RAPID STEPPED-HEATING EXPERIMENTAL METHOD FOR ROUTINE MONAZITE (U-TH-SM)/HE THERMOCHRONOLOGY

Monazite is a common accessory phase that shares many of the characteristics of zircon and apatite but is a LREE phosphate with ~1-15 wt.% Th that exhibits significant compositional variability. Previous experimental work evaluating this phase as a (U-Th-Sm)/He (He) thermochronometer yielded a large range of He diffusion parameters, and thereby effective closure temperatures (~180–290 ˚C for 10˚C/Myr cooling rate), which is hypothesized to arise from single-crystal variability in Th and LREE concentrations. Although this chronometer could fill a temperature gap between common He thermochronometers and low-temperature ⁴⁰Ar/³⁹Ar methods, such variable diffusion kinetics makes routine application of this thermochronometer challenging without performing diffusion experiments on each crystal analyzed. Here we report how the high Th content (and therefore high ⁴He content) in monazite, as well as advances in vacuum line automation, allow us to use step-heating schedules that take <24 h to measure the kinetics of He diffusion in individual monazite crystals. We use this approach to exploit and explore the system’s known kinetic variability. To compare our method to previous work, we present a series of step-heating experiments on monazite crystals collected from the 554 Standard location in the Wilderness intrusive suite, Santa Catalina Mountains, Arizona. Monotonic step-heating experiments reveal a range of diffusion parameters consistent with previously published studies. Cycled step-heating experiments, in contrast, yield a much narrower range of diffusion parameters on the retrograde heating path—a result consistent with similar experiments on He diffusion in natural zircon. The monazite retrograde heating steps typically yielded effective closure temperature for monazite of ~203 ± 20 ˚C. These results suggest that, in some cases, monazite diffusion kinetics from representative crystals may be useful for interpreting monazite He ages derived from faster single-step bulk He degassing (i.e., “routine” He dating). Not all crystals analyzed, however, conformed to simple Arrhenian behavior. Ongoing work uses natural monazite crystals from multiple localities and of a variety of ages to explore how composition and self-irradiation control the diffusivity of He in monazite.