Wednesday, March 22, 2017

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.

What actually happened is that the second author and I realized that we had different pieces of the puzzle which, with the help of some old Japanese data, could be pieced together for a coherent story. So hey, "write it up."  Most of my part of the puzzle was unpublished bits and pieces from my PhD and post doc 15+ years ago, but the SHRIMP data was actually less than a year old, as I had collected it for an entirely different reason.

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.

No comments: