From the archives: Big numbers from tiny grains

By Kate Boston (originally posted 21st December 2011)

One of the best things about my PhD project is all the cool machines I get to play with. Today I am running a Sensitive High Resolution Ion Microprobe (SHRIMP).  It is one of the more impressive gadgets I get to use.  And what a gadget!  It is a huge, silver, futuristic, impressive-looking  machine with tangles of tubes and wires connecting this thing-amy to that whats-it.  It has switches with labels like “duoplasmatron” (is it just me or does duoplasmatron sound like a space-age weapon or possibly the precursor to the Flux Capacitor?).

 So what does this awe-inspiring contraption do?  This very big machine takes a very small bit of a mineral (we’re talking a 20-30 micron spot size) and analyses the isotopes in it.  I use this for Th-Pb and U-Pb dating.  And the best bit – I can use it to find out how old different growth zones in the same mineral are. This is pretty excellent because I am interested in the rates at which processes like metamorphism occur.
In an ideal world one would simply put a sample in a machine and find out how old it is.  In the real world it will takes you weeks of preparation to get a sample ready to analyse.  Before samples are even crushed to make mineral separates, the thin sections need to be studied to see which of the samples are likely to have datable minerals of the relevant age (for example I am looking at Alpine metamorphism – I don’t care at all about anything older than about 40 million years).  Only then can samples be crushed and separated.  50 to 100 of the target mineral grains are then individually picked under a microscope to make an epoxy mount (picking minerals is no one’s favourite job).  The epoxy mount is then polished, cleaned, coated, imaged (using a scanning electron microscope), cleaned and gold coated before finally going into the machine.  Then finally (if your analyses work well and the machine behaves) will you possibly get some meaningful results.

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