Precise K-Ar, 40Ar/39Ar, Rb-Sr and U/Pb mineral ages from the 27.5 Ma Fish Canyon Tuff reference standard

The accuracy of ages measured using the 40Ar/39Ar technique is affected by uncertainties in the age of radiation fluence-monitor minerals. At present, there is lack of agreement about the ages of certain minerals used as fluence monitors. The accuracy of the age of a standard may be improved if the age can be measured using different decay schemes. This has been done by measuring ages on minerals from the Oligocene Fish Canyon Tuff (FCT) using the K-Ar, 40Ar/39Ar. Rb-Sr and U/Pb methods. K-Ar and 40Ar/39Ar total fusion ages of sanidine, biotite and hornblende yielded a mean age of 27.57 +/- 0.36 Ma. The weighted mean 40Ar/39Ar plateau age of sanidine and biotite is 27.57 +/- 0.18 Ma. A biotite-feldspar Rb-Sr isochron yielded an age of 27.44 +/- 0.16 Ma. The U-Pb data for zircon are complex because of the presence of Precambrian zircons and inheritance of radiogenic Pb. Zircons with Pb-207/U-235 < 0.4 yielded a discordia line with a lower concordia intercept of 27.52 +/- 0.09 Ma. Evaluation of the combined data suggests that the best age for FCT is 27.51 Ma. Published by Elsevier Science B.V.