Tuesday, February 17, 2009

Laser ICPMS imaging

ResearchBlogging.orgOne of the big developments in geology over the past decade or so is to turn everything into a pretty picture. If it isn’t imaged, it’s mapped, and if it ain’t hyperspectral it needs to be saturated in coffee.

In one sense, this is just a continuation of a trend that started around 1800 with the first geologic map; it’s simply the best way to present complex spatial information. What makes the last decade or so different is simply the scale, speed, and breadth with which this has been applied.

For me, the turning point was EPMA element mapping. This is where the geologist goes out on the town, and while he or she is partying all night long, the electron probe maps out the elemental variation of a thin section. In the morning, the hangover is vanquished with a pretty picture showing such crazy things as REE zoning in optically homogenous garnets, or alteration features that render a year’s worth of work pointless.

In order to map things easily and rapidly, a few things are required. First, you need a fast analytical method. If you’re taking tens to hundreds of thousands of data points, you can’t spend all day on each one.

Secondly, you need to know where you are. You can’t make a map on any scale without positional information.

Thirdly, you need a computer to process the data so that all data is somehow normalized to a useful value, and all the bits are put together in the correct places.

Woodhead et al. show how all this can be done using laser ICPMS. The key for 1 is rapid washout time, and for 2 and 3 are computer-controlled stage operation. They did a bit of extra homework, though, and ran several experiments to show that ejecta resampling produced negligible signal contamination on the scale of their measurements. This is important with destructive mapping techniques, because the sample quality can be compromised by annihilating it with laser beams.

As their example experiment, they show some lovely stalactite images, with U-Mg-Sr spectral maps. You’ll have to read the paper- or guess- to learn which element the vertical axis represents.

Jon D. Woodhead, John Hellstrom, Janet M. Hergt, Alan Greig, Roland Maas (2007). Isotopic and Elemental Imaging of Geological Materials by Laser Ablation Inductively Coupled Plasma-Mass Spectrometry Geostandards and Geoanalytical Research, 31 (4), 331-343

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