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Author Topic:   Potassium-Argon Dating
Biophysicist
Inactive Member


Message 1 of 22 (95822)
03-30-2004 1:34 AM


I'm new to this particular forum, but I've been on some others before. My question is for any geologists out there. I am a chemistry graduate student, so I should be able to handle any amount of detail you're willing to throw at me; technical information is welcome.

On what assumptions or conditions does K-Ar dating rest, other than:

* that the rock being dated has not undergone a significant heating/cooling cycle since it was initially formed
* that the rock is impermeable to Ar, and so traps what Ar-40 is a result of radioactive decay and does not leach Ar-40 from the atmosphere
* that the rock is sufficently old for some buildup of Ar-40 (100K years or so)

I'm a bit confused because Geochron labs has their minimum date on the order of 0.5M years, while some webpages I'm finding at UCSB talk about K-Ar dating possibly being useful for dating rocks as young as 20K years.

I've skimmed some reports by creation "scientists" such as Steve Austin describing a mockery of this technique by using it to derive dates of up to a million years for historic lava flows. I use the words "up to a million" advisedly, because all of the supposedly erroneous dates fall nicely at the bottom end of the spectrum K-Ar is meant to test.

I also saw a defense of those techniques by some guy at AIG, but I didn't like his assertion that the error of the measurements being 60K years and the mean being ~300K years strongly indicated that it was the test itself, not the experiment, that was the problem. Is there some natural distribution to the amount of Argon present in rocks, such that we can measure the amount actually there to a finer precision than the natural variation? If so, how does this distribution arise? If not, why do we not meaure zero Ar-40 in rocks that were just coughed out of a volcano?

If possible, I'd like to design a quiz question for my freshman chemistry students on K-Ar dating, so I'm trying to know as much background as possible.

Thanks very much for your comments,

Biophysicist


Replies to this message:
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Adminnemooseus
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Message 2 of 22 (95823)
03-30-2004 1:35 AM


Thread moved here from the Evolution forum.

Move courtesy of AM 1 Minute Topic Moving Service.

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[This message has been edited by Adminnemooseus, 03-30-2004]


    
RAZD
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Posts: 15475
From: the other end of the sidewalk
Joined: 03-14-2004
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Message 3 of 22 (95831)
03-30-2004 1:58 AM
Reply to: Message 1 by Biophysicist
03-30-2004 1:34 AM


information source
A good source of information I have found on many types of radiometric dating is:
Radiometric Dating, A Christian Perspective by Dr. Roger C. Wiens
http://www.asa3.org/ASA/resources/Wiens.html

You will see Potassium-Argon listed in the hyperlinked table of contents and there is some good information there.

Dr. Wiens also gives his e-mail address.

Enjoy.


we are limited in our ability to understand
by our ability to understand

RebelAAmerican.Zen[Deist


This message is a reply to:
 Message 1 by Biophysicist, posted 03-30-2004 1:34 AM Biophysicist has responded

Replies to this message:
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Sylas
Member (Idle past 1544 days)
Posts: 766
From: Newcastle, Australia
Joined: 11-17-2002


Message 4 of 22 (95842)
03-30-2004 2:32 AM
Reply to: Message 1 by Biophysicist
03-30-2004 1:34 AM


Biophysicist writes:

On what assumptions or conditions does K-Ar dating rest, other than:

* that the rock being dated has not undergone a significant heating/cooling cycle since it was initially formed
* that the rock is impermeable to Ar, and so traps what Ar-40 is a result of radioactive decay and does not leach Ar-40 from the atmosphere
* that the rock is sufficently old for some buildup of Ar-40 (100K years or so)

Omit that last. You don't need to assume sufficient age; though you do need to pay attention to the error bars on any calculated age. More on this below.

The other two assumptions are variations of the same thing.

Another very important assumption is that the rock did not entrap any Argon at the time of its formation. This assumption is probably the most important source of error with K-Ar dating. There are various ways in which a rock might include Argon at the time of initial formation -- such as inclusion of solid crystals which have Argon already within their own crystal matrix. This kind of error has led to Argon-Argon dating becoming more common, which is a form of isochron dating that can handle such issues. But I digress.

Another assumption -- which is not a basis for any concern -- is the that radiactive decay rates are constant over time. It is worth mentioning this, since creationists are likely to bring it up. If you are likely to be explaining this to an audience including creationists, it would be wise to check up a bit about the many methods by which constancy of decay rates is tested and confirmed. The point here is that geologists don't do any additional special checks for decay rate variations in any particular rock. Decay rate variation is the domain of particle physics. Geologists leave testing of that matter to the physicists.

