U-Th-Pb Dating of Monazite by Single-Collector ICP-MS: Pitfalls and Potential

Methods are presented for in situ determination of Pb/U, Pb/Th, and Pb/Pb ages in monazite by laser ablation, single-collector, magnetic sector inductively coupled plasma-mass spectrometry (ICP-MS). Analytical precisions for individual spot analyses are ±2–3% for ²⁰⁶ Pb/ ²³⁸ U and ²⁰⁷ Pb/ ²³⁵ U, 4% for ²⁰⁸ Pb/ ²³² Th, and 1–2% for ²⁰⁷ Pb/ ²⁰⁶ Pb (2σ, SD). For pooled analyses these precisions are ∼ ± 1–2% for ²⁰⁶ Pb/ ²³⁸ U, ²⁰⁷ Pb/ ²³⁵ U, and ²⁰⁸ Pb/ ²³² Th and ≤1% for ²⁰⁷ Pb/ ²⁰⁶ Pb (2σ, SE). When normalized to Trebilcock, LA-ICP-MS ages on other monazite standards can deviate from their thermal ionization mass spectrometry (ID-TIMS) ages by up to 1% for ²⁰⁷ Pb/ ²⁰⁶ Pb and up to 5% for ²⁰⁶ Pb/ ²³⁸ U, ²⁰⁷ Pb/ ²³⁵ U, and ²⁰⁸ Pb/ ²³² Th ages. This variability has also been observed for secondary ion mass spectrometry (SIMS) Th-Pb dating. The source(s) of these inaccuracies remains unknown. Although there are significant uncertainties inherent in this technique, U-Th-Pb monazite dating by LA-ICP-MS has important applications where this higher level of uncertainty is acceptable. Application to a large vein monazite from the Llallagua tin district of Bolivia suggests mineralization at 17–21 Ma, consistent with 21 Ma K-Ar ages from wallrock minerals but distinct from ∼45 Ma ages from apatite (Sm/Nd) and zircon (Pb/Pb).