A comparison of 87Rb/87Sr and 40Ar/39Ar dates: Evaluating the problem of excess 40Ar in Himalayan mica

Abstract

The problem of excess 40Ar (ArE) affecting 40Ar/39Ar dates, particularly in biotite, is a long-standing issue in Himalayan geology. The development of in situ Rb-Sr dating presents a convenient alternative method to extract geochronological data from the same material and avoids potential ArE complications. Herein we present a comparison between previously published 40Ar/39Ar data, including many interpreted to reflect ArE, and new in situ Rb-Sr analyses for the same specimens. This work demonstrates that Rb-Sr mica dates from across the exhumed Himalayan metamorphic core typically overlap 40Ar/39Ar dates from the same rocks that are unaffected by ArE. Where 40Ar/39Ar dates have been interpreted to reflect ArE, the Rb-Sr method invariably yields dates that are younger than the ArE date and consistent with other mineral chronometer systems. Finally, while further refinement is required, we propose a technique to extract spot ages for high Rb/low Sr biotite analyses, by correcting for common 87Sr based on the present known ratio of 87Sr/88Sr, removing the need to assume an initial 87Sr/86Sr.