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June 2008

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JEOL-Focused Probe Users List <[log in to unmask]>
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Wed, 11 Jun 2008 10:27:17 -0500
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Ellery Frahm <[log in to unmask]>
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"Andersen, Jens" <[log in to unmask]>, Travis Tenner <[log in to unmask]>
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*

Hi Jens,

There didn't appear to be boron in the light element scans.  As for  
Li, I've been assured it is unlikely because there isn't a probable  
source in their experiments.  Also, because Li is so light, the  
analyses for oxygen would've picked up half of the mass for Li2O -- I  
don't that he's seeing that when he analyzing for oxygen.  The same  
for boron... But I'll suggest that he double-checks this.

I think he's also considered trivalent iron, but I'll suggest that he  
double-checks this possibility too.

Thanks,
Ellery


On Jun 11, 2008, at 10:14 AM, Andersen, Jens wrote:

> Have you tested for the valency of Iron? Trivalent Fe would give low
> totals, right? Any boron, lithium???
>
> Jens
>
> ____________________________________________________
>
> Jens C. Andersen
> Senior lecturer
> Camborne School of Mines
> School of Geography, Archaeology and Earth Resources
> University of Exeter
> Cornwall Campus
> Tremough
> Penryn
> TR10 9EZ
> UNITED KINGDOM
>
> Tel. +44 1326 371836
> Fax. +44 1326 371859
>
> http://www.exeter.ac.uk/csm
>
> Visit the virtual Skaergaard intrusion: http://www.skaergaard.org
>
> -----Original Message-----
> From: JEOL-Focused Probe Users List  
> [mailto:[log in to unmask]] On
> Behalf Of Ellery Frahm
> Sent: 11 June 2008 16:02
> To: [log in to unmask]
> Subject: [PROBE-USERS] Hydrous melt analysis boggle
>
> JEOL Probe Users Listserver
>
> Moderator: Ellery Frahm, [log in to unmask], Electron Microprobe Lab,
> University of Minnesota
>
> Post a message: send your message to [log in to unmask]
>
> Unsubscribe: send "SIGNOFF PROBEUSERS" to [log in to unmask]
>
> On-line help and FAQ: http://probelab.geo.umn.edu/listserver.html
>
>
>
> *
>
> Probe folks,
>
> A researcher in our lab (an experimental petrologist) is trying to
> analyze a few of his samples (experimental andesitic melts/glasses
> containing between 3% and 6% water), and he is trying to (1) get good
> totals and (2) correlate measurements of the water content using FTIR
> and oxygen analysis in the microprobe.  He is getting a linear fit
> between water contents from the FTIR and microprobe, but his totals  
> are
> low on the probe when we analyzes his samples as both oxides and  
> metals.
> He is an experienced user, and we've gone through all of the usual
> procedures for testing the causes of low totals -- nothing.
>
> The FTIR measured total water contents ranging of 3.37, 4.12, 4.54,
> 5.25, 5.45, and 5.88 to 5.9 wt%.  When we analyze the glasses with the
> electron microprobe, assuming a deficit from a 100% total is  
> attributed
> to H2O, the calculated water contents are 5.29, 6.45, 7.12, 7.75,  
> 7.56,
> and 9.21 wt. %, respectively.  So his totals are about 5% low,
> occasionally more, causing overestimation of water.
>
> Here is what he has tried (based on the notes he's given me):
>
> 1)  Periodically aligned beam and checked for astigmatism on  
> willemite.
> 2)  Beam conditions: 10 nA current, 30 micron spot size.
> 3)  Always analyzed Na first (monitored counts rates, found Na counts
> did not drop for at least 30 seconds).
> 4)  Analyzed for oxygen using three different standards: enstatite,
> pyrope, and almandine -- all yielded similar results.
> 5)  Counted the area under the peak for oxygen.
> 6)  Adjusted the detector settings for oxygen: PHA Gain: 32, High V:
> 1708, Base L. (V) 0.70, Window: 2.5, Mode: Diff.  Set up this way to
> filter out high-order Al and Na peaks.  WDS scans indicated these  
> peaks
> were eliminated by this filtering method but had little effect on
> improving the water contents of the glasses when compared to an
> "unfiltered" method.
> 7)  Set oxygen backgrounds based on WDS scans and software overlap
> database: Back +: 12.000 mm, Back -: 6.500 mm.
> 8)  Total counting time on oxygen: ~44 seconds.  Other elements: 10
> seconds for peak, 5 seconds for background.
> 9)  Elements present in samples (and analyzed for): Si, Ti, Al, Cr,  
> Fe,
> Mn, Mg, Ca, Na, K, O, H (known from FTIR)
> 10)  Full WDS scans (including light elements) revealed no missed
> elements that could have led to low totals.
> 11)  Used basaltic glass to standardize MgO, CaO, FeO.  Rhyolitic  
> Glass
> (basaltic glass) for Al2O3.  SiO2 Glass (Quartz, basaltic glass,
> rhyolitic glass) for SiO2.  Benitoite for TiO2.  Chromite for Cr2O3.
> Mn-Hortonolite for MnO. Omphacite (albite) for Na2O.  K-Spar for K2O.
> Standards in brackets were also tries, and produced worse results.
> Standardized as oxides, and then measured the unknowns as a metal.
> Oxygen was standardized as a metal on the enstatite standard, and
> analyzed as a metal on the unknowns.
> 12)  Sample thicknesses were 100 microns or greater.
> 13)  Two carbon coat thicknesses tried: about 50 and 200 angstroms --
> produced similar results, so charging isn't a problem.
> 14)  Glasses were free of quench crystals, so the interaction volume
> should be homogeneous.
> 15)  Moving elements to different spectrometers had no effect on
> improving the results.
> 16)  Using these conditions, analyses on two different basaltic glass
> standards yielded matching elemental concentrations.  These glasses  
> were
> relatively dry (a few tenths of a wt.% H2O) compared to the  
> specimens in
> question.
> 17)  This technique was also performed on hydrous rhyolitic glasses  
> with
> 1.3, 3.3, 4, 5, and 6 wt. % H2O (determined by FTIR) and produced the
> similar water contents (1.06, 3.11, 4.74, 5.65, and 7.24,
> respectively) and overall chemistry.
>
> The only real issue I see above is the benitoite as a Ti standard
> because it has CL, but the TiO2 content of these glass samples isn't
> high enough to account for the difference.  Because these glasses are
> amorphous (presumably), even most of my usual "exotic" explanations  
> for
> low totals are ruled out, like X-ray polarization between the crystals
> in the sample and the dispersing crystal.  I'd like to have a better
> explanation for these low totals other than "well, something is  
> causing
> the electrons and/or X-rays to be more strongly absorbed or behave  
> in an
> unexpected way" -- that's not very satisfying.
>
> So I need to get some fresh ideas or find out if others have had  
> similar
> problems with hydrous andesitic melts.  Anyone have an ideas about  
> what
> to try next or what the problem might be (even if we can't fix it)?
>
> Thanks,
> Ellery
>
> --------------------
> Ellery E. Frahm
> Research Fellow & Manager
> Electron Microprobe Laboratory
> University of Minnesota - Twin Cities
> Department of Geology & Geophysics
> Lab Website: http://probelab.geo.umn.edu Personal Website:
> http://umn.edu/~frah0010

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