The derivation of ⁴⁰Ar/³⁹Ar age spectra of single grains of hornblende and biotite by laser step‐heating

We present the first examples of 40 Ar/ 39 Ar age spectra for hornblende and biotite derived by the laser step‐heating of individual grains. It is straightforward to use a continuous laser to produce detailed (>20 steps), reproducible and precise age spectra in this way. An infra‐red microscope is used to monitor the sample temperature changes so that a judicious heating schedule can be chosen. Such a system can be readily automated. The biotite spectra shown exhibit large age differences (∼60 m.y.) between co‐existing flakes differing in mass by a factor of ∼70.