First the samples had to be coated and the machine had to be calibrated. But before the calibration Donald had a look at the sample. And he didn’t like what he saw... the grains weren’t polished well. Some we wouldn’t be able to measure at all, and the best ones we could have a try at wouldn’t give very precise results. But we could give it a go... who knows what you find.
The sample is being coated. Notice the spooky reflection in the glass...
Calibration takes quite a while, but at some point I could start. I held my breath! We were looking for ash from a specific eruption, and we knew it was high in titanium and iron. In microprobe terms that means: a bright particle. The first turned out not to be an ash shard at all. The next was so badly polished we didn’t get a good measurement. This was going well.
I measured on. Some shards measured OK, but they always had the wrong chemical signature. Figures; most shards would be local, or spewed out by volcanic systems as Reykjanes, Torfajökull or Veidivötn. And who wants those. It was a bit disheartening. The fact that I was only just comfortable in two pairs of trousers, three T-shirts, a jumper, a scarf and a jacket may not have helped.
The machine!
Just because I had nothing to lose I measured on. I knew we, or rather, Donald, would have to come up with a better way of mounting and polishing these shards, and when that would be done I should come back and give it a proper go, with a whole sample full of measurable shards. But seeing what I had here wouldn’t hurt, now would it.
We had two types of samples. We had our normal salt marsh sediments, in which you would find all kinds of grains, and we had also taken a core in a peat bog; the sediment there is almost all organic. What isn’t would logically be ash. So in that core the ash would be easier to find... if we wouldn’t find it there we should give up; no way we’d find it in the salt marsh sediments. But if we would...
At 15.30ish I screwed up a measurement. Donald had a look, found the flaw, found a reasonably polished grain, and just aimed the beam of the machine at it. And walked away. When the results appeared on screen I gave a start. High iron and titanium!
We had 1.5 hours of lab time left. We had to think quick. I phoned Roland with the good news, and we decided it was really worth trying to prepare these samples better and come back for another go. So what to do now? With Donald I decided I should pick more, lots more grains from this same sample. He would try to develop a method to get them polished well. If it’s done well you can easily pick out the promising shards by their specific brightness. So then you can just measure away and get it done in no time! With more precise results.
A pretty picture, bt not what you want to see; they're all supposed to look homogeneous. All topography you see if bad polishing...
I left the microprobe lab, picked hundreds of shards, placed the sample in his lab (it was past 6 by then; he himself had gone home) and went for dinner with one of the local postdocs. From October on we’ll both be employed within the same project! I think I’ll see her again in the south...
I had seen the spark in Donald’s eyes; he’s keen now to get this work with very small ash particles working. And so am I! I’m really looking forward to the next step. He’ll undoubtedly find a way of making brilliant slides, and then we can measure away. And hopefully find that ash layer in the core it’s all about! And that will be one hard won data point, but it will have been worth it...
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