Dear All,
To elaborate just a tiny bit on Bruce's comments with which I fully
agree, the only rocks that conceivably "might see" the the tripe point on
their first pass by it in PTt space would be those having uncommonly
Al-rich bulk compositions, e.g. such that they had pyrophyllite present at
lower grades. Some of the rocks in Vermont are of this sort. But for
reasons stated by Bruce, it's likely that sillimanite would be the
Al-silicate phase to nucleate.
The vast majority (>90%?) of metapelitic rocks have bulk
compositions such that Al-silicates come in via reactions involving
staurolite etc. (as Bruce notes), and this is well after the rock has
passed by in PT space the triple point.
Cheers, Chuck
>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
>---------------------------------------------
>
>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:
>
>http://www.blackwell-science.com/gfl
>
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