Precise U–Pb and Pb–Pb dating of Phanerozoic baddeleyite by SIMS with oxygen flooding technique 论文

摘要

Baddeleyite has long been recognized as one of the most important U-bearing minerals for dating silica undersaturated igneous rocks. Age determination of baddeleyite calls for analysis within small volumes using high-resolution secondary ion mass spectrometry (SIMS) because of its minuscule grain size as well as potential altered domains or micro-inclusions. However, precise SIMS U–Pb dating has been hampered for baddeleyite owing to crystal orientation effects that bias Pb/U ratio measured in baddeleyite. In this study we carried out a series of tests of U–Pb and Pb–Pb measurements on Phanerozoic baddeleyite using a multi-collector Cameca 1280 IMS with oxygen flooding technique. Our results demonstrate that the oxygen flooding can not only enhance secondary Pb+ ion yield by a fact of 7 for baddeleyite, but also depress the baddeleyite U/Pb orientation effect down to ∼2% (1 RSD). Therefore, Phanerozoic (as young as Cenozoic) baddeleyite can be precisely dated by SIMS Pb–Pb and/or U–Pb measurements with precision of 1–3% (2 RSE).