Abstract

The significant capabilities of the ion microprobe for thermochronometric investigations of geologic materials remain largely unexploited. Whereas 208Pb/232Th spot analysis allows ∼10-mm-scale imaging of Pb loss profiles or overgrowths in sectioned monazite grains, the spatial resolution offered by depth profiling into the surface region of natural crystals is more than two orders of magnitude higher. We document here the ability of the high-resolution ion microprobe to detect 208Pb/232Th age differences of <1 m.y. with better than 0.05 μm depth resolution in the outer micron of Tertiary monazites from the hanging wall of the Himalayan Main Central thrust. Age gradients on this scale are inaccessible to ion microprobe spot analysis or conventional thermal ionization mass spectrometry. Interpretation of the near-surface 208Pb distributions with available monazite Pb diffusion data illustrates the potential of the approach for recovering continuous, high-temperature thermal history information not previously available.

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