VizualAge UComPbine DRS and the Reference Materials data

Chengkaide

Hello everyone
I have some question about the VizualAge UComPbine DRS.

If I use VizualAge UComPbine DRS, the Reference materials data should include the radiogenic Pb ratio value or not?
Because Chew et al., 2014 mentioned "The values for the radiogenic and common Pb ratios are retrieved from the appropriate Iolite standard file".
But in MAD standard there is a “c207Pb/206Pb”, I want to know iolite will calculate the radiogenic Pb ratio from age or not?
Can I use any materials which have ID-TIMS ages, U-Pb measured ratios to be one standard like rutile?

Joe

If you're using UComPbine, your reference material file should contain the radiogenic U-(Th)-Pb ratios as well as whichever common Pb ratios are required depending on your settings. If you're using Stacey-Kramers composition, they need to have an "Age" defined. Otherwise, C207Pb/206Pb is needed for a 207Pb-based correction, and C206Pb/204Pb, C207Pb/204Pb, C208Pb/204Pb are needed for a 204Pb- or 208Pb-based correction.
The radiogenic composition should also be specified and will not be worked out from the specified age.
Yes, you can use any material for which you know the required information.
Yes, the 207/206 reference is to allow you to use a different material to calculate the 7/6 ratio.

All the best,

Chengkaide

What the meaning of the 207Pb/206Pb reference in VizualAge UComPbine?
It means we can use another materials to calculate the 207Pb/206Pb ratio? Like SRM 612.

tara_unicorn

Hi, I'm jumping on this thread as I am having an issue with this DRS... it keeps throwing up an error saying it could not find the common lead ratios for the reference material. I am using nist614 for the pb/pb correction and in the RM data file the common lead ratios are listed like this:
C207Pb/204Pb
C206Pb/204Pb
C208Pb/204Pb
I have tried restarting the program, trying to run it on a different session but nothing works. Am I missing something?

Thanks!

Bence

Hi Tara,

You can just list the common Pb ratios for the NIST614 glass without the "C" in front. However, the DRS will look for the "207Pb/206Pb" ratio in particular, so you'll need to add that.

Kind regards,
Bence

tara_unicorn

Thanks Bence,

I have taken out the C so it just reads 207Pb/206 Pb but the software is continuing to throw up the same error message. I have tried multiple files, deleting the RM file and starting fresh, restarting software etc. Any suggestions?

Bence

Hi @tara_unicorn

I think the issue here is that you need two types of ratio for UCPb:

Pb/Pb ratios for your primary reference material to model the downhole fractionation (quite often the same mineral as your unknowns, eg apatite, etc), and
one 7/6 ratio for correcting Pb bias (quite often a NIST glass) and

For the downhole correction setup, your RM will likely have common Pb ratios listed with the "C". This is because that composition might not actually exist in your sample, but rather is a calculated value to tie one end of the data through the common Pb intercept.

For the Pb bias correction, you need Pb ratios without the "C" in it. That's because these materials pretty much only contain common Pb, and materials for this correction (e.g. NIST glasses) mostly only have the Pb ratios listed without the C by convention.

So, in summary: Add "C" to the Pb ratios of your primary RM, and leave it out for the mass bias material.

I hope that makes sense. Please let me know if you have any questions.

Kind regards,
Bence

lsday1

Hi, I was having a similar problem attempting to apply a 204Pb correction to my data. I managed to get it to run by adding C in front of 207Pb/204Pb, 206Pb/204Pb and 208Pb/204Pb in the Z_91500 reference material data file. However, the correction appears to make my data even more poor - I am wondering if this is something I am doing wrong when running the DRS or if it simply isn't applicable to my dataset?

Thanks

Bence

Hi @lsday1

Were your data collected on a multi-collector? If not, your 204Pb data might be so noisy as to make the 204Pb correction ineffective, especially if there is no 204Hg correction.

Kind regards,
Bence

LK0095

Hi,

I have encountered an interesting problem with this DRS.

I am processing an allanite U–Th–Pb dataset. One of the NIST610 analyses shows an anomalously low raw 207 Pb CPS. When I select NIST610 as the 207Pb/206Pb reference (see the attached figure), some unknown analyses located close in time to this anomalous NIST610 analysis show 207Pb_CPS intermediate values that are even higher than the raw 207Pb input values.

At first, this seemed to imply an abnormal baseline correction. However, I checked the baseline spline, and it is around 53 CPS, which is a normal positive value.

I then excluded this anomalous NIST610 analysis from the NIST610 group and reran the DRS. After doing so, both the 207Pb_CPS intermediate values and the final 207Pb/206Pb ratios of the affected unknowns became normal. The results are also normal if I select “None” instead of NIST610 for the “207Pb/206Pb reference”.

This makes me wonder whether the “207Pb/206Pb reference” correction is applied across the whole session, including both the primary reference material and the unknowns, and whether the 207Pb_CPS intermediate channel is therefore not simply a baseline-subtracted signal, but has also been modified by the NIST610-based Pb/Pb instrumental correction.

If so, could you please explain how this correction is implemented? In particular, I would be very grateful if you could provide the relevant equations or describe how the NIST610 correction factor is propagated through the dataset.

I hope I have explained the problem clearly. I have spent several days trying to understand this behavior.

Best regards,
Kai

Bence

Hi Kai,

You have identified one of the very few times in iolite where a channel is modified "in place" rather than by creating a copy and adjusting the copy. In this case Pb207_CPS initially contains the baseline-corrected Pb207 intensity. The values in this channel are then adjusted by the mass bias correction. If you had an anomalous NIST610 analysis, this would affect the Pb207_CPS channels in this region. The relevant equations equations for this process are:

1/ The Pb mass bias factor is determined by:

f = r / s

where f is the mass bias correction factor, r is the accepted 207Pb/206Pb value for the reference material and s is a spline of the raw 207Pb/206Pb ratio for the reference material. Note: the reference material mentioned here is the one selected using the "207Pb/206Pb reference" option in the DRS settings (NIST610 in your example).
This mass bias factor is not the Russell equation, but it should be precise enough for quad-based Pb isotope ratios.

2/ The Pb207_CPS values are then updated using
Pb207 = Pb207 * f

In your case, the anomalous NIST values would create an anomaly in the spline, which would then affect the calculated mass bias factor, which would then overwrite the Pb207_CPS values.

I hope this helps. Please let me know if you have any questions.

Bence