The same radioactivity that makes zircon a valuable chronometer damages its crystal structure over time and changes zircon helium kinetics. I used a zircon, titanite, and apatite (U-Th)/He dataset combined with previously published data and a new thermal model to place empirical constraints on the closure temperature for helium in a suite of variably damaged zircon crystals from the McClure Mountain syenite of Colorado. Results of this study suggest that the widely-used zircon damage accumulation and annealing model (ZRDAAM) does not accurately predict helium closure temperatures for a majority of the dated zircons. Detailed Raman maps of Proterozoic zircon crystals from the Lyon Mountain Granite of New York document complex radiation damage zoning. Models based on these results suggest that most ancient zircons are likely to exhibit intracrystalline variations in helium diffusivity due to radiation damage zoning, which may, in part, explain discrepancies between my empirical findings and ZRDAAM.
Zircon crystallography suggests that helium diffusion should be fastest along the crystallographic c-axis. I used laser depth profiling to show that diffusion is more strongly anisotropic than previously recognized. These findings imply that crystal morphology affects the closure temperature for helium in crystalline zircon. Diffusivity and the magnitude of diffusive anisotropy decrease with low doses of radiation damage.
Xenotime would make a promising (U-Th)/He thermochronometer if its helium kinetics were better known. I performed classic step-wise degassing experiments to characterize helium diffusion in xenotime FPX-1. Results suggest that this xenotime sample is sensitive to exceptionally low temperatures (∼50 °C) and produces consistent (U-Th)/He dates.
The highest elevation of the Tibetan Plateau, lying 5,700 m above sea level, occurs within the part of the Lhasa block immediately north of the India-Tibet suture zone (Yarlung Zangbo suture zone, YZSZ), being 700 m higher than the maximum elevation of more northern parts of the plateau. Various mechanisms have been proposed to explain this differentially higher topography and the rock uplift that led to it, invoking crustal compression or extension. Here we present the results of structural investigations along the length of the high elevation belt and suture zone, which rather indicate flexural bending of the southern margin of the Lhasa block (Gangdese magmatic belt) and occurrence of an adjacent foreland basin (Kailas Basin), both elements resulting from supra-crustal loading of the Lhasa block by the Zangbo Complex (Indian plate rocks) via the Great Counter Thrust. Hence we interpret the differential elevation of the southern margin of the plateau as due originally to uplift of a forebulge in a retro foreland setting modified by subsequent processes. Identification of this flexural deformation has implications for early evolution of the India-Tibet continental collision zone, implying an initial (Late Oligocene) symmetrical architecture that subsequently transitioned into the present asymmetrical wedge architecture.