Potassium 40 dating rocks
After the recrystallization of magma, more being the most abundant isotope.
Thus, the amount of calcium originally present is not known and can vary enough to confound measurements of the small increases produced by radioactive decay.
If the rock actually contained some argon-40 when it solidified then the calculated age would be too old. What he does is check his calculated age with the ages produced by other dating methods.
On the other hand, if the rock was later disturbed by a geological upheaval and lost argon the age would be too young. In other words, he checks to see if his calculated result falls into the range where he expects it to fall, given the geological situation of where he found his rock.
Due to the long half-life of Although it finds the most utility in geological applications, it plays an important role in archaeology.
One archeological application has been in bracketing the age of archeological deposits at Olduvai Gorge by dating lava flows above and below the deposits.
The amount of argon sublimation that occurs is a function of the purity of the sample, the composition of the mother material, and a number of other factors.
These factors introduce error limits on the upper and lower bounds of dating, so that final determination of age is reliant on the environmental factors during formation, melting, and exposure to decreased pressure and/or open-air.
Entrained argon—diffused argon that fails to escape from the magma—may again become trapped in crystals when magma cools to become solid rock again.
This means that the geologist can plausibly assume that all argon gas escapes from the molten magma while it is still liquid.
He thinks this solves his problem of not knowing the initial quantity of the daughter element in the past and not being able to go back in time and make measurements. He assumes that any argon-40 that he measures in his rock sample must have been produced by the radioactive decay of potassium-40 since the time the rock solidified.
He always does this check because no dating method can be trusted on its own. It’s simple; the geologist will change his assumed history for that rock.
For example, if the age is higher than he expected he will say that his rock contains ‘excess argon’ or ‘parentless argon’.