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Absolute dating techniques

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Caltech Authors


In their report on the ages of crystalline rocks from the Apollo 14 mission, Husain, Sutter, and Schaeffer present some important results using the ^(40)Ar-^(39)Ar method. This technique, which has been used by several other workers, depends critically on the use of mineral or rock standards of precisely known ^(40)Ar/K ratio. The "age" of the standard is not directly relevant. What is required is the 40Ar/K in the standard. These standards should be adequately documented somewhere in the literature. Often the point is missed that the ^(40)Ar-^(39)Ar age is not absolute, but is relative to ^(40)Ar/K of the standard sample. The uncertainty of absolute ages determined by this method must include any uncertainty in the ^(40)Ar/K ratio of the comparison sample. At the present stage of development of argon-potassium dating in particular, and geochronology in general, it is rather surprising to find that workers continue to determine ages on lunar samples, using "standards" (terrestrial or otherwise) which are themselves uncertain to several percent. In the work by Husain et al. it would appear that the actual uncertainty in age due to both analytical error and the error in the hornblende monitor is 3.77 ± 0.15 ± 0.15 eons, or 3.77 ± ~0.30 eons. Analytical techniques which have been available for some years easily permit more definitive measurements to be made, particularly on valuable lunar materials. Since the time interval over which lunar igneous activity is presently observed to occur is rather restricted (4.00 to 3.20 eons from current data), the necessity for adequately precise data is apparent.

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