Radiometric age dating examples
Note that the mere existence of these assumptions do not render the simpler dating methods entirely useless.
In many cases, there are independent cues (such as geologic setting or the chemistry of the specimen) which can suggest that such assumptions are entirely reasonable.
Isochron methods avoid the problems which can potentially result from both of the above assumptions.
especially in absence of cross-checks by different methods, or if presented without sufficient information to judge the context in which it was obtained.
The only problem is that we only know the number of daughter atoms now present, and some of those may have been present prior to the start of our clock. The reason for this is that Rb has become distributed unequally through the Earth over time.
We can see how do deal with this if we take a particular case. For example the amount of Rb in mantle rocks is generally low, i.e. The mantle thus has a low If these two independent dates are the same, we say they are concordant.
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Some of these rocks are sedimentary, and include minerals which are themselves as old as 4.1 to 4.2 billion years.