The basic assumptions can be given as follows:

  • The rock started out with no Argon.
  • Over time, no Potassium or Argon enters or leaves the rock; the atoms of the rock we measure are the same atoms present at formation from a molten state.
  • Over time, Poteassium decays to Argon according to the known decay relation.

I'm a bit confused because Geochron labs has their minimum date on the order of 0.5M years, while some webpages I'm finding at UCSB talk about K-Ar dating possibly being useful for dating rocks as young as 20K years.

It depends on the accuracy of instruments. For example, some labs may have some residual atoms of Argon lying around from the last sample measured, and this will contribute to small errors in the next.

If the amounts of Argon in the next measurement are very small, then the final Argon count will be sigificantly due to foreign Argon. Some labs charge more for young samples, because they need to prepare the measuring equipment much more carefully.

But in any case, you should get error bars for known sources of measurement error, like Argon contamination in measurement. For a very young sample, the error bars will exceed the age... which means it is not actually useful for finding the age of the rock.

You can still do the test, and this is sufficient to distinguish a young rock from an old one. But you can't get a meaningful age. You can only say that it is something younger than some lower bound.

If you have a messy rock which is likely to have included Argon, then dates you get back may be meaningless. Geologists are well aware of this potential for error, and have various ways to judge the reliability of a date. Some kinds of rock are better than others -- and this has been tested. In general, a confirming date by some independent means is very handy.

My quick on-line check suggests that Geochron labs does not currently do K-Ar dating. I know they have been used by creationists in the past to measure some of their samples, and they have done K-Ar in the past. The qualifications you list above were considered here in Message 77.

I've skimmed some reports by creation "scientists" such as Steve Austin describing a mockery of this technique by using it to derive dates of up to a million years for historic lava flows. I use the words "up to a million" advisedly, because all of the supposedly erroneous dates fall nicely at the bottom end of the spectrum K-Ar is meant to test.

I also saw a defense of those techniques by some guy at AIG, but I didn't like his assertion that the error of the measurements being 60K years and the mean being ~300K years strongly indicated that it was the test itself, not the experiment, that was the problem. Is there some natural distribution to the amount of Argon present in rocks, such that we can measure the amount actually there to a finer precision than the natural variation? If so, how does this distribution arise? If not, why do we not meaure zero Ar-40 in rocks that were just coughed out of a volcano?

A lot of the errors are due to included Argon. Austin is actually someone who has real experience in this area. This doesn't make him any more sensible (just the reverse, in fact) but it does mean we need to be careful to check what the source of errors might be.

You can check out some critiques of some of Austin's work at talkorigins; but if you have a specific claim from Austin for which you don't know some careful analysis from someone well informed in geology, then you need to be a bit careful about identifying the problems. It would be a great topic here at EvC.

We are at present suffering an infestation of people who want to overthrow conventional science but have no idea at all of the matters they purport to refute. Many of the posts look like Finnegan's Wake on drugs. We are all starving for a creationist argument with some meat on it.

Cheers -- Sylas

[This message has been edited by Sylas, 03-30-2004]


This message is a reply to:
 Message 1 by Biophysicist, posted 03-30-2004 1:34 AM Biophysicist has responded

Replies to this message:
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Biophysicist
Inactive Member


Message 5 of 22 (95859)
03-30-2004 3:46 AM
Reply to: Message 3 by RAZD
03-30-2004 1:58 AM


Re: information source
Thanks! That was a really informative website. Particularly interesting was the method by which we can get a measurement of the initial parent/daughter ratios through use of different minerals in a single rock in methods such as Rubidium dating.

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?

Weins is from my hometown. He's a welcome antidote to John Baumgardner, although I didn't know about him when I was growing up there.

Biophysicist


This message is a reply to:
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Biophysicist
Inactive Member


Message 6 of 22 (95861)
03-30-2004 3:55 AM
Reply to: Message 4 by Sylas
03-30-2004 2:32 AM


Thanks Sylas. I gleaned a number of your points from reading the previous post's suggested website. Very good information, although I did catch a couple of lexical errors.

I'm all for having a meaty discussion on some issue of science, and I appreciate your admonition to be careful about making assertions.

On another forum, more than a year ago, I was a bit of a jerk. I logged on a pretended for weeks to be a creationist, making a number of really asinine statements before the "evolutionists" finally called me on it. Then, a few weeks later, I turned right around and did it again under a different name. After I gave myself away, I think a number of the creationists were a bit sore. I won't be doing that here.

