and crushed to separate the individual mineral grains. The crushed material is then sieved to a specific grain size and subjected to separation methods that sort the mineral grains based on their density, magnetic properties, and other characteristics. Under a light microscope, we select individual grains for further analysis.

We then send the chosen grains to the nuclear research reactor at Oregon State University where they are bombarded with neutrons. This process transforms some of the ³⁹K to ³⁹Ar. Subsequently, the samples are sent back to the Geochron Lab, where we heat them either using a furnace or a laser to release the gases contained within. The argon is separated from the other gas components, and all of the five main argon isotopes (³⁶Ar, ³⁷Ar, ³⁸Ar, ³⁹Ar, and ⁴⁰Ar) are measured using a mass spectrometer. Some amounts of all of these isotopes are produced in the irradiation, so we measure them all (as opposed to just ³⁹Ar and ⁴⁰Ar) to improve the measurement. To determine the age of the analyzed sample, we compare the ⁴⁰Ar/³⁹Ar measurements with those of a standard with a known age.

Applications

The ⁴⁰Ar/³⁹Ar technique finds various applications, such as measuring the ages of volcanic rocks. When rocks are hot, argon diffuses out of them, preventing the accumulation of argon within the rock. Since volcanic rocks are erupted while still hot, they do not contain any argon initially, but they do contain potassium. As these rocks cool rapidly after the eruption, they can retain all the argon produced from the decay of ⁴⁰K. Therefore, the measured age corresponds to the time elapsed since the eruption. This method helps determine the ages of volcanic and sedimentary rocks, contributing to our understanding of volcanoes, the evolution of life over geological time, and the movement of tectonic plates.

Argon can also be used to study rocks that have cooled gradually over time. At great depths within the Earth, rocks are hot enough to lose all the argon produced by the radioactive decay of ⁴⁰K. However, as these rocks move closer to the Earth's surface due to processes like mountain-building and erosion, they gradually cool. Before the rock reaches a temperature at which argon is no longer lost, it experiences a range of temperatures at which argon is partially lost. Only when low temperatures are reached can ⁴⁰Ar fully accumulate within the crystal. The ages of samples that have undergone this process do not indicate a specific point in time but rather are a product of the temperature changes that the rock has experienced. We called this kind of data “cooling ages”. We can reconstruct the temperature history of a sample from such cooling ages using computer modeling. This method of thermochronology helps us to study how mountain ranges form and how fast they are eroded.

 

Equipment

 

We are fully equipped to produce ⁴⁰Ar/³⁹Ar ages from rock samples. The equipment of the Geochron Lab covers rock crushing, mineral separation, sample picking, sample characterization, handling of irradiated samples, Ar isotope measurement, and data reduction.

• Isotopx NGX-600 multi-collector noble gas mass spectrometer with a collector configuration optimized for argon (nine Faraday cups with ATONA amplifiers and one ion-counting electron multiplier). The new instrument was purchased with funds from National Science Foundation Major Research Instrumentation grant #2019235 and the Geophysical Institute.
• Preparation line with two GP50 SAES getters (reactive surfaces that help reduce certain gas components) and a liquid nitrogen trap for gas purification
• Teledyne Photon Machines Fusions CO₂ laser stepped heating system (100 W, 10600 nm)
• Pond Engineering TC-9 Low Blank Furnace
• PolyScience IP-100 Immersion Probe Cooler
• Norhof #905 automatic LN2 micro-dosing system
• Eaton 9PX 6k UPS (uninterruptible power supply)
• Pychron data processing and reduction software

 

The new Isotopx NGX mass spectrometer being installed (October 2022).

