Talk:K–Ar dating

message: 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)
thread_title:  Lower and upper limits of K-Ar method? 

message: 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.McDougall I. and Harrison, T.M., 1988, "Geochronology and Thermochronology by the 40Ar/39Ar Method", Oxford Monographs on Geology and Geophysics No. 9., pp. 11, 12. This book explains the ins and outs of the 40Ar/39Ar method of geochronology written by those intimately involved in its development.
"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)
replies: text: 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)
replies: text: 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)

text: 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)
replies: text:  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)

text: 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)

text: 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)
thread_title: Assumptions

message: 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.McDougall I. and Harrison, T.M., 1988, "Geochronology and Thermochronology by the 40Ar/39Ar Method", Oxford Monographs on Geology and Geophysics No. 9., pp. 11, 12. This book explains the ins and outs of the 40Ar/39Ar method of geochronology written by those intimately involved in its development.
"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)
thread_title: Assumptions - quote

message: 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. McDougall I. and Harrison, T.M., 1988, "Geochronology and Thermochronology by the 40Ar/39Ar Method", Oxford Monographs on Geology and Geophysics No. 9., pp. 11, 12. This book explains the ins and outs of the 40Ar/39Ar method of geochronology written by those intimately involved in its development.
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)
replies: text: 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)
replies: text: 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)
thread_title: Assumptions - modified

message: 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.McDougall I. and Harrison, T.M., 1988, "Geochronology and Thermochronology by the 40Ar/39Ar Method", Oxford Monographs on Geology and Geophysics No. 9., pp. 11, 12. This book explains the ins and outs of the 40Ar/39Ar method of geochronology written by those intimately involved in its development.
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)
replies: text: 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)
replies: text:  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)

text: 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)
thread_title: Assumptions (further summarized)

message: 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)
replies: text: 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)
thread_title: Assumptions section added back in

message: 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)
replies: text: 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)
thread_title: complete degassing of Ar invalid statement

message: 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)
replies: text: 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)
thread_title:  Precondition 

message: 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)
replies: text: 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)
thread_title:  Electron capture rate at ultra high pressure 

message: 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)
thread_title:  Layman's Summary 

message: 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)
replies: text: 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)
thread_title:  Positron Emission 

message: 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)
thread_title:  Calcium article need source