New Mexico Geochronology Research Laboratory

The NMGRL is fully equipped to handle all aspects of ⁴⁰Ar/³⁹Ar dating:

Mineralogical Separation Laboratory: Complete with a crusher, acid treatment setup, magnetic and density separation facilities, and a hand-picking station.

Two State of the Art Multi-Collector Mass Spectrometers: an Argus VI and a Helix MC interlinked via a shared extraction line.

Two Laser Systems: The laboratory boasts two cutting-edge laser systems, utilizing diode and ₂ technologies.

In-House Developed Software: To ensure efficient data collection, processing, and management, the NMGRL employs Pychron (Ross, 2014), an in-house proprietary software. This software plays a crucial role in streamlining various aspects of the ⁴⁰Ar/³⁹Ar dating process.

NMGRL boasts two state-of-the-art multi-collector mass spectrometers. Their configurations make them some of the most advanced mass spectrometers in the world. Their complementary differences make it possible to date all kinds of K-bearing minerals and rocks.

The multi-collection mode involves the simultaneous measurement of the five Argon's isotopes (⁴⁰Ar, ³⁹Ar, ³⁸Ar, ³⁷Ar, ³⁶Ar), with the primary advantage of enhancing precision and accuracy in isotope measurements.

Some key benefits of multi-collector mass spectrometry include:

• Enhanced Isotopic Resolution: This technique provides more detailed and accurate characterization of isotopic compositions.
• Improved Precision:The simultaneous detection of isotopes from different detectors allows for better statistical precision in isotope ratio measurements.
• Reduction of Systematic Errors:Multi-collector mass spectrometry helps minimize systematic errors, contributing to the overall reliability of isotopic data.
• High Sensitivity:The method is highly sensitive, enabling the analysis of small sample sizes without compromising data quality.

The Argus VI high-sensitivity magnetic sector mass spectrometer has a very low volume, making it exceptionally sensitive, enabling very precise measurement of feeble signals. This allows the ⁴⁰Ar/³⁹Ar dating of small, young, or low-K samples.

The Argus VI comprises five fixed Faraday detectors (with amplifier circuits of 10¹², 10¹³, and 10¹⁴ Ohm resistors) and one ion-counter CuBe electron multiplier, allowing a precise measurement of very low ³⁶Ar signals.

For further information, please consult: www.thermofisher.com - Argus IV

The Helix MC is a large radius multi-collector mass spectrometer designed to provide highly accurate results via high resolution where small isotope mass differences (such as ³⁶Ar and HCl) can be deciphered.

The Helix MC comprises ten collectors: five fixed Faraday detectors (with amplifier circuits of 10¹², 10¹³, and 10¹⁴ Ohm resistors) and five ion counter CuBe electron multipliers, allowing precise measurement of very low signals. In the NMGRL, the typical configuration is using three Faraday detectors to measure ⁴⁰Ar, ³⁹Ar, and ³⁷Ar and two ion counters to measure the small signals of ³⁸Ar and ³⁶Ar. Nevertheless, this configuration is adapted to suit the samples and the scientific questions posed.

For further information, please consult: www.thermofisher.com - Helix MC

The NMGRL uses two types of lasers and one high precision temperature furnace for sample degassing.

High precision resistance furnace

The high precision temperature furnace is mainly used for thermochronology studies.

Although ⁴⁰Ar/³⁹Ar is often associated with dating, it can also be used to study temperature variations. By measuring the distribution of argon ages in minerals such as micas or feldspar, for example, we can deduce information about the thermal evolution of a region. The resistance furnace present at the NMGRL features precise temperature control, enabling a precise degassing pattern to be obtained and the thermal history of a geological region to be elucidated.

The furnace presents some advantages such as:

• Homogenous heating of bulk samples.
• Capable of analyzing large quantities of material (e.g. very young or low potassium samples).

Nevertheless, for dating purposes, the use of lasers displays several advantages compared with the furnace:

• The blank levels are lower.
• Background levels of interfering species are lower.
• The extraction times are faster.

This provides the ability to analyze younger and smaller samples.

CO₂ Laser

The CO₂ laser is used to release argon from individual mineral grains or a grain population during incremental heating steps or total grain fusion. It produces energy in the infrared spectrum at a wavelength of 10.6 µm. The CO₂ laser is the most versatile, due to the high absorption of 10.6 µm energy by all minerals and glasses of interest. It offers excellent beam coupling with silicate minerals (transparent/clear minerals).

The CO₂ laser presents some advantages such as:

• A high efficiency with rapid sample heating.
• A very low argon background or blank levels.
• It is ideal for identifying sample heterogeneities (e.g., non-desirable mineral phases, xenocrysts)
• Capable of step-heating by using special lenses.

It also has several limitations:

• Bad coupling with mafic minerals.
• A relatively large beam (minimum diameters attainable are generally 5-10 times the laser wavelength).
• It is ideal for identifying sample heterogeneities (e.g., non-desirable mineral phases, xenocrysts)
• Not completely uniform heating.

Diode Laser

The Diode laser is used to release argon from individual mineral grains, a grain population or a whole rock sample during incremental heating steps or total grain fusion. The diode laser couples well with optically opaque minerals.

The Diode laser presents some advantages such as:

• A high efficiency with rapid sample heating.
• A very low argon background or blank levels.
• Large beam for grains population and whole rock analyses.

It also has several limitations:

• Less effectively absorbed by sub-transparent minerals (e.g., muscovite, feldspar) / did not produce partial melting for near-transparent minerals (e.g., sanidines).
• A large beam preventing precise analysis of a piece of mineral or in-situ dating.
• Not completely uniform heating.