Extending the Temperature Range of (U-Th)/ He Thermochronology

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HodgesEAR-0087382One way to explore the thermal structure of the Earth's crust over geologic timescales is by the judicious application of well-calibrated isotopic thermochronometers. Because the thermal structure of the upper crust reflects surface topography, low-temperature thermochronology also provides a powerful tool for studying paleotopography, and thus landscape evolution in mountainous terrains. A recent explosion in the application of the (U-Th)/He apatite thermochronometer, which has a nominal closure temperature of ~70 C, has catalyzed a new era of research that integrates tectonic geomorphology with isotope geochemistry. However, high-fidelity reconstruction of topographic evolution over geologic timescales requires a more complete understanding of time-temperature histories than is currently provided by existing thermochronometers. In particular, we have relatively few ways of recovering thermal histories over the 200-300 degrees C interval. This proposal presents a program of laboratory investigations aimed at evaluating two new thermochronometers that may improve the situation: (U-Th)/He monazite and xenotime. The rare-earth phosphates monazite and xenotime typically contain high concentrations of U and Th and thus have the potential of accumulating substantial radiogenic 4He over short timescales (104-105 yr). Calculations of nominal (U-Th)/He closure temperatures for these minerals using the ionic porosity model for rare-gas diffusion in minerals suggests that these systems may be useful thermochronometers over the ~200-350 degrees C temperature range. We are fortunate to have extensive collections of both monazite and xenotime at MIT as a consequence of a vigorous research program in U-Pb geochronology, and thus we are able to propose a series of basic 4He diffusion experiments that would be conducted on naturally occurring samples representing a broad range of compositions. Some of these experiments will focus on diffusivity as a function of grain size, others on the influence of compositional variability. A significant question, given the high concentrations of U and Th in these minerals, is the extent to which their 4He diffusion systematics is affected by radiation damage, and this will be evaluated explicitly through a series of experiments. Based on in-house experience with monazite and xenotime microsampling procedures, we also have designed a set of empirical tests of the computational method commonly used to correct (U-Th)/He dates for alpha ejection. Collectively, these studies should permit a comprehensive evaluation of the viability of rare-earth phosphates (U-Th)/He thermochronometry.