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A test of the 40Ar/39Ar age spectrum technique on some terrestrial materials

January 1, 1971

40Ar/39Ar age spectra were determined for 10 terrestrial rock and mineral samples whose geologic history is known from independent evidence. The spectra for six mineral and whole rock samples, including biotite, feldspar, hornblende, muscovite, and granodiorite, that have experienced post-crystallization heating did not reveal the age of crystallization in any obvious way. Minima in the spectra, however, give reasonable maximum ages for reheating and high-temperature maxima can be interpreted as minimum crystallization ages. High-temperature ages of microcline and albite that have not been reheated are approximately 10% younger than the known crystallization age. Apparently there are no domains in these feldspars that have retained radiogenic40Ar quantitatively. Spectra from two diabase samples that contain significant quantities of excess argon might mistakenly be interpreted as spectra from reheated samples and do not give the age of emplacement. The40Ar/39Ar age spectrum technique may be a potentially valuable tool for the study of geologic areas with complex histories, but the interpretation of age spectra from terrestrial samples seems to be more difficult than suggested by some previous studies.

    Publication Year 1971
    Title A test of the 40Ar/39Ar age spectrum technique on some terrestrial materials
    DOI 10.1016/0012-821X(71)90020-3
    Authors M. A. Lanphere, G. Brent Dalrymple
    Publication Type Article
    Publication Subtype Journal Article
    Series Title Earth and Planetary Science Letters
    Index ID 70010332
    Record Source USGS Publications Warehouse