Hi there again,
Frank makes some fascinating points. I have written a short paper
on the subject of the buffering of melt H2O contents by fluid-absent
granulite-facies partial melting reactions and submitted it to
Contribs. The editor is my friend Jacques Touret. He and a couple of
reviewers have given me a hard time over my claim that I can now
demonstrate plausibly that the granulite-grade melting reactions that
produced many granitic magmas must have taken place in the effective
absence of any free fluid phase. The paper is accepted but I have to
jump through a few more hoops to make it intelligible to the
readership. I wish Frank had been a reviewer!
Another fascinating point brought up by Frank's mail is the one
of the inevitability of retrograde back reaction in metamorphic
terranes that have partially melted. I have to agree, with the caveat
that the efficiency of melt extraction and chemical segregation will
be critical in determining the degree of retrogression. Certainly,
fluid ingress is not implied when one sees retrograde rehydration in
rocks. Clemens and Droop dealt, at excruciating length and detail,
with just these issues (Clemens JD, Droop GTR (1998) Fluids, P-T
paths and the fates of anatectic melts in the Earth's crust. Lithos
44: 21-36).
Yet another interesting aspect was touched on by Stevens G,
Clemens JD, Droop GTR (1997) Melt production during granulite-facies
anatexis: experimental data from "primitive" metasedimentary
protoliths. Contrib Mineral Petrol 128: 352-370. Here it was pointed
out that, if graphite were present in the fluid-absent protolith, it
may not greatly affect the prograde reactions, and may well survive
as a residual phase. However, on crystallization of any melt formed,
nearly pure H2O would be expelled, and this the graphite would not
enjoy at all! Maybe we have here the origin of some of the
high-density CO2-rich fluid inclusions in some granulites.
Cheers,
JC
--
John D. Clemens
Professor of Geosciences, Director - Centre for Earth and Environmental
Science Research,
Editor in Chief - Electronic Geosciences
http://link.springer.de/link/service/journals/10069/index.htm
School of Earth Sciences and Geography, CEESR, Kingston University,
Penrhyn Rd, Kingston-upon-Thames, Surrey, KT1 2EE, UK
phone: +44 (0)20 8547-7023 fax: +44 (0)20 8547-7497
e-mail: [log in to unmask]
personal web page:
http://www.kingston.ac.uk/geolsci/staff/clemens/jdclemens.htm
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Our passions cannot alter the facts, only hide them from us.
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