I guess I still don't understand. If it turns out isochron dating doesn't require and/or tests the assumptions you posted, then what?
Scriptural interpretations are not something I enjoy debating. Sorry, but I've had some very bad experiences that I don't want to risk repeating.
You didn't answer what I asked and that's a couple of times you've done it. But I won't push, so here's some of the material from the article:
Initial daughter product
The amount of initial D is not required or assumed to be zero. The greater the initial D-to-Di ratio, the further the initial horizontal line sits above the X-axis. But the computed age is not affected.
If one of the samples happened to contain no P (it would plot where the isochron line intercepts the Y-axis), then its quantity of D wouldn't change over time -- because it would have no parent atoms to produce daughter atoms. Whether there's a data point on the Y-axis or not, the Y-intercept of the line doesn't change as the slope of the isochron line does (as shown in Figure 5). Therefore, the Y-intercept of the isochron line gives the initial global ratio of D to Di.
For each sample, it would be possible to measure the amount of the Di, and (using the ratio identified by the Y-intercept of the isochron plot) calculate the amount of D that was present when the sample formed. That quantity of D could be subtracted out of each sample, and it would then be possible to derive a simple age (by the equation introduced in the first section of this document) for each sample. Each such age would match the result given by the isochron.
Contamination - parent isotope
Gain or loss of
P changes the X-values of the data points:
In order to make the figures easy to read (and quick to draw), the examples in this paper include few data points. While isochrons are performed with that few data points, the best ones include a larger quantity of data. If the isochron line has a distinctly non-zero slope, and a fairly large number of data points, the nearly inevitable result of contamination (failure of the system to remain closed) will be that the fit of the data to a line will be destroyed.
For example, consider an event which removes
P. The data points will tend to move varying distances, for the different minerals will have varying resistance to loss of
P, as well as varying levels of
Di:
The end result is that the data are nearly certain not to remain colinear:
Even in our simple four-data-point example isochron, a change to two of the samples...
... would require exact changes to the remaining two samples in order for the data to remain colinear:
Note: In the special case where the isochron line has a zero slope (indicating zero age), then gain or loss of
P may move the data points, but they will all still fall on the same horizontal line. In other words, random gain or loss of
P does not affect a zero-age isochron. This is an important point. If the Earth were as young as young-Earth creationists insist, then the "contamination" which they suggest to invalidate dating methods would have no noticeable effect on the results.
Contamination - daughter isotope
In the case of Rb/Sr isochron dating, the most common form of isotope migration is a preferential loss of radiogenic daughter (87Sr).
Faure (1986, p. 123) notes:
This will change the vertical position of the data points:
As with gain or loss of
P, in the general case it is highly unlikely that the result will be an isochron with colinear data points:
