R
rossum
Guest
Taking these in order:The basic assumptions made in radiometric dating are:
- Every radioactive element will decay at a constant rate. The rate at which each element decays is its half-life [ (def) ](javascript:define('Half-life: ', ‘The amount of time that it takes for 50% of a given number of radioactive atoms to dcay.’,‘275’,‘225’,‘def’))
- The rate of decay is specific to a particular radioactive element (see list of half lifes of various radioactive elements).
- When the substance containing a radioactive parent was first formed, there was not daughter element present. It is assumed that the daughter is derived solely from the decay of the radioactive parent. If daughter atoms were present that were not the result of the decay process the calculated date would be unreliable.
- From the time when the substance containing radioactive elements first came into existence until the time that the material was analyzed and dated, the system had been closed; in other words there had been in infusion or removal of either parent or daughter atoms.
- All daughter atoms contained within the radioactive substance were created by the radioactive decay of the corresponding parent atom. This is a repeat of the previous assumption that the system is a closed system.
These are not “assumptions”. They are either known experimental measurements or are known problems which are eliminated by careful selection of both methods and samples. As I said above, K-Ar dating should not be used on pillow lavas because it gives incorrect results. It gives correct results on sub-aerial lavas.
- This is not an assumption. We can measure decay rates in the past by observing radioactive decays in distant astronomical objects, particularly supernovae. This is a measurement, not an assumption.
- This is not an assumption. Decay rates can be measured here on earth. Again a measurement not an assumption.
- This can be eliminated in many cases. For example, pillow lavas cannot be K-Ar dated because of this problem, instead some other method might be used. In other cases the Isochron method can be used, which effectively measures the original amount of daughter product in the sample.
- Again, not an assumption. This can be determined geologically. If one contaminant is present, then so will others. Any measured contaminant, such as water, can render the specific sample unsuitable for dating.
- As you say, closure of the system was ensured at step 4.
ETA: Your link appears not to work after multiple tries.
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