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Author Topic:   The isochron method has been questioned?
Lili
Junior Member (Idle past 5959 days)
Posts: 11
Joined: 06-24-2007


Message 1 of 7 (436288)
11-24-2007 10:56 PM


Hi, I found this old article at Answers In Genesis, which claims that the isochron dating method has been questioned. Since it's coming from AIG, I'm skeptical of the claims and I wanted someone who knows more about isochron dating to give their opinion of the article.
quote:
Isochron dating questioned
However, it is this isochron dating method that has recently come ”under fire’. Writing in the international journal Chemical Geology,2 Y.F. Zheng of the Geochemical Institute at the University of Gottingen in Germany says:
”The Rb-Sr isochron method has been one of the most important approaches in isotopic geochronology. But some of the basic assumptions of the method are being questioned at the present time. As first developed the method assumed a system to have: (1) the same age; (2) the same initial 87Sr/86Sr ratio; and (3) acted as a closed system. Meanwhile, the goodness of fit of experimental data points in a plot of 87Sr/86Sr vs. 87Rb/86Sr served as a check of these assumptions. However, as the method was gradually applied to a large range of geological problems, it soon became apparent that a linear relationship between 87Sr/86Sr and 87Rb/86Sr ratios could sometimes yield an anomalous isochron which had no distinct geological meaning. A number of anomalous isochrons have been reported in the literature and various terms have been invented, such as apparent isochron (Baadsgaard et al., 1976), mantle isochron and pseudoisochron (Brooks et al., 1976a, b), secondary isochron (Field and Ra- Heim, 1980). inherited isochron (Roddick and Compston, 1977), source isochron (Compston and Chappell, 1979), erupted isochron (Betton, 1979; Munksgaard, 1984), mixing line (Bell and Powell, 1969; Faure, 1977; Christoph, 1986) and mixing isochron (Zheng, 1986; Qin, 1988). Even a suite of samples which do not have identical ages and initial 87Sr/86Sr ratios can be fitted to isochrons, such as aerial isochrons (Kohler and Muller-Sohnius, 1980; Haack et al., 1982).’3
He went on to say:
”Evidently, the theoretical basis of the classical Rb-Sr isochron is being challenged and some limitations of its basic assumptions are being revealed . . Some of what this paper contains is not new to isotopic geochronologists, but it is drawn together here for the first time and is placed in a context within unifying general models for Rb-Sr dating.’4
However, Zheng’s paper really isn’t the first time that these problems with the isochron dating method have been comprehensively highlighted and treated mathematically. It was in fact creation scientists who first comprehensively pointed to the problems with the isochron dating method. In a series of short articles published in the Bible-Science Newsletter in 1981, Dr. Russell Arndts, Professor of Chemistry at St Cloud State University in Minnesota, and Dr. William Overn, a former engineer and physicist with the National Aeronautics and Space Administration (NASA), showed how isochrons were in fact often a result of the mixing of the radioisotopes from different sources.’5 They also illustrated this with various examples from the geological literature. They concluded:
”It is clear that mixing of pre-existent materials will yield a linear array of isotopic ratios. We need not assume that the isotopes, assumed to be daughter isotopes, were in fact produced in the rock by radioactive decay. Thus the assumption of immense ages has not been proven. The straight lines, which seem to make radiometric data meaningful, are easily assumed to be the result of simple mixing.’(their emphasis)6
They go on to suggest that the concept of mixing a material from wide ranges seems to suggest that the earth has undergone widespread stirring. Such processes do not of course always involve the actual physical movement of rock, rock-forming components such as mineral grains, or molten materials, but more often involve the mixing of chemical components via fluxes of fluids, principally water, through the rocks. Zheng concurs with this in his paper when he speaks of geological processes such as hydrothermal (hot water) alteration, metasomatism, and metamorphism, the latter two involving changes in rocks due to fluids, temperature, and pressure. Zheng admits:
”In some cases, gain or loss of Rb and Sr from the rocks is so regular that a linear array can be produced on the conventional isochron diagram and a biased isochron results from the altered rocks to give spurious age and initial 87Sr/86Sr ratio estimates.’7
At the end of his paper, Zheng wrote:
”In conclusion, some of the basic assumptions of the conventional Rb-Sr isochron method have to be modified and an observed isochron does not certainly define a valid age information for a geological system, even if a goodness of fit of the experimental data points is obtained in plotting 87Sr/86Sr vs. 87Rb/86Sr. This problem cannot be overlooked, especially in evaluating the numerical time scale. Similar questions can also arise in applying Sm-Nd and U-Pb isochron methods.’8
And as if to make the point even more succinctly and clearly, Zheng also wrote in the abstract (or summary) of his paper:
”As it is impossible to distinguish a valid isochron from an apparent isochron in the light of Rb-Sr isotopic data alone, caution must be taken in explaining the Rb-Sr isochron age of any geological system.’9
One could hardly expect a more emphatic and complete ”demolition job’ on the isochron dating method than that! Notice also that Zheng extends his criticism to the traditional uranium-lead (UPb) and currently-in-vogue samarium-neodymium (Sm-Nd) isochron methods.
Edited by Adminnemooseus, : Added some blank lines in quote section.

