Flat plateau and impossible isochrons: Apparent Ar-40-Ar-39 geochronology in a high-pressure terrain

Until recently most ages determined by the Ar-40-Ar-39 technique have been represented by the age spectrum and isochron. Early in the history of the t echnique a series of criteria were outlined in order to discriminate betwee n spectra which were considered to reflect disturbed isotopic systems and s pectra which were considered to reflect the accurate age of the rock or min eral. With the advancement of Ar-40-Ar-39 instrumentation and the introduct ion of laser-ablation techniques, it is now possible to obtain an age from single grains and on a sub-grain scale. In this case there is no production of an age spectra or determination of a plateau age; however, isochrons ar e still readily produced and used in the discussion of Ar isotope ratios, a nd specifically excess argon. Here, new data illustrate the possible drawba ck first of the age spectrum and plateau age in the absence of an independe nt geochronological constraint in high-pressure terranes, and second, of th e accompanying isochron, which we demonstrate can be both incorrect and geo logically meaningless. (C) 1999 Elsevier Science Ltd.