Garnet LA–MC–ICP–MS In-Situ U–Pb Dating: The Impact of Water Vapor on Matrix Effects

Garnet has become a crucial mineral for constraining the timing of metamorphic and magmatic events due to its variable uranium content, low common Pb characteristics, and high U-Pb system closure temperature (> 850C).Although Laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) has been carried out on garnet U-Pb dating, its measurement accuracy remains constrained by matrix effects.This study introduces water vapor into garnet U-Pb dating and conducted comparative test under different laser spot sizes (15-50 m), yielding the following findings: 1) The introduction of water vapor reduced signal intensity during U-Pb analysis of LA-MC-ICP-MS; 2) Under small spot sizes conditions (15 and 23 m), water vapor significantly reduces Pb/U down-hole fractionation and enhances signal stability; 3) Normal ablation conditions with large spot size (50 m) effectively overcome matrix effects, achieving optimal age dates deviation of ~ 3%; 4) Water vapor-assisted small spot sizes (15 and 23 m) can also suppress matrix effects, yielding age dates deviation of ~ 4%.This study develops a water vapor-spot size synergistic method that enhances garnet non-matrix-matched calibration and enables micro-scale high-precision garnet U-Pb dating.