Two reliable calibration methods for accurate in situ U–Pb dating of scheelite†
Scheelite is an important metallic mineral in W-related hydrothermal deposits and can be utilized as a reliable geochronometer to directly date the timing of mineralization. Up to now, two previous studies have performed in situ U–Pb dating of scheelite using NIST glasses as reference materials due to the lack of scheelite standards. However, there exists a significant matrix effect between NIST612 and scheelite based on our analysis. Therefore, two reliable calibration methods have been proposed and assessed via LA-SF-ICP-MS in this study. A laser spot size of 32 μm was mainly used to satisfy the signal for 206Pb, 207Pb, and 238U and keep a high resolution to fit small grains and altered crystal zones of scheelite samples. Scheelite WX27 as a secondary standard has been accurately determined in normal ablation settings of 3 J cm−2 – 5 Hz, 3 J cm−2 – 10 Hz, 5 J cm−2 – 5 Hz (using a spot size of 24 μm), 5 J cm−2 – 10 Hz, and 10 J cm−2 – 5 Hz. Its lower intercept 206Pb/238U ages are consistent with each other within error and agree with two known ages (144.8 ± 11.7 and 141.8 ± 5.3 Ma) from the Woxi deposit with age offsets of <3.8%, no matter whether they were calibrated by using concordant wolframite YGX or the combined calibration method (i.e., NIST612 for 207Pb/206Pb and YGX for 238U/206Pb ratios). Moreover, accurate U–Pb ages, ranging from 432 Ma to 92 Ma, were also obtained for five typical W deposits. All these age results demonstrate the reliability of the two calibration methods and indicate that no matrix effect exists between scheelite and wolframite using single spot analysis mode in normal ablation settings. When all the same data from scheelite WX27 were calibrated by using NIST 612, most of the obtained ages were much younger than the known ages, with the age offsets ranging from 5.9% to 18.0%. Thus, NIST glasses are commonly not suitable as primary standards for in situ U–Pb dating of scheelite. Scheelite WX27 contains relatively high U (averaging 7.1 ppm) and low common-lead and is suitable as a candidate reference material for in situ U–Pb dating of scheelite at the moment.