Mineral separation

• Two rock saws
• Two jaw crushers
• Disk mill
• 8” and 3” test sieves and sieve shakers
• Three Frantz isodynamic magnetic separators
• Density separation equipment using LST (lithium heteropolytungstate) and SPT (sodium polytungstate)
• Vacuum oven and natural convection oven for drying samples

Sample characterization

• Reflected and transmitted light microscopes for sample picking and documentation
• Thermo Scientific Nicolet Summit Pro Attenuated Total Reflection Fourier Transform Infrared Spectrometer (ATR-FTIR) for phase identification
• Computer for analyzing and visualizing microCT data

 

Services

We offer ⁴⁰Ar/³⁹Ar dating services, including step-heating, single-aliquot total fusion, and high-resolution diffusion experiments. We can also perform all necessary mineral preparation, picking, and sample characterization. We support the AGeS initiative and welcome outside collaborators to the Geochron Lab for sample analysis. Our commitment includes offering comprehensive training and expert guidance to all visiting individuals across every phase of the process, from sample processing, mineral separation, and picking, to sample analysis, data reduction, and interpretation.

For questions or interest, please send an email to UAF-Geochron@alaska.edu.

Shipment address:

Florian Hofmann
Reichardt 369
University of Alaska Fairbanks
1930 Yukon Drive
Fairbanks, AK 99775
USA

After shipping samples, please provide us with the tracking number so that we know to look out for the package.

 

Personnel

 

Florian Hofmann
Research Assistant Professor
Geophysical Institute, University of Alaska Fairbanks
Office: Reichardt 328
fhofmann@alaska.edu

Florian Hofmann manages the day-to-day operations of the UAF Geochron Lab, including preparing and analyzing rock samples, developing and implementing measurement protocols, and training new users. His background is in noble gas geochemistry and geochronology as well as structural geology, tectonics, and surface processes. Florian completed BSc and MSc degrees at the University of Munich, and a PhD at Caltech. He was a Visiting Professor at the University of Munich and a postdoc at Caltech, the University of Southern California, and UAF before being hired as a Research Assistant Professor at the Geophysical Institute in 2023. His research uses high-precision geochronology and thermochronology to study the long-term history of fault zones, the timing of volcanic eruptions, and orogenic processes, with field areas in Alaska and beyond. He is also working on improving the precision and accuracy of geochronologic techniques and developing new target phases for the ⁴⁰Ar/³⁹Ar method.

 

Sean Regan
Assistant Professor
College of Natural Science and Mathematics and Geophysical Institute
University of Alaska Fairbanks
sregan5@alaska.edu

Sean Regan has a broad background in geochronology, with most emphasis given to U–Th-Pb systematics of a wide array of chronometers (monazite, zircon, rutile, apatite, and columbite) and has helped oversee the new Noble Gas Mass Spectrometer at the Geophysical Institute. His background is broadly in igneous and metamorphic petrology, ductile structure, and critical minerals systems. Sean has a BSc from St. Lawrence University, and an MSc and PhD from the University of Massachusetts Amherst, where he worked extensively on petrochronology of high-grade gneiss terranes. After briefly working as a research geologist at the US Geological Survey, Sean accepted an Assistant Professor position at the University of Alaska Fairbanks in 2018 and joined the Geophysical Institute in 2020. His current research covers a broad range of topics including structural evolution of the Denali fault, magma transport and emplacement, and multi-phase geochronology of Critical Mineral systems.

 

Projects

Current Projects

• Long-term offset history of major continental strike-slip faults (Denali fault, San Andreas fault)
• Modeling of exhumation derived from multiple thermochronometric systems
• Extrusive ages of young volcanic rocks
• Precise dating of impact events

 

History of the Geochronology Laboratory

Donald Turner (1970-1989)

Don Turner established the geochronological facility at UAF in 1970 by building a laboratory for K-Ar dating. He installed a custom-built Nuclide Corporation 6-60-SGA (6-inch radius) mass spectrometer, which went into operation in December 1971. Argon was released by heating the sample in an RF fusion system. Parts of this process were automated and allowed for the processing of three samples simultaneously. The gas was cleaned by reacting it with a Cu-CuO furnace, Ti getter, and zeolite. The measurement was done by isotope dilution with a pure ³⁸Ar tracer. Peak heights were measured on a Faraday cup using the peak hopping technique.

Data were acquired with a digital voltmeter, and the averaged voltages were written to punch cards. The lab featured an Interdata computer that was able to evaluate data during the analysis for instant monitoring of the mass discrimination. Bulk potassium concentrations were then measured on the same material with an Instrumentation Labs 343 digital flame photometer using the LiBO2 flux fusion technique. The combined K and Ar data was reduced to ages with a custom program on an IBM 360 computer. The data processing and sample handling systems were updated in 1985 to include more automation.

