Routine science turns clever- laser ICP vs SHRIMP analysis of Archean detrital zircons
So, last year I published a Geology paper. It is summarized
in Geosonnet 42; see link therein to the paper itself. As it turns out, the
paper deals with Archean uranium mobilization and the sedimentary history of
carbonado diamond. But what the paper doesn’t say is that I wasn’t actually
trying to do that. More professional researchers than I might know how state in
their articles that it was all just a lucky coincidence, but I don’t know how
to squeeze that into a short format journal.
Back when I was working at ASI, which had just bought the Resolution
laser ablation line from Resonetics, a few of us started looking at how the
SHRIMP and laser products could best compliment each other. One of the things
we experimented with was controlling the SHRIMP with a version of the laser
control software. Another thing we wanted to know was whether there was any
advantage to using the SHRIMP for detrital zircon provenance studies, so I
pulled out my old PhD zircons, remounted them with modern standards, and we
programmed a customized version of GEOSTAR to automatically rerun the same
zircons (if they hadn’t been blown up) to compare the results. Of course, the
laser data was old, and the SHRIMP was trying to make analyses next to laser
holes (which distort the extraction field, due to the unfortunate tendency of
holes not to be flat), but it generally worked, and the data is tucked away
deep in the supplementary section of the paper.
Since there are analytical geochemists who occasionally read
this blog, but might not think to look for microbeam comparisons in the
appendix of a diamond radiation defect luminescence paper, I thought I’d
mention it, and put up some plots that got culled due to space requirements.
The short answer is that fully metamict zircons (like half
of the Tombador grains) are open system with either technique, but for zircons
that are only a little bit metamict (most of the Jacobina zircons), the smaller
ion probe spot and better 204Pb backgrounds improve data quality. Anyone who is
interested is welcome to download the Data Repository data (it’s all there) and
ask.
Figure 1 (See data repository for full version): Tombador
zircon analyses with SHRIMP (red) and laser ICPMS (yellow). The SHRIMP data
are, in general, a little more concordant, but there isn’t much in it.
Figure 2 (See data repository for full version): Jacobina
zircon analyses with SHRIMP (red) and laser ICPMS (yellow). For this sample,
the SHRIMP data are substantially more concordant.
Figure 3: Probability
distribution curves for Tombador zircons analysed by SHRIMP (purple) and laser
(Red).
Figure 4: Probability
distribution curves for Jacobina zircons analysed by SHRIMP (tan) and laser
(Red). Note that laser peaks are generally broader and offset to younger ages
due to Phanerozoic Pb loss.
