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 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).
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