 

 

Work in the laboratory included establishing absolute ages for igneous, metamorphic, and sedimentary rocks. Don Turner and his students spent many field seasons across Alaska and beyond, collecting rock samples for which they measured K-Ar ages. Examples of this work include studying the complex metamorphic history of the Brooks Range (Turner et al., 1979), determining the timing of movement on the Denali Fault (Warhaftig et al., 1975), exploring the volcanic history of the Aleutian Arc (Nye and Turner, 1990) and dating ash layers in coal beds, which established stratigraphic dates important for evaluating economic potential (Turner et al., 1977, 1980, 1983). Turner et al. (1982) tested the hot spot hypothesis, which is an important part of the theory of plate tectonics, by measuring K-Ar ages of several islands in the Cook-Austral Islands chain, the oldest in the Pacific Ocean. Don Turner retired in 1989 and passed away in 2014.

Paul Layer (1989-2020)

Paul Layer became Don Turner’s successor in the geochronology laboratory in 1989. He modified the existing equipment in order to enable measurements with the ⁴⁰Ar/³⁹Ar technique. In 1992, he was awarded an NSF grant (#9214375) to purchase and install a new VG3600 mass spectrometer with a Coherent 6-watt argon-ion laser and a Modifications Ltd. resistance-type furnace, which became operational in May 1994. The laser system, which was one of the first of its kind, was fully custom-built and featured a system of lenses, mirrors, and filters on an optical bench, which focused the laser beam onto a sample under a light microscope. Sample heating was monitored with a CCTV camera mounted to the microscope. The resulting gas was purified using a liquid nitrogen cold trap and two SAES Zr-Al getters and then inlet into the VG3600 mass spectrometer, where it was measured by peak hopping on a Faraday cup or Daly detector. The gas handling, data acquisition, and reduction were automatically controlled by a computer. The mass spectrometer electronics were upgraded in 2004, which led to an improvement in signal stability and analytical uncertainties.

Prior to his appointment at UAF, Paul Layer had demonstrated that ⁴⁰Ar/³⁹Ar age-spectra could be obtained by laser heating (Layer et al., 1987). He built the system at UAF to his specifications in order to explore this new technique. Geological research, often involving students, was focused on dating applications for understanding plutonism, metamorphism, and mineralization in Alaska, the Lower 48, and many countries around the world. Work in Alaska was done in close cooperation with state and federal agencies, as well as exploration and mining companies. Paul Layer also maintained many international collaborations, including with researchers in Mexico, Ukraine, Senegal, Russia, Germany, Argentina, Australia, Canada, and Sweden, leading to samples from all continents being analyzed here. In addition, the Geochronology Laboratory accepted outside sample analysis requests, including mineral separation, laser total fusion, and laser or furnace step-heating analyses. Typically, over 2,000 separate mass spectrometry analyses were performed on this system every year.

 

Some of the most significant research during this time includes establishing the age of the El’gygytgyn crater from impact-related volcanic rocks (Layer, 2000). The sediments accumulated in Lake El’gygytgyn since the impact represent the most complete Arctic continental climate record, spanning the entire Quaternary. Other prominent studies include providing a temporal framework for reconstructing the history of the Earth’s magnetic field over the last 5 million years (Stone and Layer, 2006; Johnson et al., 2008), exploring Alaskan gold deposits (McCoy et al., 1997) and elucidating the geologic history of regions across Earth (Layer et al., 2001; Ring and Layer, 2003; etc.).

News & Announcements

 

 

 

 

View this post on Instagram

 

 

 

 

 

 

 

 

 

 

 

A post shared by UAF Geophysical Institute (@uaf.gi)

 

UAF awards funds for two undergrad students to help in volcano research

 

 

 

 

 

View this post on Instagram

 

 

 

 

 

 

 

 

 

 

 

 

A post shared by UAF_URSA (@ursa.uaf)