Dear Kurt,

Here is my abstract for the Epstein Session.  I could not find a place on
the GSA submittal form to indicate this Session, so it was submitted to
Geochemistry (other) as a "volunteered" abstract.  I assume that we are not
officially "inviting" abstracts from anyone, even though we have been
encouraging people to submit. (You should think about that issue; some
folks get really touchy if they are not 'invited', or if others are, but
not them.  In my opinion, the simplest way is not to officially "invite"
anyone (except SAM, of course!).

I hope this abstract is suitable.  I'll try to expand the actual material
for the session to include the results for the natural samples....

Fred


Rapid changes in the oxygen- and hydrogen-isotope compositions of
halloysite during laboratory wetting at 23°C.

LONGSTAFFE, Fred J., Department of Earth Sciences, University of Western
Ontario, London, Ontario, Canada,  N6A 5B7,  [log in to unmask];
MIZOTA, Chitoshi, Faculty of Agriculture, Iwate University, Ueda 3-18-8,
Morioka 020, Japan.

Three decades have elapsed since the seminal studies on the stable isotope
geochemistry of clay minerals first appeared.  Such data have become an
important part of research on hydrothermal alteration, diagenesis,
weathering, pedogenesis and paleoclimatic reconstruction, given that most
clay minerals retain the H- and O-isotope compositions acquired during
formation.  However,  it has long been known that halloysite readily
exchanges H-isotopes with ambient water, and contradictory opinions exist
concerning its O-isotope behaviour.  Accordingly, prior to a study of
halloysite from Japan, New Zealand, The Philippines and Hawaii, we
investigated the isotopic behaviour of three typical and one Fe-rich, 1.0
nm halloysites of low, moderate or high crystallinity during routine
laboratory procedures.  At all times being kept moist, each sample was
cleaned of organic matter, Fe-Al hydroxides and allophane, and the <1 µm
fraction separated using conventional techniques.  Three portions of each
sample (1.0 nm,  collapsed to 0.7 nm, re-expanded with formamide) were
analyzed for D- and O-isotope compositions, after aliquots of each were
washed for forty minutes at 23°C in labelled waters (hydrogen:  +319 or
-339 permill; oxygen:  -9.5 or -43.0 permill).   Centrifugation was used
after washing to remove as much water as possible prior to drying at 105°C
(experiments using freeze-drying for this final step are in progress).  The
1.0 nm sample showed 16-50% H-isotope and 7-17% O-isotope exchange,
increasing with decreasing crystallinity.  When previously collapsed
halloysite (0.7 nm) was washed, less exchange occurred (hydrogen:  4-9%,
increasing with decreasing crystallinity; oxygen: 1-5%).  When the 0.7 nm
halloysite was re-expanded using formamide and then washed, 10-19%
H-isotope and 1-2% O-isotope exchange was detected, with no relationship to
original crystallinity.   Similar and reproducible O-isotope results (±0.5
permill) were obtained for the moderate and well-crystallized halloysite
samples, regardless of treatment, by correcting for exchangeable oxygen.
For the poorly crystallized samples, calculated values varied by 2 to 3
permill among treatments, with the 1.0 nm variety being most enriched in
oxygen-18.   Caution and attention to mineralogical detail are required
when measuring and interpreting the O-isotope compositions of halloysite.
However, using exchange methods akin to those that have been successfully
employed for opal-A, it should be possible to obtain precise and
geologically significant O-isotope (and perhaps H-isotope?) results for
many halloysite samples.


==============================================================================
Fred J. Longstaffe                      ph:  519-661-3177
Department of Earth Sciences            fax:  519-661-3198
University of Western Ontario           e-mail:  [log in to unmask]

London, Ontario N6A 5B7