Recalibrated legacy 40Ar/39Ar ages for the Upper Jurassic Morrison Formation, Western Interior, U.S.A.

Abstract

As a result of recent updating of decay constants and standard ages used for ⁴⁰Ar/³⁹Ar dating, it is necessary to recalibrate legacy ages obtained with older methods. These recalibrations bring legacy ⁴⁰Ar/³⁹Ar ages into better agreement with ages obtained using ²³⁸U/²⁰⁶Pb dating methods. We present nine recalibrated ⁴⁰Ar/³⁹Ar ages for the Upper Jurassic Morrison Formation of the Western Interior, U.S.A., along with the individual geographic and stratigraphic locations for each sample. These recalibrated ages will be useful for researchers looking to place better age constraints on the flora and fauna of the Morrison Formation, as well as for those working to understand stratigraphic relationships across the formation. The recalibrated ages also can now be used reliably for comparisons with newer ²³⁸U/²⁰⁶Pb ages obtained for the Morrison Formation.