Cathodoluminescence characterization of apatite for U-Pb dating

Cathodoluminescence (CL) has the advantage that it can reveal mineralogical features depending on various intervening factors; impurities of trace elements, lattice defects and sample temperature. Apatite is a common and widespread accessory minerals with a variety of occurrences in geological environments, suggesting a geochronological application added to zircon U-Pb dating. Therefore, CL imaging of apatite can discriminate various textures during crystal growth and metamorphism for LA-ICP-MS analysis as similarly. Apatite shows many color-CL emissions derived from impurity centers of transition metal elements and REE and possibly from structural defect centers (e.g., Kempe and Götze, 2002). Recently, matrix effects in U-Pb measurements have been recognized in crystal chemical characteristics of apatite to be an important factor related to elemental differentiation during the ablation (e.g., Thompson et al., 2016). In this study, we have conducted to clarify the luminescent centers in apatite for U-Pb dating by CL spectroscopy.

Several tens samples of natural and synthetic apatite were employed for CL measurements. CL spectroscopy was made by a SEM-CL system. CL spectra were corrected for total instrumental response. Chemical compositions were determined for major elements by EPMA and for minor elements by LA-ICP-MS.

Lilac to violet color, yellow color and red color CL were observed in apatite with homogeneous, domain and oscillatory zoning textures. Lilac to violet color is attributed to predominant Ce³⁺ and trivalent REE, while yellow and red colors are derived from Mn²⁺ activator with different sites. The Ca ions occupy two sites in apatite structure; M1 site coordinated by 9 oxygens and M2 site by 6 oxygens and one F (Cl, OH). The emissions around 3.3 eV in a blue region are divided by three emission components of two Mn²⁺ ions at around 3.40 and 3.33 and possibly defect center induced by Th and U. Yellow and red emissions might be assigned to Mn²⁺ impurity centers in M1 and M2 sites. Temperature-controlled CL measurements of apatite will clarify the emission centers in detail and partition of activators between M1 and M2 sites. U ions could occupy M1 and/or M2 site as same as Ce³⁺ ions in apatite, suggesting an influence on elemental differentiation during the ablation in LA-ICP-MS measurements.