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

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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 206Pb/238U and 207Pb/235U, 4% for 208Pb/232Th, and 1–2% for 207Pb/206Pb (2σ, SD). For pooled analyses these precisions are ∼ ± 1–2% for 206Pb/238U, 207Pb/235U, and 208Pb/232Th and ≤1% for 207Pb/206Pb (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 207Pb/206Pb and up to 5% for 206Pb/238U, 207Pb/235U, and 208Pb/232Th 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).