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Hi Silvio

Some things never go away in micro-analysis (other than Na!!).

This is an old problem which keeps recurring. To answer your question properly we would need to know if you are using EDS or WDS techniques. John Clemens assumes that you are using EDS. However, your reply to Ioannis suggests to me that you are using WDS. A strategy for WDS analysis could be very different from that for EDS analysis.

JC correctly suggested a number of potential options, as did Julien. However, as both flag up, my favoured option would be to use a cryostage. There are plenty of options out there. However, they do tend to limit what you can do routinely with the instrument. (Bulky things on a specimen stage with gas tubes etc do not help any normal analytical or imaging procedures). When running a Faculty instrument we used to have "cryo weeks" when we could do analysis and imaging that required a cold stage. Put it on, calibrate it, run the required samples, take it off and go back to proper usage.

I am though intrigued that you are losing 90% of your anticipated Na total. You do say that you are analysing melt inclusions in garnet. Are these experimental products or natural ones? In either event I wonder if the glass has already devitrified prior to analysis. There could be Na loss during devitrification. Alternatively, devitrified glass could be less stable under the beam than non-devitrified glass.

Have fun

Peter

From: Metamorphic Studies Group <[log in to unmask]> on behalf of Ralf Milke <[log in to unmask]>
Sent: 30 January 2019 16:52
To: [log in to unmask]
Subject: Re: severe alkali-loss during microprobe analyses of low alkali andesitic glasses

Hi Silvio,

did you try pulsed measurements? By integrating 2-second pulses I once
could reduce the spot size from 10 to 4 µm on albite compared to standard
5-second-measurements and still get the same results, Na loss started at 3
µm.

Cheers, Ralf

> Hi Ioannis,
>
> I always measured it as first element...
>
> Cheers
>
> Silvio
>
> On 30-Jan-19 4:16 PM, Ioannis Baziotis wrote:
>> Just an additional simple thought: at which sequence do you measure Na?
>> Other than first may produce severe lossing. Measure it first and the
>> rest of the elements.
>> ----------------------------------------------------------------
>> Ioannis Baziotis [log in to unmask]
>> Assistant Professor of Mineralogy and Petrology
>> Agricultural University of Athens
>> Laboratory of Mineralogy and Geology
>> Athens, 11855 Greece
>> (+30)210-529-4155
>> Twitter: https://twitter.com/EGU_GMPV

EGU GMPV Division (@EGU_GMPV) | Twitter

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The latest Tweets from EGU GMPV Division (@EGU_GMPV). Geochemistry, Mineralogy, Petrology, Volcanology Division (GMPV) of the European Geosciences Union (EGU). Munich, Germany

