| Argon-argon Dating |
Website Links For Dating |
Information AboutArgon-argon Dating |
| CATEGORIES ABOUT ARGON-ARGON DATING | |
| radiometric dating | |
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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. APPLICATIONS The primary use for 40Ar/39Ar geochronology is dating metamorphic and igneous minerals. 40Ar/39Ar can provide ages of intrusion of Granite , or even ages of movement on Fault systems. 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 including all three minerals will record three different "ages" of emplacement as it cools down through these closure temperatures. 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. |
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