⁴⁰Ar ³⁹Ar thermochronology of isotopically zoned micas: Insights from the southwestern USA proterozoic orogen

We have used three different ⁴⁰Ar ³⁹Ar laser microprobe methods to explore the distribution of radiogenic ⁴⁰Ar in 1.0-1.5-mm biotite crystals from the ca. 1680 Ma Horse Mountain monzogranite of central Arizona. Incremental heating of two single crystals with a defocused laser beam produced flat age spectra with near-plateau ages of ∼1190 Ma, showing no indication of intracrystalline ⁴⁰Ar inhomogeneity. In contrast, total fusion of twenty-five biotite fragments (∼ 100 pm) yielded apparent ages ranging from 1006.7 to 1212.0 Ma. Detailed age mapping in the {001} plane of two crystals, with the laser focused to a minimum spot size, confirms that the age dispersion in the fragment data reflects the existence of 200 m.y. age gradients in single crystals. The two mapped crystals display very different age distribution patterns that suggest radiogenic ⁴⁰Ar loss through two mechanisms: volume diffusion on a scale comparable to that of the grain radius, and more rapid diffusion along discrete zones of high crystal defect density. Simple inverse modeling of the overall age dispersion in the two mapped crystals and the fragment population is consistent with the development of the observed age gradients by slow cooling at an average rate of ∼0.5 K/m.y. The Horse Mountain results, as well as previously published studies, indicate that conventional, incremental heating of hydrous phases can homogenize intracrystalline gradients in ⁴⁰Ar, thereby masking important details of the thermal history of analyzed samples. In contrast, detailed isotopic mapping studies offer a wealth of information, and will become more powerful with continued improvement in the spatial resolution of ⁴⁰Ar ³⁹Ar laser microprobes. Total-fusion studies of crystal fragment populations can be readily automated, making them less labor-intensive than mapping studies. Our preliminary experiment on a limited Horse Mountain fragment population suggests that this procedure has great potential as a reconnaissance tool for thermal history research.