Hi,
Just wanted to clarify a point made in Horst's summary... I hope I wasn't
seen as recommending the use of traditional geobarometers over
internally-consistent data sets... a view that would make me few friends
down here in Australia.
My work using Grt-Opx barometers was done ten years ago, and since then all
my P-T estimates have used the data set approach, which has to be a better
way to go and is improving all the time as data sets become more and more
refined. Of course, any conclusions concerning the relative merits of
different traditional thermobarometric calibrations also have implications
for the use of data sets, which still depend on a careful choice of
thermodynamic data and solution models.
In my case I have used the THERMOCALC program/Holland & Powell data set,
rather than TWQ, mainly because of my UK origins, but this choice was
obviously fortuitous given my current computing capability... yes I too
have a mac!
Returning to Horst's summary. It sounds as if he is basing his P-T
estimate on the intersection of the Fe and Mg Grt-Opx-Pl-Qtz reactions. Is
this correct? If so this estimate is very poorly constrained in terms of
temperature. The "conventional" Fe-Mg pairs (e.g. Perkins and Chipera)
have fairly similar slopes in P-T space and (presumably the same holds for
TWQ), and their intersection would be a far more reliable estimate of
pressure than temperature. So making conclusions on the basis of whether
the intersections fall in the sillimanite or kyanite field is a little
dangerous given that the Ky/Sil reaction is both temperature and pressure
dependent. Other equilibria are needed to try and constrain the
temperature, such as Fe-Mg exchange between garnet and orthopyroxene,
and/or alumina solubility in orthpyroxene. As mentioned by Rob Berman,
these equilibria are variably reset during cooling as a result of
retrograde Fe-Mg exchange, and care is needed to interpret the results (see
Fitzsimons & Harley 94, JPet. 35 543-576 and Pattison & Bégin 94, J. Met.
Geol. 12, 387-410 for detailed discussion of this).
Cheers
Ian
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Ian Fitzsimons
Tectonics Special Research Centre
School of Applied Geology
Curtin University of Technology
GPO Box U1987, Perth, WA 6845
AUSTRALIA
Phone (Direct line): +61 8 9266 2455
Phone (Geology Office): +61 8 9266 7968
Fax: +61 8 9266 3153
E-mail: [log in to unmask]
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