Mathematics – Logic
Scientific paper
Sep 1974
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1974gecoa..38.1341s&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 38, Issue 9, pp.1341-1358
Mathematics
Logic
5
Scientific paper
The K---Ar isochron method is an attempt to obtain the initial age or reset age and the extraneous argon isotopic ratio of a suite of cogenetic samples of different K-contents. Some samples contain excess argon-40 and others lose argon-40. The resultant ages on single samples are not significant geologically. We have discussed the principles of three types of isochrons commonly used, the 40 Ar vs 40 K isochron, the 40 Ar/ 36 Ar vs 40 K/ 30 Ar isochron, and the 40 Ar/ 36 Ar vs 39 Ar/ 36 Ar isochron, and evaluated the first two types for ten different hypothetical cases. If a straight line is obtained in the 40 Ar vs 40 K isochron, a positive intercept indicates an approximately constant amount of excess argon, whereas a negative intercept indicates argon loss. A curved line or scattering of points indicates that the basic assumptions are not valid for the set of samples under consideration. The 40 Ar/ 36 Ar vs 40 K/ 36 Ar isochron method is valid, rigorously, only when all samples of the system under consideration have the same non-radiogenic argon isotope composition. This requires that either no excess argon is present in the system, or else each analysis contains the same proportion of excess and atmospheric argon. If these conditions do not hold, approximately, invalid ages and invalid intercepts are obtained. Any K---Ar isochron needs to be used with caution.
Damon Paul E.
Shafiqullah Mohammed
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