Zircon 91500 Reference Material Results Output Age

Sebastian1

Hi, I’ve been using zircon 91500 as reference material for UPb measurements of “known” and “unkown” samples. Based on the “known” samples I know that DRS process has been executed correctly and its accurate. Nonetheless I get a constant average (238U-206Pb) output age for zircon 91500 (the reference material) of ₁,062 Ma (instead of the published value 1,065 Ma).

Does this occurs as I’m not directly measuring 91500?
Or, am I missing something?

PCarr

Hi Sebastian,

I'm not sure if I understand your problem entirely, but have you checked the assigned age in the reference folder for the 91500? The age of your primary reference material will be normalised to this value.

On a side note, in one of the recent IAG G-Chron inter-lab comparisons, the organisers noted that different labs uses a range of ages for 91500 zircon. Some use the 206/238 age (1062.4 Ma), others the 207/206 age (1065 Ma), and others use more recent data. I would suggest that you confirm that the age and 206/238 ratio in your reference file are coherent.

Cheers
Pat

JakubS

Hello,

I'd like to follow up with a similar question, so I'll use the same thread.

We noticed some very slight irregularities in our age calculations with 91500. Our 6/8 ratio for the RM is 0.17917, which should give an age of 1062.44--but some sessions are yielding weighted means of 1061. At first I thought it was a rounding error, but on closer inspection, there seems to be something I don't understand with the Iolite age calculation. In detail:

After data reduction with VizualAge (no common Pb correction, no disequilibrium correction, exponential DFH spline, _linear spline for RM's), I calculated a (weighted and non-weighted) 6/8 ratios for 10 analyses of 91500 and checked the weighted mean from Iolite (no outlier rejection): 0.1792 in both cases, consistent with the database value--but the age output seems to be slightly off, since it's outputting 1061 instead of 1062 (Isoplot gives ages of 1062.44 and 1062.6 for 6/8 = 0.17917 and 0.1792, resp.).

Ultimately, it's an offset of 0.1%, which I'm not worried about with laser data, but I would like to understand the math behind Iolite's conversion from 6/8 ratio to 6/8 age.

Thanks in advance!
Jakub

Bence

Hi Jakub,

There is a full post about this effect here: https://notes.iolite.xyz/posts/agenonlinearity/

If you have any questions or concerns after reading that, please let us know.

Kind regards,
Bence

JakubS

This is great! Thanks, Bence!