Estimation of radiation damage in titanites using Raman spectroscopy

Abstract

Abstract Recent studies have shown that α-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 α‑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 α-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 α-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 α-dose. A simplified fitting approach considering 15 peaks is sufficient to describe the accumulated α-dose. For α-doses below 5 × 1016α/g the iw-FWHM is independent of α-dose and ranges from 25 to 50 cm-1. Above this value the iw-FWHM increases linearly with increasing α-dose up to 3 × 1018α/g. The linear correlation can be described as iw-FWHM[cm-1] ≈ 39(±1.2)[cm-1] + 3.84(+0.61,–0.26) × 10-17[cm-1/(α/g)] × α-dose[α/g]. The approach provides a pre-selection method to optimize the range of α-doses of titanite crystals to be dated by (U-Th)/He thermochronology.