COUPLING RAMAN SPECTROSCOPY AND ZIRCON (U-TH)/HE DATING TO IMPROVE INTERPRETATION OF THERMAL HISTORIES AND CRUSTAL PROCESSES

Radiation damage strongly influences ⁴He diffusion in zircon and therefore the closure temperature of the zircon (U-Th)/He (ZHe) thermochronometer. Closure temperatures delimit when and how to extract rates of orogenesis, burial, and exhumation from thermochronometric data. Analysis of multiple zircon grains with variable damage accumulation and thus a range of closure temperatures from the same sample can allow for deciphering of detailed thermal histories if the radiation damage-He diffusivity relationships are well understood. The effects of damage on ZHe dates are especially pronounced in areas with protracted thermal histories, such as in the Ancient Gneiss Complex of the eastern Kaapvaal Craton.

We acquired ZHe data for two samples from this region, and we used Raman Spectroscopy to characterize the variable magnitudes of accumulated radiation damage in the zircons. The ZHe results show negative correlations between date and effective uranium (eU) concentration in each sample, thus manifesting the influence of radiation damage on the data. After acquiring multiple Raman spectra for each zircon grain, we used the full width at half maximum (FWHM) of the 1008 cm^-1 ν³(SiO₄) Raman band as a proxy for radiation damage. Measured FWHM values in our samples spanned from ~12 cm^-1 to completely amorphous. We used these FWHM values to estimate alpha dose, compared these alpha dose estimates with the zircon damage-diffusivity relationship of Guenthner et al. (2013), and estimated that the closure temperatures for zircons in each sample vary from ~180 ºC to ~120 ºC. This range of closure temperatures can explain our observed negative ZHe date-eU correlations. ZHe dates to the southwest from the Steynsdorp pluton range from 193±14 Ma to 340±20 Ma with an eU span of 311.4 to 528.2. Those from the Usutu suite ~45 km to the northeast vary from 103±7 Ma to 175±18 Ma over an eU range of 107.6 to 573.3. Thus, the two samples record somewhat different thermal histories that likely reflect spatially variable erosion. Our work shows the potential utility of pre-characterization of zircon grains by Raman spectroscopy to allow for selection of zircons with a suitable span of closure temperatures for analysis.