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Author Topic:   Potassium-Argon Dating
JonF
Member (Idle past 168 days)
Posts: 6174
Joined: 06-23-2003


Message 7 of 22 (95899)
03-30-2004 8:46 AM
Reply to: Message 5 by Biophysicist
03-30-2004 3:46 AM


Re: information source
It's worthwhile pointing out that K-Ar dating isn't used a lot any more. It is used, because it's pretty low-cost and is understood very well, but it is seldom used as the sole determination of an age. And, of course, it is beloved of creationists because they understand it and there are some real potential problems.
Do I surmise correctly that you don't need to know the propensity of Rubidium and Strontium to ihabit the different minerals, but merely isolate separate crystals from the rock and measure the abundances of each element?
Yes, with one minor correction ... what you refer to is called a "mineral isochron". There is also such a thing as a "whole rock isochron" in which each sample is a rock from the same source, but each rock/sample consists of many different minerals.
What is actually required is a set of "co-genetic" (that is, from the same source) samples in which the ratio of 87Rb (the parent isotope) to 86Sr (the non-radiogenic isotope of the radiogenic daughter isotope) differ. This allows the isochron method to not require the assumption of zero-initial-daughter and also allows it to almost always indicate when the assumption of a "closed system" has been violated or when the samples are not actually co-genetic. A terrific example of the power of the method Precise dating of the destruction of Pompeii proves argon-argon method can reliably date rocks as young as 2,000 years, in which the Ar-Ar isochron method was used to date rocks of well-known age. There was "excess argon" (the technical term for argon being incorporated into the rock at solidification) present, but the method worked anyway.
The basic isochron method is discussed in significantly more detail at Isochron Dating. Isochron methods are "age-diagnostic" methods; they not only give us an age, they give an indication of how likely it is that the age is correct. The vast majority of dates analyzed today and recently are done with age-diagnostic methods.
The other most commonly used method, probably the most commonly used method, is Uranium-Lead concordia-discordia dating. I don't know of any good explanations of that on the Web, but I sketched one out in this mewsage.
The canonical non-specialist reference is "The Age of the Earth", G. Brent Dalrymple, Stanford University Press, 1991. There are some useful collections of links at Radioisotope dating links and information and A Radiometric Dating Resource List.

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JonF
Member (Idle past 168 days)
Posts: 6174
Joined: 06-23-2003


Message 15 of 22 (186310)
02-17-2005 5:14 PM
Reply to: Message 12 by techristian
02-17-2005 4:56 PM


Re: Mount st. Helens lava rock over 300,000 with a/p dating
Sigh.
Those samples were specifically and dishonestly chosen to fool the method. They had "xenoliths" in them; pieces of older rock that hadn't completely melted. For such samples, the date obtained by K-Ar dating is a weighted average of the age of the new stuff and the age of the old stuff. Real geologists select samples without xenoliths. And there were other difficulties; such as, if the lab isn't warned that the samples may be young, they will not spend the heroic effort necessary to clean every last trace of argon out of the mass spectrometer before the run (such effort isn't necessary for older samples which have more argon).
For the New Zealand samples (Mt. Ngauruhoe) see DR. SNELLING'S "RADIOACTIVE 'DATING' FAILURE" and for the Mt. St. Helen's stuff see Young-Earth Creationist 'Dating' of a Mt. St. Helens Dacite: The Failure of Austin and Swenson to Recognize Obviously Ancient Minerals.
There are lots of other similar examples; when you read of a creationist crowing about failure of radioisotope dating, there's essentially always dirty work hidden somewhere
The vast majority of radioisotope dates obtained today and recently are performed by methods that are more robust than "plain vanilla" K-Ar dating, and they provide trustworthy indications of whether the dates are reliable. And thedates are, indeed, reliable. If you are interested in learning the truth about radioisotope dating, see Radiometric Dating: A Christian Perspective which is accurate and not too technical.

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JonF
Member (Idle past 168 days)
Posts: 6174
Joined: 06-23-2003


Message 18 of 22 (186314)
02-17-2005 5:23 PM
Reply to: Message 13 by crashfrog
02-17-2005 5:02 PM


Re: Mount st. Helens lava rock over 300,000 with a/p dating
Did those samples contain xenoliths?
Yes. See the links in my previous message.
And why do I find your claim that this "just happened" rather suspect,
He claimed that he had just heard which is something quite different.
since K-Ar dating hasn't been widely used since the 70's?
Depends on what you mean by "widely". It's still used, maybe on the order of 10-15% of all dates. It's low-cost, easy, well-understood, and known to be reliable almost all the time. Often K-Ar dates are cross-checked with other methods to eliminate the possibility of excess argon.
Did they do 40Ar/39Ar dating to eliminate test errors from nonradiogenic argon? If not, why not?
Surely you jest. Ar-Ar wouldn't give them the answer they wanted. But you knew that.
Since that's the most commonly used procedure for dating rocks these days?
The most commonly used procedure these days, over 50% of the cases, is U-Pb concordia-discordia dating, for several reasons. Ar-Ar is widely used but may suffer a decline in usage; new regulations for handling irradiated material post-9/11 make it more cumbersome and expensive.

This message is a reply to:
 Message 13 by crashfrog, posted 02-17-2005 5:02 PM crashfrog has replied

Replies to this message:
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JonF
Member (Idle past 168 days)
Posts: 6174
Joined: 06-23-2003


Message 19 of 22 (186315)
02-17-2005 5:25 PM
Reply to: Message 14 by techristian
02-17-2005 5:04 PM


Re: Mount st. Helens lava rock over 300,000 with a/p dating
WHY DO I NEED TO KNOW THE MECHANICS OF THE DATING PROCESS ANYWAY?
If you are going to discuss the dating proces, you need to know enough to understand the answers and figure out who's lying to you. (Hint: its not us).

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JonF
Member (Idle past 168 days)
Posts: 6174
Joined: 06-23-2003


Message 22 of 22 (186326)
02-17-2005 5:47 PM
Reply to: Message 21 by crashfrog
02-17-2005 5:27 PM


Prevalence of various dating methods
See the end of Message 1.

This message is a reply to:
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