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Mass bias corrections for U‐Pb isotopic analysis by secondary ion mass spectrometry: Implications for U‐Pb dating of uraninite
Author(s) -
Sharpe Ryan,
Fayek Mostafa
Publication year - 2016
Publication title -
rapid communications in mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.528
H-Index - 136
eISSN - 1097-0231
pISSN - 0951-4198
DOI - 10.1002/rcm.7595
Subject(s) - uraninite , uranium , chemistry , isotope , analytical chemistry (journal) , accelerator mass spectrometry , mass spectrometry , geochronology , calibration curve , secondary ion mass spectrometry , radiochemistry , geology , nuclear physics , detection limit , geochemistry , environmental chemistry , metallurgy , chromatography , materials science , physics
Rationale Uranium (U)‐lead (Pb) geochronology of uraninite is critical to the study of the genesis of U deposits throughout the world. Previous attempts at developing a technique to date uraninite using secondary ion mass spectrometry (SIMS) have, however, had limited success. Improper correction of mass bias results in incorrect reported U‐Pb ratios. When these results are plotted on Concordia diagrams they produce erroneous ages, complicating the study of U deposits. Methods Uranium and Pb isotope ratios were measured in three uraninite reference materials (RMs) with varying Pb content and three samples with unknown U‐Pb isotope compositions using a CAMECA 7f SIMS instrument. Measurements were made using a primary beam of O – accelerated at 12.5 kV. A mass resolving power of 1300 and a 50‐V offset were used to minimize interferences. Results The study demonstrates that the mass bias for U‐Pb isotope ratio measurements in uraninite by SIMS varies as a function of Pb content. A three‐point calibration curve was developed using uraninite RMs with low‐, intermediate‐ and high‐Pb contents. Corrected ratios for both concordant and discordant uraninite were plotted on a Concordia diagram to demonstrate the effect that different correction techniques have on the resulting age. Conclusions Accurate determination of U‐Pb ratios in uraninite using a SIMS instrument requires a suite of RMs with varying Pb content and the construction of a calibration curve, or a uraninite RM with a Pb content similar to that of the unknowns. Failure to standardize correctly will result in erroneous ages being calculated using Concordia diagrams. Copyright © 2016 John Wiley & Sons, Ltd.