⁴⁰ Ar/ ³⁹ Ar 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 ⁴⁰ Ar/ ³⁹ Ar ages obtained through incremental-heating experiments on minerals with different Ar retentivities. Laser incremental 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 ⁴⁰ Ar 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 ⁴⁰ Ar gradients indicative of very slow (<1 K/m.y.) cooling and a closure interval for Ar diffusion lasting >400 m.y. Inconsistency between the laser mapping and incremental-heating results for these micas suggests caution when interpreting incremental-heating data for slowly cooled samples. -from Authors