I must ask, though. I've noticed moderated debates with "evaluations" of each side's performance. Are the moderators truly "fair and balanced" or are they like Fox News (something you Aussies haven't had to deal with first-hand)?

Biophysicist


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JonF
Member
Posts: 2602
Joined: 06-23-2003
Member Rating: 3.7


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.


This message is a reply to:
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RAZD
Member
Posts: 15475
From: the other end of the sidewalk
Joined: 03-14-2004
Member Rating: 4.4


Message 8 of 22 (95928)
03-30-2004 10:27 AM
Reply to: Message 5 by Biophysicist
03-30-2004 3:46 AM


Rubidium and Strontium dating services
You're welcome.

I am not conversant with the intricacies of radiometric dating, so wouldn't presume to answer your question on Rubidium and Strontium other than to suggest you contact Dr Weins.

There are others on this forum that are scientists in related fields that may know the answer.


we are limited in our ability to understand
by our ability to understand

RebelAAmerican.Zen[Deist


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Coragyps
Member
Posts: 5050
From: Snyder, Texas, USA
Joined: 11-12-2002
Member Rating: 5.1


Message 9 of 22 (95948)
03-30-2004 11:35 AM


The book "The Age of the Earth" by G Brent Dalrymple gives a mighty good treatment of a bunch of dating methods. It may be a little out of date - published in 1991, I think - but it gives plenty of details.
Replies to this message:
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diggerdowner
Inactive Member


Message 10 of 22 (181373)
01-28-2005 1:02 PM
Reply to: Message 9 by Coragyps
03-30-2004 11:35 AM


Time/Age
No scientific brain here-
I can't understand DATING at all. I think it's OFF by 85%!
DD

This message has been edited by diggerdowner, 01-31-2005 13:09 AM


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AdminNosy
Administrator
Posts: 4741
From: Vancouver, BC, Canada
Joined: 11-11-2003


Message 11 of 22 (181386)
01-28-2005 2:25 PM
Reply to: Message 10 by diggerdowner
01-28-2005 1:02 PM


t o p i c !
This is not on topic.

If you can't find a topic where it does belong you may propose a new one. But you'll have to flesh out your post some before it would be promoted.


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techristian
Member (Idle past 387 days)
Posts: 60
Joined: 04-03-2002


Message 12 of 22 (186301)
02-17-2005 4:56 PM


Mount st. Helens lava rock over 300,000 with a/p dating
I heard today that several rock samples were sent to GEOCHRON labs for Argon Potassium dating. These samples were from MODERN lava sites including a 16 year old sample from Mt. St. Helens and a 40 year old sample from a site in New Zealand. These were both modern day eruptions which produced modern rock but when the results came back from Geochron, the rocks from Mt. St. Helens were dated between 300,000 years and 1.5 million years old. The rocks from New Zealand were dated between 300,000 and 3 million years old !!!!

HUH???

Dan

http://etnot.com/ootw.html


Replies to this message:
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crashfrog
Inactive Member


Message 13 of 22 (186303)
02-17-2005 5:02 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
Did those samples contain xenoliths?

And why do I find your claim that this "just happened" rather suspect, since K-Ar dating hasn't been widely used since the 70's? Did they do 40Ar/39Ar dating to eliminate test errors from nonradiogenic argon? If not, why not? Since that's the most commonly used procedure for dating rocks these days?

Why would they date them at all, come to think of it? I mean, if you wanted to know how old some lava at St. Helens was, why wouldn't you just look up the date of the eruption? Why bother with an expensive procedure that is rarely used nowadays?

There's a lot of obvious questions here that you do not address. Why is that?


This message is a reply to:
 Message 12 by techristian, posted 02-17-2005 4:56 PM techristian has responded

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techristian
Member (Idle past 387 days)
Posts: 60
Joined: 04-03-2002


Message 14 of 22 (186304)
02-17-2005 5:04 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
Here are some links with a more in-depth answer to your questions.

http://www.answersingenesis.org/creation/v23/i3/radiodating.asp
http://www.creationism.org/articles/swenson1.htm
http://www.icr.org/research/sa/sa-r01.htm
http://www.reall.org/newsletter/v06/n04/visit-to-the-icr-part-1.html

Now that I think about it, WHY DO I NEED TO KNOW THE MECHANICS OF THE DATING PROCESS ANYWAY? If I sent a rock sample to the "best" lab for dating, their results should show the date for the sample in question, or mankinds best attempt to date.

Dan

http://teachmedrums.com

This message has been edited by techristian, 02-17-2005 17:11 AM


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JonF
Member
Posts: 2602
Joined: 06-23-2003
Member Rating: 3.7


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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