Abstract

A brief history of the development of the potassium-argon method of geochronometry is presented and the evidence for the decay scheme is reviewed. A survey of the experimental measurements of the β and γ activity of K40 indicates that there is fairly general agreement on a λγ/λβ = 0.12.

The isotope dilution technique and equipment employed at the Lamont Geological Observatory for the argon analysis are described. The A40/K40 ratio can be determined with an accuracy of better than ±5 per cent. Radioactive A37 has been used as a tracer to check the yield of the liberation and purification procedure. These experiments indicate that within an experimental error of 2 per cent, all the argon present in a sample is released during fusion; that spike equilibration is achieved, and that no argon is lost during purification.

The problem of argon retentivity in various minerals is discussed. For minerals of known age the fraction of argon retained can be calculated. Micas retain about 80–100 per cent of their argon, whereas feldspars vary in retentivity from 40–85 per cent for the samples listed. Much more data are needed to evaluate the possible causes of argon loss.

In the Southern Appalachians K-A ages have been determined for the pegmatites and several host rocks. The micas of the Spruce Pine District pegmatites yield ages of 340 ± 15 m.y. in excellent agreement with U-Pb ages for the same pegmatites (350 ± 20 m.y.). The host rocks give uncorrected ages of about 400 m.y. which when corrected for the argon loss from the feldspars increase to at least 600 m.y. This establishes the primary regional metamorphism which produced the Cranberry, Carolina, and Roan gneisses and the Beech granite as being Precambrian rather than Paleozoic.

In the Bear Mountain area of New York State the K-A ages corrected for retentivity are about 800 m.y. for the Storm King granite and about 1400 m.y. for the Canada Hill gneiss. This proves that the Storm King could not be Paleozoic and suggests that the New Jersey-New York Highlands are older than the Blue Ridge Province of the Southern Appalachians. The spread in ages is interpreted as evidence for the igneous origin of the Storm King granite.

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