U‐Th‐Pb dating of monazite by single‐collector ICP‐MS: Pitfalls and potential

Abstract

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).