APPLICATION OF SPECTROSCOPY TO ZIRCON U-PB GEOCHRONOLOGY

Non-destructive, light spectroscopy techniques (e.g., Raman and luminescence emission spectroscopy) allow one to detect quantitatively the degree of radiation damage of micrometer-sized areas, or interior regions, within crystals of accessory minerals. One recent example for an application related to zircon U-Pb dating is a study addressing the question why the “Mattinson technique” (i.e., Chemical Abrasion TIMS) works so well in reducing, and often eliminating, U–Pb discordance? In his fundamental 2005 Chemical Geology paper, Jim Mattinson has proposed to remove zircon domains affected by Pb loss through a treatment consisting of dry annealing and HF etching steps. Systematic Raman spectroscopic analyses showed that the dry annealing treatment at 1000 °C results in significant but still incomplete structural reconstitution of zircon. The partial “fixing” of the structural state of the heat-treated grains is needed to avoid their over-etching and preferential leaching of Pb relative to U, in the following HF treatment. The incompleteness of the recovery, in contrast, ensures that structural differences among previously less and more metamict domains, which were variably susceptible to Pb loss, are partly preserved. Further, the partial recovery of initially more radiation-damaged areas is connected with more pronounced volume shrinking, compared to the recovery of their neighboring, less damaged areas, resulting in enhanced strain and often the development of sub-micron pores in the former. This, and elevated trace element concentrations, result in preferred HF etching of the initially more damaged domains. The HF treatment thereby virtually “follows the footsteps” of the natural fluid that transported the radiogenic Pb away from more radiation-damaged and well accessible domains; and results in dissolving and removal of these Pb-deficient volume areas from the specimens to be dated.

Other examples discussed include spectroscopic studies of effects of heterogeneous radiation damage on the intensity distribution patterns observed in BSE and CL images of grains to be dated, and possible effects of alpha particles on the existing self-irradiation damage.