Estimation of radiation damage in titanites using Raman spectroscopy

International audienceRecent studies have shown that a-damage in titanite influences He diffusivity and thus the closure temperature of the (U-Th)/He system in titanite. We compare different methods for measuring the alpha-dose in titanite by Raman spectroscopy. Raman spectra of randomly oriented titanite fragments from the Archean Karelian domain in eastern Finland along with some well-studied young titanites and U-Pb standard reference materials were analyzed and related to the concentration of alpha-emitting elements (U and Th) that generated damage in the respective grains. Automated curve-fitting was performed by the IFORS software and different curve-fitting protocols were tested and compared.The Raman bands at 424 and 465 cm(-1) show a good correlation of full-width at half maximum (FWHM) and position with the alpha-dose. However, these bands are not always present because titanite is highly anisotropic implying that Raman spectra are sensitive to orientation. The intensity-weighted mean FWHM (iw-FWHM) of all Raman bands of a spectrum proves to be the most robust measure of the alpha-dose. A simplified fitting approach considering 15 peaks is sufficient to describe the accumulated alpha-dose. For alpha-doses below 5 x 10(16)alpha/g the iw-FWHM is independent of alpha-dose and ranges from 25 to 50 cm-1. Above this value the iw-FWHM increases linearly with increasing alpha-dose up to 3 x 10(18)alpha/g. The linear correlation can be described as iw-FWHM[cm(-1)] approximate to 39(+/- 1.2)[cm(-1)] + 3.84(+0.61,-0.26) x 10(-17)[cm(-1)/(alpha/g)] x alpha-dose[alpha/g]. The approach provides a pre-selection method to optimize the range of alpha-doses of titanite crystals to be dated by (U-Th)/He thermochronology