Rho value calculation for U-Pb zircon ages

Ashu527

Hi All,
When exporting data after reduction in iolite, the rho values are provided as rho 206Pb/238U vs 207Pb/235U and rho 207Pb/206Pb vs 238U/206Pb. However, to plot in isoplotR, rho 207Pb/235U vs 206Pb/238U and rho 206Pb/238U vs 207Pb/206Pb are needed. I am wondering if these ratios can be deduced or calculated in iolite, because the ellipses in VizualAge concordia plots are not matching those obtained in Isoplot. Additionally, I read that rho values can be calculated by taking the ratio of the relative errors, but in my case, many of my data have ratios above 1. What does that imply?

Joe

Ashu527

The error correlation of X vs Y is the same as Y vs X:

So, I suspect that the ellipses are not matching for some other reason. Have you tried using the IsoplotR -> Send data menu item? When I use that I can't spot any differences between the two:

Maybe you're plotting one with propagated error and one with the internal error (configurable in VizualAge via the right click menu, in export settings and in IsoplotR settings)? Or maybe the format of the data you're copying in is not what IsoplotR is expecting (you can configure the data format in the IsoplotR options)?

If you'd still like to calculate an error correlation from the measurements, you can do that as:

(from Schmitz, Mark D., and Blair Schoene. "Derivation of isotope ratios, errors, and error correlations for U‐Pb geochronology using 205Pb‐235U‐(233U)‐spiked isotope dilution thermal ionization mass spectrometric data." Geochemistry, Geophysics, Geosystems 8, no. 8 (2007).)

I'm not sure what you're referring to when you ask what does your ratios being > 1 imply... Can you please clarify that if it is still relevant?

All the best,