Talk:K–Ar dating

Religious objections

Many religious folk are opposed to the use of this kind of dating. Do they have a argument. How acurate is this kind of dating and how do we know this happens over the time frames suggested?

Personally I think that as we have had many technological advancements using theoretical science, that it is true and accurate. Anybody else have ideas/suggestions.

there are still assumptions which we must assume to be true for potassium-argon dating to work. We must assume that there was no argon already trapped in these lattices, none escaped or was added until we found the rock and that decay rates have always been constant. No one was there to observe these things, so can we really trust their dates? Alisyd 17:01, 18 April 2006 (UTC)

Standard creationist boilerplate is not appreciated and not helpful. Of course there are ::assumptions involved in K/Ar dating. Every radiometric dating technique relies upon similar ::assumptions--the point is that they're justified assumptions. A few seconds of googling will ::show you various sites that confirm that decay rates are constant and remain so. That is not an ::issue here.

But for completeness's sake, read here for K/Ar basics:

http://anthro.palomar.edu/time/time_5.htm

and here for an example of the technique used appropriately:

http://www.ajsonline.org/cgi/content/abstract/262/2/145

and here for an acknowledgment that yes, there are problems with the technique, but that by no ::means undermines the whole process:

http://jchemed.chem.wisc.edu/hs/journal/issues/2005/jul/clicSubscriber/V82N06/p1094.pdf

Finally, can we please stop using the "you weren't there!" argument? I wasn't there for your ::mother's birth, either, but I can infer that she was indeed born from the fact of your existence. ::Oh, and yeah, unless a particular religious objection to radiometric dating techniques is notable, ::there is no reason to include it in an article. This same discussion has gone on ad nauseum ::over at Radiocarbon dating.

Lower and upper limits of K-Ar method?

Hi all. I don't have my copy of Harrison and McDougall handy and I don't work on young rocks, so the lower limit to K-Ar dates that I inserted (>100 000a) might be wrong.Rickert 18:11, 3 February 2006 (UTC)

Hi all, is the upper limit (5 mil yrs) given at the end of the first para correct? It is odd given that the half life is in bln years, and the technique is used for dated very old rocks. Could somebody having a copy of Harrison and McDougall check?

Assumptions

The following currently is on another wikipage, but I think it ought to be here instead.

Assumptions - quote

In order to do Potassium/Argon dating there are some assumptions that must be accepted as true. The following list of assumptions comes from McDougall I. and Harrison, T.M.

"As with all isotopic dating methods, there are a number of assumptions that must be fulfilled for a K-Ar age to relate to events in the geological history of the region being studied. These same assumptions also apply to the 40Ar/39Ar method, although this latter technique provides greatly increased opportunities for their testing. The principal assumptions are given below with brief comment as to their validity, and will be further discussed implicitly or explicitly in the following chapters."

1. "The parent nuclide, 40K, decays at a rate independent of its physical state and is not affected by differences in pressure or temperature. This is a major assumption, common to all dating methods based on radioactive decay; the available evidence suggest that it is well founded (Friedlander et al., 1981). Although changes in the electron capture partial decay constant for 40K possibly may occur at high pressures, theoretical calculations by Bukowinski (1979) indicate that for pressures experienced within a body of the size of the Earth the effects are negligibly small."

2. "The 40K/K ratio in nature is constant. As 40K is rarely determined directly when ages are measured, this is an important underlying assumption. Isotopic measurements of potassium in terrestrial and extraterrestrial samples indicate that this assumption is valid, at least to the extent that no differences greater than about 3% have been reported in the 39K/41K ratio. The evidence for the essentially constant isotopic ratios for the potassium isotopes will be presented in more detail subsequently."

3. "The radiogenic argon measured in a sample was produced by in situ decay of 40K in the interval since the rock crystallized or was recrystallized. Violations of this assumption are not uncommon. Well-known examples of incorporation of extraneous 40Ar include chilled glassy deep-sea basalts that have not completely outgassed preexisting 40Ar*, and the physical contamination of a magma by inclusion of older xenolitic material. Further examples will be discussed later, as the 40Ar/39Ar method allows the presence of extraneous argon to be recognized in some cases."

4. "Corrections can be made for nonradiogenic 40Ar present in the rock being dated. For terrestrial rocks the assumption generally is made that all such argon is atmospheric in composition with 40Ar/36Ar = 295.5, and although this commonly is so, there are exceptions. Various ways of assessing this assumption are available including the use of isotope correlation diagrams. Extraterrestrial samples such as meteorites and lunar rocks have nonradiogenic argon of quite different composition to that of atmospheric argon, but corrections often can be made satisfactorily, particularly as the nonradiogenic contributions usually are minor."

