Analysis - U-Pb
SHRIMP U-Pb analysis consists of two separate measurement types.
1) The isotopic composition of the Pb isotopes is taken as measured. Molecular interferences are well resolved (in zircon, 180Hf28Si from 208Pb requires 5,500). Hydride contributions are generally less than 0.1%. Isotopic mass fractionation is taken as negligible but could be as high as 0.2% per amu.
2) The measurement of the U-Pb ratio (206Pb/238U) is
an interelement ratio and so is affected by the different ionization yields
of Pb and U. Moreover, the ratio is variable from standard materials with
known ratios. It has been found that changes in the Pb/U ratio are reflected
in the abundances of U, UO, and UO2 ion abundances. Hence, measured U-Pb
isotope ratios must be calibrated to a standard to get age information.
The first measurements on SHRIMP used U/UO covariance to monitor U/Pb
The measured ion ratios of Pb/U covary with the concurrent measurement of UO/U. This allows the regression of the standard data to define a standard point for the session (B). Treating the unknown sample in the same way and regressing to the same UO/U will yield another point (A). For calibration to the 1099 Ma FC1 standard, the age of the unknown will be
Age(A) = (Pb/U)A/(Pb/U)B*1099