LA-ICP-MS U-Pb dating of garnet has become one of the important geochronometric methods in recent years. Both the matrix-matched and non-matrix-matched external standards were used for dating. However, under which conditions reliable ages can be obtained and the difference between the matrix-matched and non-matrixmatched methods have not been well constrained. In this contribution, a series of ablation experiments (including spot analysis and line scanning) were conducted on different grandite garnets using various ablation parameters (spot size, repetition rate and fluence) under both the matrix-matched (garnet as external standard) and non-matrix-matched (zircon as external standard) conditions, with the aim to obtain the optimal conditions for precise LA-ICP-MS garnet U-Pb dating. The results show that U-Pb fractionation associated with downhole depth, intrinsic properties of U (more refractory) and Pb (more volatile) and radiation damage (alpha dose) of crystal lattice occurs in zircon and garnet. The fractionation can be minimized or even eliminated by optimizing the ablation parameters (e.g., large spot size and low repetition rate) or the analytical modes (e.g., line scanning). This makes it feasible to use zircon as external standard for garnet U-Pb dating. Line scanning is most favorable for garnet U-Pb dating at broad ablation parameters regardless of matrix-matched or non-matrix-matched. In spot analysis mode, all the garnets can be precisely dated using a large spot size (e.g., >= 90 mu m) coupled with moderate-low repetition rate (e.g., 2-5 J/cm(2)) and fluence (e.g., 2-5 J/cm(2)) under matrix-matched condition. Under non-matrix-matched condition, all the garnets can also be precisely dated when a larger spot size (e.g., >= 120 mu m) coupled with moderate repetition rate (e.g., 5 Hz) and fluence (e.g., 5 J/cm(2)) is used. Too high or too low repetition rates (e.g., <2 Hz and > 10 Hz) and fluences (e.g., <2 J/cm(2) and > 10 J/cm(2)) were found to be unsuitable for garnet U-Pb dating. Low-U and high-U samples are best treated differently due to their different U-Pb fractionation coefficients, especially at the high fluence and repetition rate conditions. Equally, U-concentration matched garnet standards are required to improve the accuracy and precision of the U-Pb dating

Natural grandite garnets selected from Fe, W and Cu-Mo skarn deposits can be precisely dated using the optimized methods. These garnets have compositions varying widely between andradite and grossular with variable U (1-100 ppm) and common Pb (f(206) of 0-0.7) concentrations, indicating that the methods are reliable and thus suitable for wide applications.

View here for the details