Replies

"Even a suite of samples which do not have identical ages and initial 87Sr/86Sr ratios can be fitted to isochrons, such as areal isochrons. [p. 1] ...The theoretical basis of the classical Rb-Sr isochron is being challenged and some limitations of its basic assumptions are being revealed. [p. 2] 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." [from Abstract, p. 1]" [Y.F. Zheng, "Influences of the Nature of the Initial Rb-Sr System on Isochron Validity," Chemical Geology, Isotope Geoscience Section, Vol. 80, No. 1 (December 20, 1989), pp. 1-16 (emphasis added).]

Zheng's paper

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:

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).

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.

Mixing isochron Discussed previously; in most cases detected by the mixing plot test.

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.