5. "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 40Ar*, other than by radioactive decay of 40K. 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. "

"These basic assumptions must be tested and assessed in each study that is undertaken. This is usually best done by measuring a suite of rocks or minerals from the area under study. The consistency or lack of consistency of the results, together with knowledge of the geology of the area, allows assessment of some of these assumptions, and provides the basis for conclusions as to the reliability and meaning of the measured ages. As will become evident later, an important advantage of the 40Ar/39Ar dating method is that the assumptions underlying calculation and interpretation of an age are more readily assessed than is the case for conventional K-Ar age measurements."

---Christian Skeptic (talk) 14:26, 18 July 2008 (UTC)

Summary for use in article.

Assumptions - modified

In order to do Potassium/Argon dating there are a number of assumptions that must be accepted as true for a K-Ar age to relate to events in the geological history of the region being studied.

1. The parent nuclide, 40K, decays at a rate independent of its physical state and is not affected by differences in pressure or temperature. This is a major assumption, common to all dating methods based on radioactive decay; the available evidence suggest that it is well founded (Friedlander et al., 1981). Although changes in the electron capture partial decay constant for 40K possibly may occur at high pressures, theoretical calculations by Bukowinski (1979) indicate that for pressures experienced within a body of the size of the Earth the effects are negligibly small.

2. The 40K/K ratio in nature is constant. As 40K is rarely determined directly when ages are measured, this is an important underlying assumption. Isotopic measurements of potassium in terrestrial and extraterrestrial samples indicate that this assumption is valid, at least to the extent that no differences greater than about 3% have been reported in the 39K/41K ratio.

3. The radiogenic argon measured in a sample was produced by in situ decay of 40K in the interval since the rock crystallized or was recrystallized. Violations of this assumption are not uncommon. Well-known examples of incorporation of extraneous 40Ar include chilled glassy deep-sea basalts that have not completely outgassed preexisting 40Ar*, and the physical contamination of a magma by inclusion of older xenolitic material.

4. Corrections can be made for nonradiogenic 40Ar present in the rock being dated. For terrestrial rocks the assumption generally is made that all such argon is atmospheric in composition with 40Ar/36Ar = 295.5, and although this commonly is so, there are exceptions. Various ways of assessing this assumption are available including the use of isotope correlation diagrams. Extraterrestrial samples such as meteorites and lunar rocks have nonradiogenic argon of quite different composition to that of atmospheric argon, but corrections often can be made satisfactorily, particularly as the nonradiogenic contributions usually are minor.

5. 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 40Ar*, other than by radioactive decay of 40K. 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.

--Anything less than this will give an incomplete and inaccurate picture of what McDougall and Harrison said. Christian Skeptic (talk) 18:37, 18 July 2008 (UTC)

Removed. First, if you're going to include Harvard references from the original - then you must provide a ref for them. Most importantly the addition of the boolean true comment and conclusions derived are probably WP:OR and are a bit misleading as stated. I don't disagree with the inclusion of the five assumptions - just need to summarize more and some comment should be included noting that all geochronology labs and researchers are aware of and consider the assumptions in their work. Not sure how to source that, but the inclusion of detailed assumptions without that kind of context is inherently misleading. Vsmith (talk) 23:57, 24 July 2008 (UTC)

I did some summarizing, but the original is so succinct and plain it is extremely hard to summarize any further. And if you remove too much, it will be claimed that the article is attempting to hide something. There was no intent to imply that labs and researchers are not aware of the assumptions. But, rather that everyone should know what those assumptions are, not just the labs and researchers. And they are easy to comprehend. The Boolean expression was simply added to make it easier to see the relationship between assumptions and method. It is not WP:OR, but simply a graphic paraphrase of the assumptions and the first paragraph. Christian Skeptic (talk) 04:05, 25 July 2008 (UTC)

Assumptions (further summarized)

In order to do Potassium/Argon (K/Ar) dating, labs and researchers make a number of assumptions that must be accepted as true for a K-Ar age to relate to events in the geological history of the region being studied.

1. The parent nuclide, ⁴⁰K, decays at a rate independent of its physical state and is not affected by differences in pressure or temperature. This is a major assumption, common to all dating methods based on radioactive decay; the available evidence suggest that it is well founded. Although changes in the electron capture partial decay constant for ⁴⁰K possibly may occur at high pressures, theoretical calculations indicate that for pressures experienced within a body of the size of the Earth the effects are negligibly small.

