U dating com
In contrast to the SHRIMP II results of Stern and Amelin (2003), we find no discernible geometry effect resulting from secondary ion steering in the UCLA data ("LT1 Def A").
This suggests that variations in interelement fractionation during ion bombardment are largely independent of instrumental design and that interelement fractionation in ion microprobe U-Pb analysis can be monitored to an external reproducibility of ~1-2%.
At the beginning of an analytical session or when analytical parameters such as primary beam intensity or spot size were changed, energy offsets for all measured ion species must be determined.
Pb have characteristic energy distributions (see Figure) and therefore optimizing intensities and finding a region in the energy distribution sufficiently stable to minimize interelement fractionation is a prerequisite for analysis.
To better characterize instrumental fractionation and to evaluate the potential for improved standardization techniques, we analyzed 54 of a total of 62 fragments of zircon megacryst z6266 (TIMS weighted mean age of 559.0±0.2 Ma) mounted on a single epoxy disk.
The same mount was also analyzed by Geological Survey of Canada (GSC) researchers using the GSC SHRIMP II ion microprobe (Stern and Amelin, 2003).
In our comparison, we determined that variations in U and Pb instrumental fractionation, sputter yield, detection efficiency (useful yield) and analytical reproducibility are very similar for both, the UCLA and GSC instruments, despite prominent differences in design and analytical conditions.
Subsequently, intensities were measured in 15 magnet cycles.
Isotopic ratios are then calculated using raw intensities corrected for electron multiplier dead-time and drift-corrected mid-analysis values.