Measurement of the 39 Ar produced by bombardment is made by mass spectrometer at the same time as measuring the amount of 40 Ar present. Gondwana Research. Almost like a reversed CL imagery, as the brightness correlates to atomic number. The atomic number decreases by one, but there is no change in the atomic weight. The samples of rock collected for radiometric dating are generally quite large several kilograms to eliminate inhomogeneities in the rock. If we dated a number of individual grains in the sedimentary rock, we would likely get a range of different dates, all older than the age of the rock.
8.4 Isotopic Dating Methods
The amount of argon measured is therefore commonly less than the total amount produced by the radioactive decay of potassium.
Detrital zircon geochronology
We have no reason to think that the white layers are formed in any other way. Afterwards, the decay rate gradually decreases with time as fewer and fewer parent isotopes remain. This results in an underestimate of the age of the rock.