Evidence for initial36Ar in volcanic rocks, and some implications

doi:10.1016/0012-821X(75)90006-0

Earth and Planetary Science Letters

Volume 26, Issue 3, July 1975, Pages 312-318

https://doi.org/10.1016/0012-821X(75)90006-0 Get rights and content

Abstract

This paper extends the evidence that volcanic rocks may contain initial argon, and investigates some of the implications of the results.

The incorporation of initial argon with a composition different to that of air at the time of crystallisation may be an important cause of scatter of K-Ar dates in areas such as the British Tertiary igneous province, but in favourable circumstances the correct age may be recovered.

An age of about 60 Ma is indicated for the lavas of Northern Ireland.

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The linear disposition of data in ⁴⁰Ar-⁴⁰K correlation diagrams can be accounted for using a single-stage concomitant K-Ar exchange model. Such a model avoids the implausible mechanism of “equal Ar loss” in accounting for a negative intercept on the ⁴⁰Ar axis, and demonstrates that no information of geological significance relates directly to the gradient of the correlation line. Bounds to the “primary formation age” and “age of exchange event” may be established using extreme values of the observed K content and using estimates of its value based on petrographic type. Application of the model method to discordant age data from an upper Cretaceous/lower Tertiary rhyolite—dacite—pitchstone sequence from well 2094-1A drilled on the Erlend Complex (61° 55′N, 00° 18′W) on the SE margin of Faeroe-Shetland Channel yields a Palaeocene formation age perturbed in the Neogene by an event which may relate to seawater penetration following regional subsidence. This apparent Palaeocene K-Ar formation age may be real, or may be a consequence of thermal overprinting of an actual Cretaceous age by the overlying lower Tertiary basalts.
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Evidence for initial36Ar in volcanic rocks, and some implications

Abstract

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Geochim. Cosmochim. Acta

On the basic assumptions in K-Ar dating method

Geophys.

A redetermination of the relative abundances of the isotopes of carbon, nitrogen, oxygen, argon and potassium

Phys. Rev.

Evidence for initial³⁶Ar in volcanic rocks, and some implications