Dear All,
We are experiencing almost complete alkali loss during microprobe
analyses of andesitic glasses with narrow beam (diameter around 1
micron), at 9 nA and 15 kV. The glass under investigation is hydrous
(around 5 wt% H2O), and based on other data we were expecting 1-2 wt%
Na2O. Our analyses show instead a remarkably constant amount of 0.05-0.1
wt% Na2O over 20 analyses. Analyses on granitic standards during exactly
the same session show a very limited loss for Na (around 20% relative),
and in both cases the Na is only measured for 6 seconds. Few analyses
were conducted with diameter of 3-4 microns, and still the Na was in the
same range of values. Larger beams cannot be used because we are
analyzing small melt inclusions in garnet.
I´m fully aware of the alkali loss problem in alkali-rich glasses, but I
never experienced a almost complete (around 90% relative I would say)
loss of Na in any previous case study. Anyone has experienced such a
problem analyzing hydrous andesitic glasses with very narrow beams?
Any insights, suggestion or comment is welcome!
Silvio
--
Silvio Ferrero
tel. 0049(0)3319775705
Universität Potsdam
Institut für Erd- und Umweltwissenschaften
- Dept. of Earth and Environmental Sciences -
Haus 27, Karl-Liebknecht-Straße 24-25
14476 Potsdam
"In those early days people laughed at me. They quoted Saussure who had
said that it was not a proper thing to examine mountains with microscopes,
and ridiculed my action in every way. Most luckily I took no notice of
them"
(Henry Clifton Sorby)
To many petrologists a volatile component is exactly like a Maxwell
daemon; it does just what one may wish it to do.
(The evolution of the igneous rocks, N.L. Bowen, 1928)
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