Replies to this message:
 Message 3 by kongstad, posted 11-25-2007 3:36 AM Lili has replied

  
AdminNosy
Administrator
Posts: 4754
From: Vancouver, BC, Canada
Joined: 11-11-2003


Message 2 of 7 (436310)
11-25-2007 1:42 AM


Thread moved here from the Proposed New Topics forum.

  
kongstad
Member (Idle past 2870 days)
Posts: 175
From: Copenhagen, Denmark
Joined: 02-24-2004


Message 3 of 7 (436320)
11-25-2007 3:36 AM
Reply to: Message 1 by Lili
11-24-2007 10:56 PM


Talkorigins has the story
If you look at talkorigins, the article in question is discussed:
Chris Stassen, Talkorigins writes:
Lately it seems that some creationists have latched onto Zheng (1989), and reference this paper as if it disproved isochron dating and made room for a young Earth. The paper is a discussion of potential problems of Rb/Sr isochron dating, with examples of instances where these problems are known to have occurred.
However, the paper is not terribly helpful to the young-Earth cause. Zheng discusses four ways in which an incorrect isochron could result:
1. Protracted fractional crystallization
Requires a slow cooling period on order of ten million years, which is not possible on a young Earth. Also, the effect is very slight: in the only example which Zheng produces (first entry in Table II on p. 14), the "incorrect" age (437 10 Ma) is not very different from the actual age (415 10 Ma).
2. Inherited (for example, by partial melting)
Discussed previously; requires special circumstances and almost always induces a fair amount of scatter in the isochron plot. Requires ancient source material (the "inherited" age matches the age of the source), which is not available on a young Earth.
3. Mixing isochron
Discussed previously; in most cases detected by the mixing plot test.
4. Apparent isochron by metamorphism
Discussed previously; requires special circumstances and results in an age in between original time of crystallization and the metamorphic event that partially reset the isochron. Requires ancient source material, which is not available on a young Earth.
While each of these processes can be invoked to explain a few confusing or conflicting dating results, none could reasonably be expected to account for all (or even most) isochron dating results which are incompatible with a young Earth.
Isochron Dating
To this I will add some comments.
The purpose of AIGs criticisms of agemeasuring methods is to cast doubt on whether the earth is old.
Zheng discusses som ways that the isochron dates may be wrong. Now if you look at the four methods of producing confusing results, three of them require an old age for the earth.
Method on requires cooling over millions of years, this is incompatible with a 6.000 year old earth.
The second is inheriting age from other materials, but the other materials can only have that age if they themselves are old.
The fourth describes resetting the isochron, which would show a younger age than the real date.
Only the third method would give older ages without reuiring the earth to be old, but as is said, this will most often be detacted by other means.
/Soren

This message is a reply to:
 Message 1 by Lili, posted 11-24-2007 10:56 PM Lili has replied

Replies to this message:
 Message 4 by Lili, posted 12-02-2007 5:16 PM kongstad has replied

  
Lili
Junior Member (Idle past 5959 days)
Posts: 11
Joined: 06-24-2007


Message 4 of 7 (438073)
12-02-2007 5:16 PM
Reply to: Message 3 by kongstad
11-25-2007 3:36 AM


Re: Talkorigins has the story
Yes, Zheng's article cannot be used to support a young earth. However, based on the excerpts from his paper, the isochron method seems to be quite unreliable.

