APPLICATIONS AND LIMITATIONS OF U-PB THERMOCHRONOLOGY (Invited Presentation)

Traditionally, the thermal evolution of the lithosphere is recovered through the interpolation of discrete temperature-time points, generated by assigning estimates of nominal closure temperatures to volume-averaged radiometric ages. Whilst informative, bulk thermochronology results in low-resolution thermal history information. The highest resolution record of thermal history information is harnessed by intracrystalline concentration profiles [1, 2, 3]. With the advent of LA-ICPMS depth profiling, U-Pb age gradients can now be measured routinely at sub-micron spatial resolution in single grains of apatite, rutile and titanite. Given that Pb diffusion in these minerals operates at temperatures characteristic of the middle and lower crust, this represents an opportunity to refine understanding of deep lithospheric dynamics. With a particular focus on the U-Pb systematics of rutile, I will present examples of intracrystalline U-Pb gradients that are consistent with formation by Fickian-style Pb-loss during slow cooling or reheating. Such profiles can be inverted to yield near-continuous thermal histories and improve understanding of deep lithosphere dynamics. The importance of non-Fickian processes, including U zonation, multiple diffusion pathways and recrystallization on the formation of U-Pb age gradients will also be discussed with reference to case studies from the Archean Slave province, the Mauléon granulite terrane and the Ivrea Zone.

[1] Dodson (1986) Mat. Sci. Forum 7, 145-154; [2] Harrison et al. (2005) Rev. Min. Geochem. 58, 389-409; [3] Smye and Stockli (2014) Earth Planet. Sci. Letters 408, 171-182