Groundwater-mediated U–Th–Pb differential migration in monazites during hydrochemical weathering

Abstract

The influence of near-surface weathering on U–Th–Pb isotope systematics in accessory minerals remains poorly understood. Here, we investigate monazite samples (DFS225, DFS277 and DFS620) collected from the Duanfengshan (DFS) pegmatites (Hubei Province, South China), which exhibit supergene alteration driven by oxidizing groundwater. Combined optical microscopy, scanning electron microscopy (SEM), and TESCAN Integrated Mineral Analyzer (TIMA) analyses were used to characterize high-porosity textures and minor mineral inclusions (MIs) within the monazite grains. Results indicate that MIs are enriched in U and Th (e.g., U–Th–rich silicate and oxide phases) relative to host monazite and possess lower Th/U ratios. Porous domains frequently occur spatially proximate to these inclusions. Based on these observations, DFS monazites were classified into two distinct types: Type I (porous domains) and Type II (MI-bearing domains).

LA–ICP–MS U–Th–Pb isotope analyses revealed that Type I monazites yield U/Pb ages of 135–138 Ma, identical to published magmatic ages. In contrast, Type IIa monazites (MI-bearing) show younger U/Pb ages (125–128 Ma). Laser ablation analyses of Type IIa samples show elevated U signals coinciding with low ²⁰⁶Pb/²³⁸U ratios, suggesting age resetting via incorporation of secondary U–rich phases. Type-IIb monazites (porous domains) display significant common Pb contamination (up to 81 %), resulting in imprecise intercept ages on the Tera-Wasserburg (T-W) plot with low ²⁰⁷Pb/²⁰⁶Pb intercepts (DFS225: 0.781; DFS620: 0.719) and artefactually older common Pb-corrected UPb ages (142–152 Ma). These samples also exhibit elevated Cu signals and positive correlations between ²⁰⁷Pb/²⁰⁶Pb ratios and U depletion, interpreted as evidence for late–stage U leaching coupled with external Pb infiltration. Remarkably, Th/Pb ages for both monazite types remain consistent (131–134 Ma), highlighting systematic discordance between U/Pb and Th/Pb chronometers in these weathering-affected samples. Based on U–Th–Pb isotope analyses and binary mixing-based forward modeling, we identify three distinct scenarios of differential U–Th–Pb mobility in monazite grains during groundwater leaching: (1) Unaltered monazite—no visible textural modification; proportional UPb loss preserves true crystallization ages (t_U/Pb = t_True > t_Th/Pb), demonstrating the relative robustness of the UPb chronometer to hydrochemical attack. (2) Porous monazite—intense U loss yields spuriously old U/Pb ages (t_U/Pb > t_True > t_Th/Pb). Concurrent Pb contamination positively correlates with U loss, producing imprecise UPb isochrons with low ²⁰⁷Pb/²⁰⁶Pb intercepts on Tera–Wasserburg diagrams. (3) MI-bearing monazite—secondary low-Th/U phase admixture induces preferential U/Pb disturbance (t_True > t_Th/Pb > t_U/Pb), leaving Th/Pb ages as the most reliable record of crystallization.

Our findings demonstrate the significant impact of groundwater-mediated alteration on monazite U–Th–Pb chronometry, and the UPb system in monazite is more resilient to supergene leaching than the ThPb system. Geochronologists should therefore exercise caution when interpreting discordant U–Th–Pb datasets from weathering-affected samples. Subtle textural and geochemical criteria, combined with Th/Pb constraints, are essential for distinguishing primary from secondary isotopic signatures.