Argon Geochronology for the Earth Sciences (AGES) Lab
The primary instrument in the lab is a Micromass VG 5400 mass spectrometer attached to a CO2 laser extraction system. The system is fully automated and controlled by the MassSpec software developed by Al Deino at the Berkeley Geochronology Center. While some samples require greater attention and manual supervision, routine samples can be run in large batches entirely under computer control.
Minerals are hand picked under a microscope following disaggregation of rock or sediment samples and separation using a combination of size, density, and magnetic properties.
Selected minerals are irradiated offsite to produce 39Ar from 39K (as a proxy for radioactive 40K). Samples are co-irradiated with monitor standards to define the "J-value", used for age calculations.
The 5400 is used to measure the Ar isotope compositions and the ratio of 40Ar*, the daughter product produced by 40K decay over geologic time, to 39Ar, the proxy for parent potassium. This parent-daughter 40Ar/39Ar ratio is then used in conjunction with the standard radiometric age equation to produce an age for the sample. Ratios are calibrated against the known 40Ar/36Ar ratio of atmospheric air to account for mass discrimination in the mass spectrometer.