The 36Ar content of a 50 m thick lava flow shows minima at a depth of around 37 m, as well as at the bottom chilled margin, suggesting that most of the atmospheric argon entered the solidifying lava flow from the air above it. Measurement of the 36Ar/38Ar ratios indicates that little or no isotopic fractionation occurred during this process. The K–Ar 'isochron' age of the S.P. Flow is about 70 000 years.

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