>> Personal site: http://ibaziotis.wix.com/ioannis-baziotis
>> ----------------------------------------------------------------
>>
>> ----- Original Message -----
>> Από: Allaz Julien <[log in to unmask]>
>> Ημερομηνία: Wednesday, January 30, 2019 8:57 am
>> Θέμα: Re: severe alkali-loss during microprobe analyses of low alkali
>> andesitic glasses
>> Προς: [log in to unmask]
>>
>>
>>> Hi Silvio,
>>>
>>> Yes, Na (or K) in hydrous glass can be tricky. Using the EDS and a low
>>> current is probably the safest way to get decent analysis (we do this
>>> regularly on our 30 mm2 Thermo Ultradry detector at 1.5 to 2 nA and 15
>>> keV), but if you work on the electron microprobe here are a couple
>>> suggestions (in addition to the one already suggested, notably the use
>>> of a cryostage which is your best chance):
>>>
>>> 1) Lower the electron beam even more - 2 nA and 5 um is our normal
>>> routine for very sensitive and small-sized material, especially
>>> hydrous alkali glass, and it is acceptable in most cases, even highly
>>> hydrous andesitic glass.
>>>
>>> 2) Lower the counting time on Na. I had in the past analysed even more
>>> challenging minerals (jarosite and alunite, which are alkali-rich
>>> hydrosulfate), and the most accurate analyses were obtained with a 3
>>> second counting time on Na at 2 to 3 nA with an almost focused beam
>>> (we couldn’t even afford a 5 um beam size due to strong zoning in
>>> these 10 um sized minerals). Beyond this, the analyses were
>>> inaccurate… Such results are very low precision, but you have here to
>>> choose between accuracy and precision, and once again, if you can get
>>> multiple analysis point, then the average will be more acceptable in
>>> term of precision.
>>>
>>> 3) If you are using a Cameca instrument, and if you have access to the
>>> PC0 crystal (2d = 45 Å), then go for it! You will have a much higher
>>> count rate, and can get very good results even with just 3 seconds
>>> counting time.
>>>
>>> 4) Use a Time Dependent Intensity correction. I believe JEOL and
>>> Cameca now offers this option (on the newest instrument at least, and
>>> often as an additional software package), otherwise it is available on
>>> the software "Probe for EPMA” from John Donovan.
>>>
>>> 5) If none of the above works, lower even more the beam current. I
>>> would strongly advise to monitor the count rate of Na using the Chart
>>> Recorder and determine at which point the Na intensity is decreasing.
>>>
>>> Note that I am not certain that lowering the acceleration voltage
>>> would help: doing so, you will strongly reduce the analytical volume,
>>> and the beam damage will be increase. The damage is a function not
>>> just of the counting time and the current, but also the impacted
>>> volume (electron density). If you increase the electron density, you
>>> will increase the heat effect and thus the damage…
>>>
>>> Best,
>>>
>>> Julien
>>>
>>>
>>> P.S. You can also address your question on one of the following
>>> probe-specific email list which regroup many microprobe users:
>>> - Cameca: [log in to unmask]<
>>> - JEOL: [log in to unmask]<
>>>
>>>
>>> ###########################
>>> Dr. Julien Allaz
>>> Head assistant for SEM/EPMA
>>> Inst. für Geochemie und Petrologie
>>> ETH Zürich
>>> NW E 84
>>> Clausiusstrasse 25
>>> 8092 Zürich
>>> Switzerland
>>>
>>> Tel: +41 44 632 37 20
>>> Fax: +41 44 632 16 36
>>> Email: [log in to unmask]<
>>> ###########################
>>>
>>>
>>> On 30 Jan 2019, at 13:16, Clemens, JD, Prof [[log in to unmask]< wrote:
>>>
>>> Hi Silvio,
>>>
>>> I am guessing that the granitic glasses you were referring to may not
>>> have been hydrous or perhaps they had much lower H2O contents than
>>> your andesitic sample. Na counting losses correlate positively with Na
>>> abundance and with H2O content of the glass.
>>>
>>> I assume you are using EDS. You can virtually eliminate the problem if
>>> you:
>>>
>>> lower the beam voltage (10 kV?)
>>> move the nose piece of the detector closer to the sample to get more
>>> counts and shorten the time for analysis
>>> most critically, use a cooling stage to lower the sample temperature
>>> to that of liquid N2.
>>>
>>> I have tested an albite-composition glass with 9 wt% H2O in it under
>>> these conditions and easily got stoichiometrically perfect Ab analyses
>>> out of it.
>>>
>>> So, you may have to invest in the freezing stage if you want to
>>> continue using a finely focused beam. There really is very little
>>> choice about that.
>>>
>>> I don’t know who might make these things, but people do have them.
>>> Manchester and Stellenbosch both have (or had) such facilities, for
>>> example.
>>>
>>>
>>> Prof. John D. Clemens
>>> Dept of Earth Sciences
>>> University of Stellenbosch
>>> +27 (0)21 808 3159
>>>
>>> On 30 Jan 2019, at 13:44, Silvio Ferrero
>>> <[log in to unmask]< wrote:
>>>
>>> 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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> --
> 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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>
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