|Title:||In situ U–Pb dating of micro‐baddeleyite by secondary ion mass spectrometry|
|Authors:||A. K. Schmitt, K. R. Chamberlain, S. M. Swapp, and T. M. Harrison|
|Publication:||Chem. Geol., v. 269, p. 386‐395.|
We introduce a technique for U–Pb dating of baddeleyite using secondary ion mass spectrometry (SIMS) in situ analysis of ng–mass crystals that cannot be efficiently extracted by conventional mineral separation techniques. Average 207Pb/206Pb ages for Precambrian baddeleyite crystals are within b0.3% of the respective isotope dilution thermal ionization mass spectrometry (ID–TIMS) ages. 206Pb/238U ratios are corrected for instrumental fractionation calibrated through linear regression in a Pb/U relative sensitivity vs. UO2 +/U+ calibration plot. Calibration is performed on separated baddeleyite crystals (–100–200 µm in maximum dimension) mounted in random crystallographic orientation. 206Pb/238U ages for baddeleyite from Duluth gabbro (FC4b) and Kovdor are accurate within 1–2% when averaging 15–30 individual spot analyses and relative sensitivities calibrated on Phalaborwa baddeleyite. The relative difference of 206Pb/238U between large crystals and micro–baddeleyite from FC4b is within 1%. Comparison between silicate glass and baddeleyite, as well as replicate analysis of the same grains in different orientations relative to the incidence direction of the primary beam support previous evidence for bias in Pb/U sensitivity in baddeleyite due to variable crystal orientations. We successfully utilized oxygen flooding and a UO2 +/U+ –based calibration to significantly reduce orientation dependent bias.