2. The ⁴⁰K/K ratio in nature is constant. As ⁴⁰K is rarely determined directly when ages are measured, this is an important underlying assumption.

3. The radiogenic argon measured in a sample was produced by in situ decay of ⁴⁰K in the interval since the rock crystallized or was recrystallized. Violations of this assumption are not uncommon. Well-known examples of incorporation of extraneous ⁴⁰Ar include chilled glassy deep-sea basalts that have not completely outgassed preexisting ⁴⁰Ar*, and the physical contamination of a magma by inclusion of older xenolitic material.

4. Corrections can be made for nonradiogenic ⁴⁰Ar present in the rock being dated. For terrestrial rocks the assumption generally is made that all such argon is atmospheric in composition with ⁴⁰Ar/³⁶Ar = 295.5, and although this commonly is so, there are exceptions. Various ways of assessing this assumption are available.

5. The sample must have remained a closed system since the event being dated. Thus, there should have been no loss or gain of ⁴⁰K or ⁴⁰Ar*, other than by radioactive decay of ⁴⁰K. 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.

The relationship of these assumptions to the method and age as stated above can be paraphrased in a simple Boolean expression:

(Asm. 1: TRUE) AND (Asm. 2: TRUE) AND (Asm. 3: TRUE) AND ( Asm. 4: TRUE) AND (Asm. 5: TRUE) = (Method: TRUE) = (Age: TRUE)

The Boolean restatement clarifies the obvious point that all assumptions must be true for the method to be valid. Yet we read that violations of Assumption 3 are "not uncommon". Accordingly, McDougall and Harrison, based on Assumption 4, allow for corrections to the amounts of "nonradiogenic ⁴⁰Ar" such that Assumptions 3 becomes TRUE. So, for the common case, the relationship of assumptions to method and age, as expressed by McDougall and Harrison, can again be paraphrased in the Boolean expression:

(Asm. 1: TRUE) AND (Asm. 2: TRUE) AND ((Asm. 3: FALSE) OR (Correction: TRUE)) AND ( Asm. 4: TRUE) AND (Asm. 5: TRUE) = (Method: TRUE) = (Age: TRUE)

Also, departures from Assumption 5 are 'quite common'. McDougall and Harrison state that this falsifying of Assumption 5 still provides useful data for "elucidating thermal histories."

--- Christian Skeptic (talk) 15:36, 25 July 2008 (UTC)

This proposed change has been here for review for a week. Unless further objections arise I'll put it in the article soon. Christian Skeptic (talk) 15:57, 31 July 2008 (UTC)

The boolean stuff is ridiculous. What makes this any easier to understand than "all the assumptions have to be true"? You can't assume every wikipedia reader understands boolean logic, especially in an article completely unrelated to computer science. The rest of this section is probably unnecessary anyway. Pwrong (talk) 15:59, 9 August 2008 (UTC)

Anyone who has taken High School or even Jr. High (USA) algebra is acquainted with simple Boolean logic which includes most readers of Wikipedia. Boolean logic extends far beyond and existed long before computer science. This last section is important. Christian Skeptic (talk) 03:50, 10 August 2008 (UTC)

Assumptions 3 and 5 explicitly state that they are commonly false. And their being commonly false makes the method commonly false and the dates commonly false. The boolean expressions clearly show HOW falsified assumptions are corrected to make the method true again for the common case. This is not clear in the summaries of the assumptions and many people can get lost in the verbage. The boolean paraphrases are easy to understand. What are you afraid of? Christian Skeptic (talk) 03:56, 11 August 2008 (UTC)

Assumptions section added back in

I added the Assumptions section back in. Although the revised Method section mentions some of these assumptions, they are scattered throughout. I thought the orginial section did an excellent job of succinctly summarizing the assumptions involved. This addresses an unintended consquence of Eldereft's major rewrite in which the assumptions (and the fact that there are assumptions) was, for the non-technical reader, buried in the technical discussion of the method. Moreover, where assumptions are mentioned in the Method section, they are discussed in terms of how the method is done. The Assumptions section focuses on what the limitations are. The result is that while some assumptions get mentioned twice, they are mentioned in different ways, with a different focus, and with different information. --Scott English (talk) 18:48, 9 November 2008 (UTC)

