A common claim among creationists' material is that, in order to be radiometrically dated, a rock sample must include information such as what stratigraphic position was the rock, what fossils were near it, what age does the one submitting the sample expect it be, etc.
I actually haven't seen those claims. But the ones about stratigraphic position and fossils are just plain wrong. Stratigraphic position is, of course, recorded for the overall report of results. Fossils nearby are often not even recorded. The lab that does the dating doesn't want to hear about stratigraphic position, nor do they want to hear about fossils. They just don't care; it won't have any effect on their measurements or the results of those measurements.
The dating lab would usually like a rough estimate of what age is expected, to optimize their equipment setup. For example, in K-Ar dating, they have to clean the argon from the equipment between runs. If they have no information about the age of the sample, or if the sample is expected to be old as suitable samples for K-Ar dating go, then they can do a normal cleaning (because a little stray argon won't make any difference unless the sample is very young). If the sample is expected to be young (i.e. not much argon), they have to do a super (and expensive) cleaning to avoid contamination.
Of course, if a sample for which they don't do a supercleaning turns out to be somewhat young
and the investigator wants an accurate date, they have to do a supercleaning and re-run the test. The typical creationist measurement of recent lavas should be done with a supercleaning, either because the creationsist tells the lab it shold be young or because the results come out young and another run should be made after supercleaning. But for some reason this never happens. Perhaps the creationists aren't interested in the best that the method can do.
(for the same sample I presume)
Depends on exactly what you mean. It is pretty rare for different tests to be run on the same physical chunk of matter. A sample suitable for one kind of test is often unsuitable for another kind of test, and the preparation and testing of samples is often destructive, leaving you afterwards with nothing to measure by another method.
It is not unusual for different tests to be run on samples from one rock, and it is downright common for different tests to be run on samples from the same outcrop or stratum or rock.
Different tests involve very different sample preparation and measurement equipment, and involve two or three wildly different mechanisms of radioactive decay. Agreement between methods is a very strong evidence of the correctness of the measurements.
It is not unusual for the results of such tests to differ slightly. Technically, what is measured as the age of a rock is the age of
closure, at which the rock was cool enough to "freeze" the relevant elements in place in the crystal lattice. The temperature at which this happens is different for different elements and minerals. So if a rock cooled slowly (and plutons can take millions of years to cool) you can get different ages from different methods. Other factors can affect the agreement between methods, but overall the agrement is very good.
One thing is worth mentioning; U-Pb concordia-discordia methods are applied only to minerals that strongly reject lead at solidification. When we find such a mineral with a significant amount of lead, that lead is the result of radioactive decay. Period. That's basic physics. This is accepted by some well-known creationists. In
HELIUM DIFFUSION RATES SUPPORT ACCELERATED NUCLEAR DECAY (PDF document), Humphreys, Austin, Baumgardner and Snelling write on page 3:
quote:
Samples 1 through 3 had helium retentions of 58, 27, and 17 percent. The fact that these percentages are high confirms that a large amount of nuclear decay did indeed occur in the zircons. Other evidence strongly supports much nuclear decay having occurred in the past [14, pp. 335-337]. We emphasize this point because many creationists have assumed that "old" radioisotopic ages are merely an artifact of analysis, not really indicating the occurrence of large amounts of nuclear decay. But according to the measured amount of lead physically present in the zircons, approximately 1.5 billion years worth — at today's rates — of nuclear decay occurred.
{emphasis added - JonF}
Unfortunately for Humphreys et al, their hypothesis of variable decay rates is a non-starter for all sorts of reasons.
All that said, here are some pointers to consistent results on the Web:
Consistent Radiometric datesRadiometric Dating
(ABE: fixed link to PDF file)
This message has been edited by JonF, 05-10-2006 09:26 AM
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