Precise K-Ar, (super 40) Ar/ (super 39) Ar, Rb-Sr and U/Pb mineralages from the 27.5 Ma Fish Canyon Tuff reference standard

Abstract

The accuracy of ages measured using the (super 40) Ar/ (super 39) Artechnique is affected by uncertainties in the age of radiationfluence-monitor minerals. At present, there is lack of agreement aboutthe ages of certain minerals used as fluence monitors. The accuracy ofthe age of a standard may be improved if the age can be measured usingdifferent decay schemes. This has been done by measuring ages onminerals from the Oligocene Fish Canyon Tuff (FCT) using the K-Ar,(super 40) Ar/ (super 39) Ar, Rb-Sr and U/Pb methods. K-Ar and (super40) Ar/ (super 39) Ar total fusion ages of sanidine, biotite andhornblende yielded a mean age of 27.57+ or -0.36 Ma. The weighted mean(super 40) Ar/ (super 39) Ar plateau age of sanidine and biotite is27.57+ or -0.18 Ma. A biotite-feldspar Rb-Sr isochron yielded an ageof 27.44+ or -0.16 Ma. The U-Pb data for zircon are complex because ofthe presence of Precambrian zircons and inheritance of radiogenic Pb.Zircons with (super 207) Pb/ (super 235) U<0.4 yielded a discordialine with a lower concordia intercept of 27.52+ or -0.09 Ma.Evaluation of the combined data suggests that the best age for FCT is27.51 Ma.

22 samples from this reference