Downhole corrected UPb ages and general workflow of the U-(Th)-Pb Geochronology DRS

I was recently asked on the forum to explain the downhole-corrected ages produced by the UPb DRS, and Luigi suggested that I turn my reply into a blog post – so here it is…

To give a bit of context (the original forum thread can also be found here), Luigi noticed that his downhole-corrected ratios (e.g., “DC_206_238”) and their related ages were quite inaccurate, and was naturally concerned that this was affecting his results. He observed that the raw ratios were reasonably close to the accepted ratios, but that despite having significantly better uncertainties, his downhole-corrected ratios were about double what they should be, and that his final ratios and subsequent ages were nevertheless coming out with the right numbers.

And here’s my reply (augmented a bit and with some pictures added):

What you’re observing is perfectly ok, it’s a natural consequence of the different steps of processing that the DRS uses.

The channels for ratio calculations are divided into three groups – “raw”, “DC” (down-hole corrected), and “final”.

Raw ratios
The raw ratios are hopefully pretty obvious – they’re the observed ratio, and are generated by simply dividing one baseline-subtracted intensity (e.g., 206Pb) by another (238U). Here’s an example of the raw 206/238 ratio from a single spot analysis, showing clear down-hole fractionation in the ratio as the analysis progresses:

 

Down-hole corrected ratios
The next step is obviously the one that’s causing the confusion – down-hole corrected ratios are corrected for fractionation induced by drilling an increasingly deep laser pit into the sample (also referred to as LIEF, which stands for laser-induced elemental fractionation).
In the Iolite DRS, this correction is made using a model of downhole fractionation that is produced interactively by the user using the reference standard analyses (I’m assuming that if you’re reading this you’ll know the general concept – if not then I’d suggest reading our 2010 G-cubed paper). Now here’s the punchline – the correction is typically made using an equation (in Iolite it’s an exponential curve by default), and depending on the type of equation used, either the y-intercept or the asymptote will be the reference point from which the correction is made. So in the case of a linear fit the correction will be zero at the y-intercept, and increase linearly with hole depth.

A slight twist on this is the y-intercept method, which regresses the data back to its intercept with the y-axis (where downhole fractionation is assumed to be zero, or at the very least constant between the reference standard and the sample). This obviously also results in ratios being corrected down to their starting value.

The result of both of these is that the slope of the raw ratio gets flattened down to the value that was measured at the start of the ablation. This typically results in down-hole corrected ratios that are reasonably close to the accepted values.
In contrast, an exponential equation will be flattened up to the asymptote, meaning that the observed ratios will change quite a lot as they’re shifted up towards the values measured at the end of the analysis, or likely even higher.

Of course this then means the down-hole corrected ratios will be much higher than the accepted values. I know that as you read that you’re probably screaming in horror, but it’s ok, it doesn’t make any difference to the end result. The down-hole corrected ratios have been flattened, and whether they’re accurate or not is not yet relevant.

Final ratios
This is where the reference standard spline is used to normalise down-hole corrected ratios to the accepted values of the reference standard. If for example the 206/238 ratio of the reference standard is twice as high as it should be then all 206/238 ratios in the session are halved to correct for this bias. Of course by using a spline any variability within the session can also be accounted for. It is this correction that also absorbs the high values potentially produced by using an exponential equation – if all of the flattened DC ratios were corrected 15% too high, then this step will bring them back down to accurate values.

So to bring it back to your specific questions – the DC ratios are “flattened”, and that’s all. The intention of this step of data reduction is to remove the down-hole effect that systematically affects every analysis, so the end result should be ratios that do not vary with hole depth (unless they really varied in the sample of course!).

The reason that you noticed a decrease in uncertainties relative to the raw ratios is that the effects of downhole fractionation have been removed, and that the resulting analysis has less variability (i.e., it’s flat, not sloped). So it’s a very good thing that the uncertainties are smaller, and a sign that the downhole correction is beneficial.

And finally, just to address the mention you made about dispersion – if a good downhole correction model is used then each analysis should be flattened, which is great. However, scatter between analyses is only minimally affected by this correction, so if you’re seeing a lot of scatter in your DC ratios (or final ratios) then this is most likely real, and not something that you will be able to fix by playing with the downhole fractionation model. The variability may be due to differences in ablation characteristics between zircons (e.g., the effect you’re seeing of systematically older/younger ages between 91500 and plesovice, which is very commonly observed). Or it may be due to changes in the conditions of ablation (e.g., different gas flows in different parts of the cell, different heights of the sample relative to the laser focus depth, etc.).
Note also that at least some of these causes of scatter will not be identified by the uncertainty propagation, and the only way to really get a grip on your overall uncertainty is by extended tests using a range of reference zircons.

If you have any questions about the above feel free to post questions either here or on the forum.

And we’re open to suggestions for future blog posts, so if there are any other topics you’d like to know more about feel free to make a request!