Query about VisualAge's treatment of standards with excess 206Pb

chini13

Dear Iolite,

I have a question for you about the way that VisualAge UCompbine DRS does reference material normalisations after downhole fractionation correction🙂

Would you be able to elaborate on the exact method employed by the DRS to calculate isotope ratios of unknowns using the isotope ratios of the primary standard after the downhole fractionation correction is done? Would you be able to share the specific equations used that show how the input isotope ratios of the primary standard are used to carry out normalisation of the unknowns?

For instance, if a downhole fractionation corrected 206/238 ratio for the primary standard comes out as 0.1 but the actual 238/206 is 0.2, does iolite simply find the factor to convert 0.1 to 0.2 (multiply by 2) then multiply all the downhole fractionation corrected 206/238 ratios for unknowns by 2 as well? Is that something like how it works? I'd love to see the exact equations to be able to understand it better. Are only the isotope ratios (206Pb/238U)r,(207Pb/235U)r ,(208Pb/232Th)r ,(207Pb/206Pb)r , and(207Pb/206Pb)i of the primary standard used in these normalisation calculations or is the input age of the primary reference material used as well? I suspect the input age of the primary standard is not used? But I'd love to just check!

I'm curious to know this because I'm doing some analyses using a reference material with a high Th/U ratio (the tara allanite) and therefore this material has some excess 206Pb in it produced by the decay of 230Th. This gives my primary reference an apparently older age than it's actual age. When measuring its age, a correction normally has to be applied to reduce its age slightly to account for the excess 206Pb. When using this tara allanite as my primary reference material, I have been using (206Pb/238U)r, (207Pb/235U)r , (208Pb/232Th)r , (207Pb/206Pb)r , and (207Pb/206Pb)i as measured directly in this material (not model values or values corrected for the excess 206Pb). If all the normalisation on iolite is done using these ratios (206Pb/238U)r,(207Pb/235U)r ,(208Pb/232Th)r ,(207Pb/206Pb)r , and(207Pb/206Pb)i measured directly in this material, then I think is valid to use it as a primary reference and there is no issue in using it even though it may have some excess 206Pb produced by excess Th. However, if the input age of my primary standard is somehow incorporated into the normalisation, then the mismatch between the actual age of the standard and the apparently older age due to excess 206Pb would probably cause an issue!

I hope all of this makes sense!! In summary, I'd love if you could share with me the specific calculations that iolite does to normalise isotope ratios of unknowns using the primary standard. I'm interested to know this because I want to know whether the presence of excess 206Pb in my primary standard would cause inaccuracies in the isotope ratios of unknowns measured using that primary standard.

Many thanks,

Chini

Joe

chini13

Hi Chini,

Although the python-based U-Pb example is significantly simplified from the built-in U-Pb DRS, the fundamentals are largely the same. So, let me point out a few parts of that code that I think will help to answer your questions. You can find the python example here.

The first part you should pay attention to is how the 'DC' ratio is calculated from the raw ratio, ~ line 140:

dc = rawRatio/(1 + (params[1]/params[0])*beamSeconds + (params[2]/params[0])*np.exp(-params[3]*beamSeconds))

where the params come from the fit. This just flattens out the raw ratio, but the value may be way off.

Next around line ~ 160:

rm = data.referenceMaterialData(settings["ReferenceMaterial"])
rmValue = rm[ratio['rmName']].value()
rmSpline = data.spline(settings['ReferenceMaterial'], ratioToCalibrateName).data()
finalRatio = (rmValue/rmSpline)*ratioToCalibrate

Here ratio['rmName'] would be something like 'Pb206/U238', settings['ReferenceMaterial'] would be something like 'Z_91500' and ratioToCalibrateName would be something like 'DC Pb206/U238'. Making those substitutions helps to see what is happening:

rm = data.referenceMaterialData('Z_91500')
rmValue = rm['Pb206/U238'].value()
rmSpline = data.spline('Z_91500', 'DC Pb206/U238').data()
finalRatio = (rmValue/rmSpline)*ratioToCalibrate

Here, ratioToCalibrate is the actual array of data for the 'DC Pb206/U238' channel. So, the final ratio is simply the RM value divided by the RM spline (an array of data that models how the RM DC 206/U238 data is changing throughout the session) multiplied by the entire array of DC Pb206/U238.

So, to answer your question: yes, it does just figure out the (time resolved) factor to multiply by to get the specified value for your reference material. And to answer your second question, no, the age is not used for the calibration, just the ratios.

All the best,

chini13

Hello Joe,

Thank you very much for the detailed answer!! That makes sense 😃

-Chini