⁴⁰Ar/³⁹Ar age gradients in micas from a high-temperature-low-pressure metamorphic terrain: Evidence for very slow cooling and implications for the interpretation of age spectra

Estimates of the time-temperature history of geologic samples are often based on ⁴⁰Ar/³⁹Ar ages obtained through incremental-heating experiments on minerals with different Ar retentivities. Laser incre mental heating of single crystals of muscovite and biotite from an ∼1700 Ma monzogranite from a low-pressure- high-temperature metamorphic terrain in central Arizona suggested an essentially uniform distribution of radiogenic ⁴⁰Ar in each crystal and yielded ages of 1412 ±5 Ma for the muscovite and 1410 ±10 Ma for the biotite. The similarity in these ages implies rapid (∼25 K/m.y.) cooling of the monzogranite after a reheating episode slightly before 1412 Ma. In contrast, detailed age mapping of single crystals from the same samples with the laser microprobe revealed large radiogenic ⁴⁰Ar gradients indicative of very slow (<1 K/m.y.) cooling and a closure interval for Ar diffusion lasting >400 m.y. Evidence for such low cooling rates has important implications for the longevity of high thermal gradients in the Early-Middle Proterozoic lithosphere. In addition, inconsistency between the laser mapping and incremental-heating results for these micas suggests caution when interpreting incremental-heating data for slowly cooled samples.