Hi all

We have been restandardizing for D/H  mineral 
analyses on our TCEA unit  (running at 1450°C) 
connected to a CF mass spec in Alan Mix's lab. 
In parallel the U of Oregon lab has been doing 
the same.  We have run into a problem in that 
biotite standards are yielding different D/H 
ratios compared to one muscovite standard.  We 
are using NBS-30 biotite (nominal values 
something between -65 and -67 ”) and as a primary 
standard, and Misasa sericite/muscovite 
(Kusakabe, person commun. 2007;  -59.1 ± 0.3 (2 
std error of mean) as a secondary standard.

Both the OSU and U of O labs have gotten results 
for H2 gases derived from Misasa sericite that 
are isotopically lighter than the gases derived 
from NBS-30 biotite.

For example, in a 2005 run, using NBS -30 as a nominal standard, we got:

		NBS-30 Biot	Misasa Sericite
accepted value 	(~-66”)		(-59”)
measured		-67.3±2.9		-73.6±1.4
analyses		n=4		n=5

I am puzzled because the Misasa sericite is a 
well-behaved standard according to Dr. Kusakabe, 
and in our experience running it for several 
years as a secondary standard in our conventional 
D/H lab (using a 1400°C resistance furnace, and U 
reduction of water to H2).

So far, our working hypothesis is that in the 
TCEA, the H2 gas liberated from biotite at 1450°C 
has a significantly different offset 
(fractionation?) from original biotite, compared 
to H2 gas liberated from muscovite.   Maybe this 
has something to do with Fe-bearing minerals?

Anyone else seen this, or have any suggestions for an explanation?

Cheers,

JOHN Dilles

-- 
John Dilles, Professor of Geology of Mineral Resources
Department of Geosciences,  Wilkinson Hall 104
Oregon State University
Corvallis OR 97331-5506
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