Paper No. 6
Presentation Time: 9:25 AM
LASER HEATING OF MONAZITE TO SIMULTANEOUSLY DETERMINE HE DIFFUSION PARAMETERS AND (U-TH)/HE AGES
PETERMAN, Emily M., Department of Geological and Environmental Sciences, Stanford University, 450 Serra Mall, Bldg. 320, Stanford, CA 94305, HOURIGAN, Jeremy, Earth and Planetary Sciences, University California Santa Cruz, Santa Cruz, CA 94305 and GROVE, Marty, Department of Geological Sciences, Stanford University, Stanford, CA 94305, emily.peterman@stanford.edu
Previous studies have shown that the diffusivity of He in monazite is a function of both grain size and composition. While grain dimensions are easily assessed, the composition of natural monazite is often highly variable. Accurate determination of the He diffusion properties of monazite for (U-Th)/He thermochronology therefore requires diffusion data to be gathered for individual samples. Although this should occur while He concentrations are measured by isotope dilution, past He diffusion experiments have generally used either resistance heating or incandescent light that required dedicated single sample experiments. We have recently developed a diffusion cell that uses a diode laser with a thermocouple feedback loop that can be set up to run multiple samples positioned within the same sample chamber. After He analysis, samples are extracted for U-Th + REE analysis to yield a (U-Th)/He age. Because the system is configured to measure the Arrhenius parameters and the (U-Th)/He age of each monazite, it provides an experimentally determined closure temperature estimate for each age measurement.
Our experiments demonstrate that the laser is highly responsive between 50-1200ºC. Under typical conditions, temperature can be maintained with better than ± 0.1ºC precision, and it invariably achieves temperature stabilization (defined as ± 0.5ºC) within 5-10 seconds. The fast response is afforded by the PID controlled system, which can yield ramp-up rates of 100ºC/second; we conservatively limit the rate to 25ºC/second to minimize setpoint overshoot (currently <2ºC). Furthermore, simultaneous imaging with a CCD camera permits adjustment of the sample position in-run to maintain optimal heating conditions. Preliminary measurements of monazite reference material ‘554’ indicated E = 40.66 ± 1.38, which is consistent with previous estimates. Work is currently in progress with reference materials that represent the range of compositional variability in monazite; these measurements will greatly enhance our understanding of He diffusion in natural monazite.
