Colleagues - I am commenting on this one in full greybeard mode - drooling
over my keyboard as I stare mistily through thickened classes to identify
the letters.

It is a quarter of a century since I used the changes in zoning patterns in
prograde metamorphic garnets between c. 600 and 650C to make an empirical
estimate of major cation diffusion rates in garnet (which was reasonably
robust!), so it is nice to see that the idea of using natural garnets still
has credibility. In hindsight, I think that the major weakness (a major
weakness...) of my earlier work was that it did not allow for the small
number of garnets which seem to survive prograde metamorphism to high
temperatures while retaining low-T bell-shaped profiles, even in areas like
Connemara where 99.99% of grains have reset. What I now think is the case
is that in a few cases it is possible for garnets to grow in a dry setting,
or to be left hot and dry after they have grown, in contrast to the high
water fugacity environment that normally attends prograde metamorphism of
garnetiferous rocks, and that this may explain the occassionally more
refractory behaviour of garnet in the 650 - 750C range. Now if this hunch
is right, it means that the contact overprint experiment, if it works, is
likely to measure the slow dry diffusion rates, because the regional
garnet-bearing rock will have lost all pore fluid as it cooled from the
metamorphic peak, and the contact event will not have replaced it. Even if
water was given off by further dehydration at the contact metamorphic peak,
it is far from clear how effectively it would penetrate garnet, and so
affect diffusion rates. Thus I fear that the results obtained would be very
relevant to diffusion rates in thermally-overprinted regional garnets, but
not to prograde garnet growth.

Unfortunately it is also the case, that although there are countless
aureoles affecting garnet-bearing regionally metamorphosed rocks, they tend
to be either rather deep aureoles, in which case the thermal constraints
can be modelled with less precision, or else to be so shallow that all
garnet is destroyed at temperatures below the diffusion threshold. It is
not normal for garnet to survive at temperatures for volume diffusion to be
effective, without participating in reactions with the surrounding matrix
that result in recrystallisation, new growth, or other processes that
invalidate the experiment! In fact I cannot think of an example where this
natural experiment will work reliably, and I look forward to seeing if
anyone else can.

Sorry to be so negative....
Bruce Yardley


>Dear all,
>
>I am looking for a rapidly cooled metamorphic terrane, containing garnet,
>that was intruded at a much later date (in the hundreds Ma) by a granite
>(or other heat source). It would be nice if garnet was found in most of
>the isograds associated with the granite aureole. In addition, alteration
>associated with cooling and or hydrous fluids should be minimal.
>
>The reason for this specific setting is that I want to use the granite
>thermal overprint as a natural experiment to determine the closure
>temperature of garnet by looking at diffusion profiles in garnet. This has
>a much better chance of success than carrying out laboratory diffusion
>experiments because REE diffuse slowly.
>
>Cheers
>
>Alfredo
>
>Alfredo Camacho
>School of Geology,
>The University of New South Wales,
>UNSW SYDNEY, NSW 2052,
>AUSTRALIA
>
>Tel no: (02) 9385 (6142 or 6134)
>Fax no: (02) 9385 5935
>Email: [log in to unmask]
>
>


--------------------------------------------
Professor Bruce Yardley
School of Earth Sciences
University of Leeds
Leeds LS2 9JT
UK

Tel. 0113 233 5200 Fax 0113 233 5259
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