January 2009


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Ellery Frahm <[log in to unmask]>
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JEOL-Focused Probe Users List <[log in to unmask]>
Mon, 19 Jan 2009 19:26:11 -0600
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I've been doing a lot of analyses of Smithsonian Standard VG-568, USNM  
72854, Rhyolite Glass, Yellowstone National Park.

For years, our lab has simply used the elemental concentrations  
published in:  Jarosewich, Nelen, and Norberg, 1980, "Reference  
Samples for Electron Microprobe Analysis," Geostandards Newsletter,  
vol 4, no 1, pp. 43-47.

Those values are:

SiO2	76.71
Al2O3	12.06
FeOT	1.23
MgO	< 0.1
CaO	0.50
Na2O	3.75
K2O	4.89
TiO2	0.12
P2O5	< 0.01
MnO	0.3
H2O	0.12

Unfortunately very little information is included in that publication  
about these data, including the estimated error.

After doing my own repeated analyses of VG-568, I sought out other/ 
updated compositional data because my data was a bit different.

I found some reported values in Streck and Wacaster, 2006,  
"Plagioclase and pyroxene hosted melt inclusions in basaltic andesites  
of the current eruption of Arenal volcano, Costa Rica," Journal of  
Volcanology and Geothermal Research, vol 157, pp 236-253.

The Streck and Wacaster data for VG-568, based on nine analyses, are:

SiO2	76.96  0.53
Al2O3	12.17  0.12
FeOT	1.08  0.05
MgO	0.03  0.02
CaO	0.45  0.03
Na2O	3.52  0.11
K2O	4.93  0.20
TiO2	0.08  0.03
P2O5	0.00  0.01
MnO	0.02  0.02
S		20  16 ppm
Cl		1013  53 ppm

The only other relevant data set I found was Rowe, Thornber, Gooding,  
and Pallister, 2008, "Catalog of Mount St. Helens 20042005 Tephra  
Samples with Major- and Trace-Element Chemistry," U.S. Geological  
Survey Open File Report 2008-1131.

The Rowe et al. data for VG-568, based on 17 analyses over 6 different  
sessions in 2 months, are:

SiO2	76.55  0.7
Al2O3	12.44  0.19
FeOT	1.11  0.07
MgO	0.03  0.02
CaO	0.39  0.02
Na2O	3.51  0.24
K2O	4.91  0.13
TiO2	0.08  0.01
P2O5	0.01  0.01
MnO	0.02  0.02
SO2	0 ?
Cl		0.12  0.01

To some extend, these later data sets fit my results better (e.g., the  
Ti content), so I was pleased to find them.  Yet these aren't the  
"official" values for VG-568, and perhaps a lot of us have been coming  
up with different values in-house.  Then there are the difficulties  
with analyzing a hydrous rhyolite glass (e.g., Na and K decrease, Si  
and Al enrichment), and there is often too little information to  
evaluate the analytical procedures used.

Does anyone use one of these data sets over the other, or does anyone  
use yet another set of elemental values?  How about trace elements?   
Are there are any data for VG-568 for Zn, Zr, etc?  I know I can  
always go to the literature for Yellowstone obsidian, but I don't know  
from which exact source this particular material came -- there is at  
least a dozen obsidian outcrops in Yellowstone.  Anyone know that?

Thanks in advance!


Ellery Frahm
Manager & Principal Analyst, Electron Microprobe Lab
Senior Research Fellow, Department of Geology & Geophysics
University of Minnesota - Twin Cities