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Author Topic:   The isochron method has been questioned?
Lili
Junior Member (Idle past 5960 days)
Posts: 11
Joined: 06-24-2007


Message 1 of 2 (436291)
11-24-2007 11:06 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.
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.

Replies to this message:
 Message 2 by Adminnemooseus, posted 11-24-2007 11:20 PM Lili has not replied

Adminnemooseus
Administrator
Posts: 3974
Joined: 09-26-2002


Message 2 of 2 (436293)
11-24-2007 11:20 PM
Reply to: Message 1 by Lili
11-24-2007 11:06 PM


Duplicate PNT - This version rejected and closed
The other (second and better formatted) version is here.
Adminnemooseus

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This message is a reply to:
 Message 1 by Lili, posted 11-24-2007 11:06 PM Lili has not replied

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