Why 207/206 ratios by Iolite are different from [207/235]/([206/238]*137.818)?

RomanV

Dear friends,

Could you please clarify why the 207/206 ratios calculated by Iolite (Final Pb207/Pb206_mean) differ from those obtained using the equation [Final Pb207/U235_mean]/([Final Pb206/U238_mean]*137.818)? Is this discrepancy related to Iolite's "time-point by time-point" principle?

Thank you very much!

Bence

Hi RomanV ,

The 207/206 ratio does not have any downhole fractionation correction applied to it, whereas the 206Pb/238U and 207Pb/235U ratios will.

I just had a look at the DRO4 example dataset, and looking at the Plesovice results, it appears that, on a measurement-to-measurement basis, the difference is about 0.0005. This is about an order of magnitude less than the 2SD of the 7/6 measurements.

Is that similar to what you're seeing?

Kind regards,
Bence

RomanV

Bence
Hi Bence,

thanks! Yes, you are right, but it would be very interesting to know why there is such a difference? For example, there is no such difference when exporting results from Glitter or Saturn. In my case, this difference varies up to 0.0168 (see attached screenshot).

Bence

RomanV

I think I've explained the difference above: there is a downhole fractionation correction applied to the U/Pb ratios. I can't speak for Glitter or Saturn.

I should not have added my comment about the size of the difference, as this depends on the amount of U/Pb fractionation, and what model you've used to correct for it. This will be different from system to system, as well as from DHF model to model.

Kind regards,
Bence

RomanV

Bence
That's clear. Thank you, Bence!

Best regards and many thanks for the Iolite's development!