This message is a reply to:
 Message 3 by kongstad, posted 11-25-2007 3:36 AM kongstad has replied

Replies to this message:
 Message 5 by Chiroptera, posted 12-02-2007 5:18 PM Lili has not replied
 Message 6 by Percy, posted 12-02-2007 9:48 PM Lili has not replied
 Message 7 by kongstad, posted 12-03-2007 9:48 AM Lili has not replied

  
Chiroptera
Inactive Member


Message 5 of 7 (438074)
12-02-2007 5:18 PM
Reply to: Message 4 by Lili
12-02-2007 5:16 PM


Re: Talkorigins has the story
However, based on the excerpts from his paper, the isochron method seems to be quite unreliable.
Sure, that is why dishonest creationists quote-mine excerpts. A true understanding of the paper and the science in general shows that the isochron method can be trusted.

Progress in human affairs has come mainly through the bold readiness of human beings not to confine themselves to seeking piecemeal improvements in the way things are done, but to present fundamental challenges in the name of reason to the current way of doing things and to the avowed or hidden assumptions on which it rests. -- E. H. Carr

This message is a reply to:
 Message 4 by Lili, posted 12-02-2007 5:16 PM Lili has not replied

  
Percy
Member
Posts: 22392
From: New Hampshire
Joined: 12-23-2000
Member Rating: 5.2


Message 6 of 7 (438126)
12-02-2007 9:48 PM
Reply to: Message 4 by Lili
12-02-2007 5:16 PM


Re: Talkorigins has the story
Hi Lili,
I'd like to echo what Chiroptera just said, but by focusing on just one part of your post:
Lili writes:
However, based on the excerpts from his paper...
Exactly. Based upon what this creationist chose to quote from the paper, you've concluded it argues that isochron dating is unreliable. The paper does no such thing.
For me, one of the biggest mysteries of creationists is their extensive use of misrepresentation. Quoting out of context to make something seem supportive of creationism when it absolutely isn't is one of the most common tools in the creationist toolbox. Presumably creationists sincerely believe in all the positive qualities expressed in the Bible, especially honesty, yet to battle evolution they will pause not an instant to mischaracterize what scientists say.
To listen to creationists quoting scientists and scientific papers you'd think there must be a huge number of geologists who reject radiometric dating, of cosmologists who reject the Big Bang, and of biologists who reject evolution, but it just isn't so. When you consider that creationists have even quoted what they purport to be anti-evolutionary statements from as strident an anti-creationist as Stephen Jay Gould then it becomes obvious that their characterizations just can't be trusted.
Doesn't seem like the way to earn a ticket through the pearly gates.
--Percy

This message is a reply to:
 Message 4 by Lili, posted 12-02-2007 5:16 PM Lili has not replied

  
kongstad
Member (Idle past 2870 days)
Posts: 175
From: Copenhagen, Denmark
Joined: 02-24-2004


Message 7 of 7 (438182)
12-03-2007 9:48 AM
Reply to: Message 4 by Lili
12-02-2007 5:16 PM


Dating methods aren't like ATMs
In a way you are right. Isochron methods aren't reliable.
That is, even when using Isochron methods of dating you cannot always be sure on the date, unless you try to evaluate your results.
What the referenced paper does is it helps increase the reliability of the Isochron dating methods, by identifying some instances where it could fail.
The thing is, measuring age is not like taking cash out of an ATM machine. You will always have to consider a lot of parameters before you can reach an age, and an estimate of how precise your result is.
Some of the potential error sources in the article can be tested for by analysis on the isochron, and some of them can be tested for age by using other age determining methods.
So yes, isochron dating is unreliable, but more reliable than C14, and in many cases it is very reliable. Determining cases where it is less reliable, does not make other cases less reliable.

This message is a reply to:
 Message 4 by Lili, posted 12-02-2007 5:16 PM Lili has not replied

  
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