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Radiometric dating falsely assumes rocks are closed systems (Talk.Origins)
From CreationWiki, the encyclopedia of creation science
Claim Claim CD001:
 Radiometric dating falsely assumes that the rocks being dated are closed systems. It inappropriately assumes that no parent or daughter isotopes were added or removed via other processes through the history of the sample.
Source: Morris, Henry M., 1974. Scientific Creationism, Green Forest, AR: Master Books, p. 139.
CreationWiki response: It needs to be noted that this book is 30+ years old and it is recognized that new methods have been developed in that time, some of which are intended to address this problem. (Talk.Origins quotes in blue)
1. Absolutely closed systems do not exist even under ideal laboratory conditions.
This is exactly the point that Morris is making.
Nevertheless, many rocks approximate closed systems so closely that multiple radiometric dating methods produce consistent results, within 1 percent of each other.
And there are also many rocks with which multiple radiometric dating methods produce radically different dates. The main question is how likely are close dates to occur by chance.
2. Some rocks may be open to outside contamination, but not all of them are.
In discussing the assumptions required for computed dates to be accepted as accurate, McDougall states:
"The sample must have remained a closed system since the event being dated. Thus, there should have been no loss or gain of potassium or ^{40}Ar, other than by radioactive decay of ^{40}K. Departures from this assumption are quite common, particularly in areas of complex geological history, but such departures can provide useful information that is of value in elucidating thermal histories."^{[1]}
"Quite common" likely means more than 50% of the time. Anything less than 50% would not be common. This means that more than half of the time, the computed dates are incorrect, despite their positive twist at the end. And further, the only way it is known that "departures are quite common" is because the results are not what were expected.
Most ages are determined from multiple mineral and rock samples, which give a consistent date within 1 and 3 percent.
An actual list of radiometric dates from Alaska shows that 97% of them were made with only one method and 62% were dated by only one measurement.
It is extremely unlikely that contamination would affect all samples by the same amount.
If most multiple dates were within 13% then Talk Origins would have a point, but an actual list of radiometric dates from Alaska shows that among those with multiple dates, but multiple methods, have no statistical tendency for dates to be within 3% of Each other. Another study shows that given enough attempts the odds of finding multiple dates, but multiple methods so close together are actually quite good.
 Reference: Concordance of dates
 Reference: CD010
 Reference: CD020
 Reference: Mythology of Modern Dating Methods
3. Isochron methods can detect contamination and, to some extent, correct for it. Isochrons are determined from multiple samples, and contamination would have to affect all of the samples the same way in order to create an isochron that appeared okay but was wrong (see CD002).
Isochron dating assumes models for rock formation that do not include the Genesis Flood, so that could throw them off right from the start. False isochrons do occur, so contamination does affect the samples in a way that creates a good looking isochron that is wrong. The main way uniformitarian geologists tell a "good" isochron from a "bad" one is by seeing if it agrees with how old the Geological column says it should be.
 Reference: Isochron dating
With uraniumlead dating, closure of the system may be tested with a concordia diagram. This takes advantage of the fact that there are two isotopes of uranium (^{238}U and ^{235}U) that decay to different isotopes of lead (^{206}Pb and ^{207}Pb, respectively). If the system has remained closed, then a plot of ^{206}Pb / ^{238}U versus ^{207}Pb / ^{235}U will fall on a known line called the concordia. Even if samples are discordant, reliable dates can often be derived (Faure 1998, 287290). 
While Concordia dating does not assume a closed system, it does make assumptions about the rock's formation and history. Like all uniformitarian dating methods it does not consider affects of the Genesis Flood and its associated Accelerated decay. Furthermore, according to the creation model, these rocks would have been created on day three so even without Accelerated decay the assumed starting point is different.
4. Geochronologists are well aware of the dangers of contamination, and they take pains to minimize it. For example, they do not use weathered samples. 
No kidding. Contamination is the uniformitarian excuse for ignoring dates that do not fit the Geological column.
References
 ↑ McDougall and Harrison, 1988, Geochronology and Thermochronology by the ^{40}Ar/^{39}Ar method. Oxford University Press, New York, p. 12
See Also
