Home Forums UPb Geochron DRS Inconsistent 206Pb/238U and 238U/206Pb in 208(noTh) corrected U-Pb data

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    Tanya Ewing

    This is really a question about the VizualAge_UcomPbine DRS, not Iolite itself, but I’m hoping that other Iolite users may have some insight (so I hope it’s not inappropriate that I post this here).

    I am new to Iolite and am processing some rutile U–Pb data with VizualAge_UcomPbine, using a 208(noTh) correction for unknowns but no common Pb correction for the standard. I have carefully gone through the various steps of data treatment in Iolite and get concordant, accurate data for my secondary standard, so I think that things are more or less working as they should. However, in the exported data, the FinalNoTh238_206 is not equal to 1/FinalNoTh206_238, which seems incorrect to me. The difference is sometimes very large: up to 30% difference, equating to tens of Ma. I noticed this because for my samples the 208-corrected ages (FinalAgeNoTh206_238) are very nice and consistent, and the common Pb corrected data also look good on a conventional concordia (which uses FinalNoTh206_238) but on a Tera-Wasserburg (using FinalNoTh238_206) they look much worse and clearly show a huge amount more scatter in 238U/206Pb in particular. This mainly affects analyses that had high common Pb, so for example my secondary standard (basically common Pb free) looks fine on a Tera-Wasserburg. Has anyone else observed this type of behaviour? Can anyone cast any light on why this is the case? It’s probably unrelated but I also noticed that in the exported data UNCORRECTED for common Pb, the Final238_206 column was blank (so for uncorrected data I calculated this value myself as 1/Final206_238). I’d be curious to know if it is normal that this column is blank.

    My suspicion (/hope) is that the FinalNoTh238_206 is somehow wrong, and the FinalAgeNoTh206_238 and 206Pb/238U ages are correct, but I would like to (a) understand why this is the case, and (b) have some justification before throwing out the exported values for FinalNoTh238_206 and re-calculating them from FinalAgeNoTh206_238 in order to be able to plot a Tera-Wasserburg that agrees with the 206Pb/238U ages.

    A related (possible) issue is that I would have thought that the FinalAgeNoTh206_238 and and the FinalNoTh238_206 would have the same percentage error – although I’m not 100% sure I am right about this. When I calculate % errors from the absolute 2SEs in the exported data they are significantly different for both internal and propagated errors (in the worst case 8% vs 40% errors). For the data uncorrected for common Pb, the Final206_238 and Final238_206 do not have identical percentage errors but they are similar – not the huge differences seen in the 208noTh ratios, which mean that the data look completely different on Tera-Wasserburg and Conventional concordias when common Pb corrected.

    I am using Iolite 3.32, IgorPro and Windows 7. I downloaded VizualAge_UcomPbine late 2016. I had a couple of errors in the error log which I don’t understand, not sure if they could be relevant:
    (1) “Warning: P or X oustide of wave assignment loop”
    (2) “**Warning** User’s window hook function tried to put up dialog”
    (3) “Could not locate the global Database Overwrite. It’s value has not been changed”

    Any insights anyone can give me would be much appreciated!

    Tanya Ewing
    Postdoctoral researcher
    University of Lausanne, Switzerland

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