a càrrec de John Fournelle, Cameron Electron Microprobe Lab, Dept of Geology & Geophysics University of Wisconsin, EE.UU.
Cicle de seminaris de la Facultat de Geologia i l'Institut de Ciències de la Terra "Jaume Almera".

Universitat de Barcelona (UB)
Consell Superior d'Investigacions Científiques (CSIC)

Dia: Dimarts 4 de març
Hora: 12:00 h.
Lloc: Sala d'actes de l'Institut Jaume Almera

Resum: I became interested in magmatic anhydrite in 1989 when working on samples from the 1986 catastrophic eruption of Nevado del Ruiz (Colombia), and when Mt. Pinatubo erupted with a massive cloud of SO2, I immediately guessed that anhydrite might be present in the pumices - and it was. I did some research, but then got my probe job and had to put aside that research. A few years later I found a student who was interested in crystallography and he agreed to do single crystal diffraction of volcanic anhydrite - something never done before. 300 micron samples were put on tape and imaged in the SX51, and EDS used to find the anhydrite grains. We immediately saw that the anhydrite had a" disease" - small round bumps over many surfaces. We acquired high resolution images and found them to be small ~micron pyramids - and we had no idea of their significance until a visiting probe lab director (and material scientist) said they resembled chemical vapor deposition artifacts. At that point the story became clear: what we were observing was a process we called "magmatic vapor deposition", where deep in the magma chamber a separate SO2-rich gas phase was precipitating small "hillocks" of anhydrite.
A few years later a professor called and asked if I would review a study a collaborator had made of small eutectic phases present in an annealed Nb-Pd-Hf-Al sample. He had been told that the Pd3Hf and Pd2HfAl had significant amounts of Nb in them, which he questioned. Upon looking at the sample and learning how the results had been determined (by EDS, running at 30 kV) I began to get suspicious that secondary fluorescence might explain these results. Using WDS, and evaluating the Nb La line at 18 kV, I got zero Nb, though if I used the Nb Ka line at 28 kV, I reproduced the ~10 wt% Nb the others had found. I used two approaches to prove secondary fluorescence was the problem: a difficult experimental approach, and the much easier "Barcelona" approach, which I will discuss. The lesson is that electrons behave well like a little child in a play pen, but once you have an x-ray, you have a wild teenager who thinks he can go wherever he wants whenever he wants.