Precise U[sbnd]Pb dating of grandite garnets by LA-ICP-MS: Assessing ablation behaviors under matrix-matched and non-matrix-matched conditions and applications to various skarn deposits

Abstract

LA-ICP-MS UPb 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-matrix-matched 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 UPb dating. The results show that UPb 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 UPb dating. Line scanning is most favorable for garnet UPb 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 μm) coupled with moderate-low repetition rate (e.g., 2–5 J/cm2) and fluence (e.g., 2–5 J/cm2) 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 μm) coupled with moderate repetition rate (e.g., 5 Hz) and fluence (e.g., 5 J/cm2) is used. Too high or too low repetition rates (e.g., <2 Hz and > 10 Hz) and fluences (e.g., <2 J/cm2 and > 10 J/cm2) were found to be unsuitable for garnet UPb dating. Low-U and high-U samples are best treated differently due to their different UPb 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 UPb datingNatural grandite garnets selected from Fe, W and CuMo 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 (f206 of 0–0.7) concentrations, indicating that the methods are reliable and thus suitable for wide applications.