Argon-argon dating

Argon-argon (or ⁴⁰Ar/³⁹Ar) dating is a radiometric dating technique similar to potassium-argon (K-Ar) dating. In this technique, the decay of ⁴⁰K to ⁴⁰Ar* (* indicates radiogenic) is used to date geological events, particularly the eruption and cooling of igneous rocks and minerals. This technique differs from the K-Ar technique in that prior to measurement in a mass spectrometer, the sample is irradiated with neutrons in a nuclear reactor and some of the ³⁹K (present as a known fraction of the total K in the rock) is converted to ³⁹Ar. The ratio of the radiogenic daughter product, ⁴⁰Ar*, to ³⁹Ar as a proxy for ⁴⁰K) can be measured in the same sample, giving a practical advantage over K-Ar dating, where the ⁴⁰K and ⁴⁰Ar must be measured separately, and improving accuracy of the measurement.

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Method

The sample is generally crushed and single crystals of a mineral hand-selected for analysis. These are then irradiated to produce ³⁹Ar from ³⁹K. The sample is then degassed in a high-vacuum mass spectrometer via a laser or resistance furnace. Dating relies on the conversion of K to Ar, and accurate measurement of this conversion. The sample is heated in increments (step heating) which releases argon from different reservoirs within the crystal grain. Each step produces argon with a certain ⁴⁰Ar:³⁹Ar ratio, and only when 80% or more of these steps are within acceptable error is the crystal's age known. Dating via ⁴⁰Ar/³⁹Ar geochronology is generally accurate to within 1-2% for properly collected and irradiated and treated samples.

Age equation

The age of a sample is given by the age equation:

where λ is the radioactive decay constant of ⁴⁰K, J is the J-factor (parameter associated with the irradiation process), and R is the ⁴⁰Ar*/³⁹Ar ratio.

Applications

The primary use for ⁴⁰Ar/³⁹Ar geochronology is dating metamorphic and igneous minerals. ⁴⁰Ar/³⁹Ar is unlikely to provide the age of intrusions of granite as the age typically reflects the time when a mineral cooled through its closure temperature. However, in a metamorphic rock that has not exceeded its closure temperature the age likely dates the crystallization of the mineral. Dating of movement on fault systems is also possible with the ⁴⁰Ar/³⁹Ar method. Different minerals have different closure temperatures; biotite is ~300°C, muscovite is about 400°C and hornblende has a closure temperature of ~550°C. Thus, a granite containing all three minerals will record three different "ages" of emplacement as it cools down through these closure temperatures. Thus, although a crystallization age is not recorded, the information is still useful in constructing the thermal history of the rock.

Dating minerals may provide age information on a rock, but assumptions must be made. Minerals usually only record the last time they cooled down below the closure temperature, and this may not represent all of the events which the rock has undergone, and may not match the age of intrusion. Thus, discretion and interpretation of age dating is essential. ⁴⁰Ar/³⁹Ar geochronology assumes that a rock retains all of its ⁴⁰Ar after cooling past the closing temperature and that this was properly sampled during analysis.

This technique allows the errors involved in K-Ar dating to be checked. Argon-argon dating has the advantage of not requiring determinations of potassium. Modern methods of analysis allow individual regions of crystals to be investigated. This method is important as it allows crystals forming and cooling during different events to be identified.