And I just took it back out; I included an admonition to take it to talk without noticing that you already had - sorry about that. I believe that I included all of the information from the old Assumptions section when I revised and expanded the Methods section. This, in my opinion, is the correct way to present the information, as the samples for and conditions under which the method is applicable are in direct consequence to how the method functions. Additionally, an embedded list like that is bad encyclopedic style for information that does not lend itself to bullet points. I believe that converting this list to prose would produce a section quite similar to portions of the Methods section, so why not just present it where it makes sense? - Eldereft (cont.) 13:40, 10 November 2008 (UTC)

Bottom line on assumptions (here and in any other radiometric dating technique, is that they are about what should be checked for before a sample is dated. And that is standard operating practice in geochronology. If some geological material violates some of the assumptions, it does not mean the method is inherently wrong, but that the applying that method on that sample will not tell you the original age of formation of the rock, but something else. Which is what McD&H are saying. For example, if someone dated a rock from a metamorphic belt using K-Ar and stated that the age they determined could only be the age of formation of the rock, it would be a questionable interpretation. But if they said the age they determined showed a thermal event at such and such a time, it would be quite reasonable, especially if it fit with other data on the metamorphic belt. Babakathy (talk) 08:52, 18 December 2008 (UTC)

Major rewrite

I believe that I did not discard any information, but there was some amount of rearranging and expansion. The lead now more accurately summarizes the article; it could still use another sentence or two - once we find a source for use deriving the geomagnetic polarity time scale, I think that that should be included. I added the formula for relating the ⁴⁰
Ar
/⁴⁰
K
ratio to time elapsed given the half life of K-40. Separated the sections more cleanly - Decay series states the relevant decays and nothing else. Included some more information from McDougal & Harrison. Stylistically it is much cleaner and less of a copyright violation to have the former Assumptions section integrated into the text. It feels pretty silly to even mention the assumption that nuclear decays are unaffected by the temperatures and pressures available on Earth, but the source felt it important enough to list, so the information that K-Ar dating is not influenced by this remains. Also, the current reference 5 (Aronson and Lee, (1986)) requires more information. - Eldereft (cont.) 02:55, 15 September 2008 (UTC)

I have no major problem with the rewrite. I did notice a couple points that needed to be clarified according to McDougal and Harrison. Christian Skeptic (talk) 14:10, 15 September 2008 (UTC)

Alright, I can see how the problem of finding a sample that will yield an accurate date is not fully covered by the last three paragraphs of Method. What if we were to add a sentence or two (or more, if you think it proper) to that effect to the second to last paragraph of that section? I think something along the lines of the final paragraph of chapter 2.1 in McDougal and Harrison would be appropriate, as it discusses the difficulty in assuring the validity of the date obtained.

Some additional information about the history and composition of a sample is required before K-Ar isotopic comparison may be used to produce accurate dates.[4] Great care is needed in selecting a sample for dating, as many potential samples have been contaminated by various means. The sample must have remained a closed system since it cooled enough to retain argon, neither admitting nor emitting either of the isotopes of interest. A deficiency of ⁴⁰Ar in a sample of a known age can indicate a full or partial melt in the thermal history of the area. Molten magma can be contaminated by inclusion of older xenolithic material such as chilled deep sea basalts that may not completely outgas preexisting ⁴⁰Ar before cooling. Reliability in the dating of a geological feature is increased by sampling disparate areas which have been subjected to slightly different thermal histories.[McD & H, p.11-12]

Real citations to be included, of course. The second and the final sentences are new, the former subsuming the point from what is now the third sentence that the method cannot be applied willy-nilly to just any sample with the expectation that it will produce an accurate date. - Eldereft (cont.) 20:05, 15 September 2008 (UTC)

Its my impression that geochronologists always try to take great care in securing uncontaminated samples. But the only sure way to discover it if was a contaminated sample is after it has been tested and found not to give the expected results. I don't know of any way to be certain about contamination before hand. I believe that is why McDougal and Harrison make the upfront statement that getting uncontaminated samples is not common. It may be tempting to gloss over problems with the methodology, but I think it is better to be upfront about them, than be accused of cover up. The Ar/Ar method was invented to eliminate the common contamination problem in K/Ar methodology. Christian Skeptic (talk) 21:13, 15 September 2008 (UTC)

So is that a yes, then? - Eldereft (cont.) 22:59, 15 September 2008 (UTC)

OK Christian Skeptic (talk) 12:30, 16 September 2008 (UTC)

Done. - Eldereft (cont.) 20:17, 16 September 2008 (UTC)

complete degassing of Ar invalid statement

Sorry, but that statement is incorrect, it has no referenced source to back it up, and appears to be OR. As is quoted above from McDougall and Harrison; It is assumed "The radiogenic argon measured in a sample was produced by in situ decay of 40K in the interval since the rock crystallized or was recrystallized. Violations of this assumption are not uncommon. " Besides the cases mentioned of quickly chilled obsidians which trapped Ar gas in them, basalts of known age have been misdate by the K/Ar method because not all the gas was eliminated from the magma.

