U–Pb geochronology of wolframite by laser ablation inductively coupled plasma mass spectrometry†
Wolframite series minerals often occur in tungsten deposits and other hydrothermal ore deposits. Because of its relatively high U content (from several to hundreds of ppm), wolframite has shown potential for U–Pb dating. In this study, we present the first laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb geochronological analysis of wolframite samples. The lack of a matrix-matched wolframite standard is the major limitation of its in situ geochronological analysis. Therefore, the matrix effects between zircon and wolframite were investigated with 193 nm excimer nanosecond (ns) LA in this study. The wolframite lower intercept ages in the Tera–Wasserburg concordia diagram obtained using zircon 91500 as the external standard in ns-LA-ICP-MS were approximately 12% younger than their isotope dilution thermal ionization mass spectrometry (ID-TIMS) ages. By adding water vapor before the ablation cell, the measured lower intercept ages of wolframite samples show excellent agreement with the ID-TIMS ages in ns-LA-ICP-MS. Our data obtained from different dates under different laser ablation conditions have shown the accuracy and long-term analytical reproducibility of the water vapor-assisted ns-LA-ICP-MS wolframite U–Pb dating method. This method is an effective and notably simple method to obtain accurate dating results, which will significantly accelerate the application of LA-ICP-MS for non-matrix-matched determination of the wolframite U–Pb age. Our results also show that wolframite samples collected from La Bosse (LB) and Les Montmins (MTM) localities are promising candidate reference materials for in situ U–Pb dating by using LA-ICP-MS.