AR-40 AR-39 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 ofte n based on Ar-40/Ar-39 ages obtained through incremental-heating exper iments on minerals with different Ar retentivities. Laser incremental heating of single crystals of muscovite and biotite from an approximat ely 1700 Ma monzogranite from a low-pressure- high-temperature metamor phic terrain in central Arizona suggested an essentially uniform distr ibution of radiogenic Ar-40 in each crystal and yielded ages of 1412 /- 5 Ma for the muscovite and 1410 +/- 10 Ma for the biotite. The simi larity in these ages implies rapid (approximately 25 K/m.y.) cooling o f the monzogranite after a reheating episode slightly before 1412 Ma. In contrast, detailed age mapping of single crystals from the same sam ples with the laser microprobe revealed large radiogenic Ar-40 gradien ts indicative of very slow (< 1 K/my.) cooling and a closure interval for Ar diffusion lasting > 400 m.y. Evidence for such low cooling rate s has important implications for the longevity of high thermal gradien ts in the Early-Middle Proterozoic lithosphere. In addition, inconsist ency between the laser mapping and incremental-heating results for the se micas suggests caution when interpreting incremental-heating data f or slowly cooled samples.