Abstract

One of the fundamental assumptions of conventional potassium-argon dating is that the rock or mineral being dated was initially devoid of argon, i.e. that the argon sample prepared for analysis consists of radiogenic argon-40 accumulated during the life of the rock or mineral and a variable amount of modern atmospheric contamination alone. As the isotopic abundance ratio of argon in the atmosphere is known, it follows from this assumption that a measurement of the respective volumes of the isotopes argon-36 and argon-40 in the gas sample will enable the degree of atmospheric contamination to be established. Knowing this, the radiogenic component of the measured argon-40 volume can then be calculated. In young and/or low potassium-content samples atmospheric contamination becomes the major part of the gas sample and precise dating of such materials requires extremely accurate and precise measurements of the necessary atmospheric correction. Only mass spectrometers with very high stability and freedom from mass discrimination (such as the omegatron) are suitable for such work. Additionally, the assumption that initial argon is absent is probably never strictly true. The presence of even quite small quantities of initial argon invalidates the conventional atmospheric correction and can make the apparent ages obtained from young and especially from low potassium-content rocks drastically discrepant.

Initial argon may be present in young volcanic rocks and minerals in either or both of two ways:—

(i) inherited: e.g. from only partially outgassed (i.e. partially overprinted) xenoliths, xenocrysts or other non-juvenile components, or

(ii) incorporated: i.e. occluded or otherwise

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