Sub-20 μm Non-Matrix-Matched U−Pb Dating by LA-Q-ICP-MS Using Pre-Cell Water-Vapor-Assisted Laser Ablation and Quadratic Polynomial Regression Intercept Correction

Abstract Accessory-mineral U−Pb geochronology by laser ablation quadrupole inductively coupled plasma mass spectrometry (LA-Q-ICP-MS) provides valuable temporal constraints on Earth and planetary processes. However, its application at sub-20 μm spatial resolution is hindered by matrix-dependent Pb/U downhole fractionation, which compromises the accuracy of non-matrix-matched calibration. Here, xenotime BS-1, allanite AMK, and titanite MKED1 were analyzed at spot sizes of 16 and 10 μm using Tanz zircon as the external calibration reference material. Under dry-ablation, the conventional mean-of-ratios approach produced relative 206Pb/238U ratio biases of 14.2−21.8%, reflecting pronounced matrix-dependent differences in downhole Pb/U fractionation between zircon calibrant and the other accessory minerals. To overcome this limitation, we developed a quadratic polynomial regression intercept method that extrapolates the time-resolved Pb/U fractionation profile to the onset of ablation. The absolute bias reduced to 2.19−11.1% using the quadratic polynomial regression correction under dry ablation conditions. When combined with pre-cell water-vapor-assisted laser ablation, the method enables accurate non-matrix-matched U−Pb dating of xenotime, allanite, and titanite using spot sizes as small as 10 μm, with bias of only 0.05−0.74%. These results establish a practical strategy for routine sub-20 μm multimineral U−Pb geochronology by LA-Q-ICP-MS.