This is a perfect example of why the assumptions required for K/Ar dates to be valid must be made more promenant. They are now hidden in the text where superfical readers and editors will miss them. Christian Skeptic (talk) 03:37, 25 April 2009 (UTC)

Please cease removing sourced and relevant information. Explaining how the technique works should be presented first and prominently, with the limitations following naturally from this explanation. Presenting the other way 'round, as you seem to suggest, has the effect of insinuating that all dates obtained from K-Ar dating are suspect, and may be thrown in the scientific rubbish bin any day now. This is patently untrue. Can we discuss specific suggestions for improving the article here? - Eldereft (cont.) 05:14, 26 April 2009 (UTC)

Precondition

I think it is important to note the difference between accuracy and validity. The measurement of rare earth elements (REEs -- such as radioactive isotopes) has become very accurate. However, accuracy of measurements does not guarantee validity. The preconditions (or assumptions) are concerned about if the method of computing ages from the accurately measured REEs has validity. This difference ought to be made plain in the article. MTDinoHunter (talk) 23:47, 22 October 2009 (UTC)

The assumptions must be met. The following quote comes from the above quote section from the book: "As with all isotopic dating methods, there are a number of assumptions that must be fulfilled for a K-Ar age to relate to events in the geological history of the region being studied." These guys know what they are talking about. Anything less is POV. MTDinoHunter (talk) 02:16, 21 January 2010 (UTC)

Electron capture rate at ultra high pressure

I added the "clarification needed" tag after the statement about electron capture at ultra high pressure. The ultra high pressure doesn't actually affect the dating.This fact must be mentioned in the article.I don't have McDougall's and Harrison's book, but I can't believe they didn't know this.So whoever has the book should make the clarification.Stefan Udrea (talk) 13:45, 13 May 2011 (UTC)

You can read the quote from McDougall and Harrison in a section above. You can see that they don't think high pressure makes much difference. Someone edited the text to make it look like it might. AshforkAZ (talk) 13:38, 14 May 2011 (UTC)

Layman's Summary

I wanted to see if I understood the following sentence:

In these materials, the decay product 40Ar is able to escape the liquid (molten) rock, but starts to accumulate when the rock solidifies (recrystallizes).

Does this mean that, of the pair, we should essentially only find Potassium in molten rock with virtually no Argon? "Able to escape" does not equate "Does escape".

Or does a significant amount or argon remain? We mention "entrained argon" but if a significant amount of argon cannot escape the molten rock, wouldn't that throw off the entire ratio? How can this be used to measure, then? --Bertrc (talk) 00:03, 5 May 2014 (UTC)

With the exception of volcanic glass (obsidian), magmatic rocks are made up of intergrown crystals. When a magmatic rock crystallises, the mineral lattices incorporate many other elements including Potassium but not 40Ar because it is a noble gas and therefore cannot be chemically bound to a crystal lattice. While very small amounts of argon can be included in crystal defects and inclusions, these are generally extremely minor or negligeable compared to the volume of the crystal itself, and if crystals have many defects, you may observe them with a microscope in a thin section. Geochron (talk) 09:21, 3 December 2014 (UTC)

Positron Emission

In the first para of the 'Decay Series' section, 'Positron Emission' is described as 'inverse beta decay' - is this correct? Should it be 'Electron Capture?

Hope this is the correct way to present a correction - it's my first try. Apologies if I've got the protocol wrong!

I've corrected my own error in my original post on Aug 16.

Alan-sci-reader (talk) 22:20, 16 August 2014 (UTC)

_Alan-sci-reader (talk) 11:00, 17 August 2014 (UTC)

It winds up emitting a positron so that is how it gets measured, but electron capture is the more standard term. Thank you for the suggestion. You can also just boldly edit the article yourself. - 2/0 (cont.) 15:34, 17 August 2014 (UTC)

Calcium article need source

Calcium#Isotopes says "While K–Ar dating is used extensively in the geological sciences, the prevalence of ⁴⁰Ca in nature has impeded its use in dating. Techniques using mass spectrometry and a double spike isotope dilution are used for K-Ca age dating." but neither statement seems explained or sourced even here. - Rod57 (talk) 12:23, 25 January 2017 (UTC)