Dear Pulak
I'd reinforce the caution - there are no true triple point assemblages,
because there are no rocks of any sort anywhere, outside the laboratory, in
which all the phases grew at a single P-T condition and also chemically
equilibrated with respect to all components at that condition. Lots of
pelitic rocks have passed by the triple point pretty closely during their
metamorphic history, but the vast majority developed no Al-silicate at all
at that stage, because the breakdown of staurolite + muscovite + quartz
lies at higher temperature. (If a suitable rock were to be metamorphosed
exactly at the true triple point, whatever that may be, I guess it would
contain sillimanite but not andalusite, nor kyanite, because sillimanite
nucleates so much easier!) BUT the only reason we can extract useful
information from metamorphic rocks about how they formed and evolved is
precisely because of the patchy, incomplete and long drawn out record that
they contain. This is a positive advantage for our subject (and our
careers), and learning how to unravel it objectively and quantitiatively is
where future advances will come from......
Bruce Yardley
--------------------------------------------
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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NEW! GEOFLUIDS - an interdisciplinary journal for research on the role of
fluids in all aspects of the evolution of the Earth's crust. Details on:
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