The Arizona Noble Gases Laboratory (ANGL) offers a number of analytical options. For samples where a simple thermal history is anticipated, such as unmetamorphosed volcanic rocks containing K-bearing minerals known to be retentive of argon, such as sanidine, a single step heating may be appropriate, whereby all the argon is released during one high temperature step (generally subsequent to a low temperature step designed to drive off atmospheric argon). For other samples where simple closed system behavior may not have been strictly adhered to during the thermal history of the sample, an incremental step-heat analysis may offer better age resolution with the identification of consecutive steps where the radiogenic age is consistent. The adjacent steps that show agreement are referred to as a plateau and are interpreted to result from argon diffusion from more argon retentive (closed-system) domains within the sample. Typically this will involve 10 to 20 steps ranging from around 300°C to total fusion of the sample, generally in excess of 1300°C.
There are two heating options employed to step-heat the sample. A double-vacuum furnace equipped with a tantalum crucible surrounded by a computer-controlled resistance heating element is used for samples low in K (and hence radiogenic Ar) wherein a relatively large quantity of sample needs to heated uniformly. Temperature is controllable and monitored to within 2-5°C.
For high K minerals, a single grain can provide sufficient argon for a statistically meaningful age. In this case it is sometimes preferable to heat samples with an Argon laser (514nm, 6W). Heating increments are attained by ramping the power to the laser, but actual temperature is not monitored. This method involves the heating of a much smaller volume within the extraction line, and hence, produces lower background corrections. However, since actual temperatures are not measured, diffusion studies are not permitted by this technique.
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