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GEO-METAMORPHISM 2001

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Subject:

Re: Cordierites + H20,CO2

From:

Simon Harley <[log in to unmask]>

Reply-To:

No title defined <[log in to unmask]>

Date:

Thu, 6 Sep 2001 09:08:44 +0000

Content-Type:

text/plain

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text/plain (91 lines)

Hello all,

I have been alerted to the recent exchange of emails re
cordierite volatiles. With Damian Carrington and Pauline
Thompson I have been (experimentally and theotertically)
working on H2O and CO2 uptake in cordierite for the past
n years, where n>6. The good news is that we have
determined the uptake of H2O and CO2 as functions of
P,T, aH2O and aCO2, and have working models that I
think do a pretty good job of allowing one to estimate
aH2O and aCO2 from cordierite volatile contents providing
a) P and T are known/estimated independently, and b)
the cordierite channels have remained 'closed' since that -
T condition (I could go on about leakage vs non-leakage
and also about reequilibration ad nauseum).

Basically it is clear that aH2O is proportional to n/(1-n) in
'normal' cordierites (low Na, Be, Li), and that aCO2 is
proportional to m/(1-m). n is the number of molecules of
H2O per unit formula, m is the number of molecules of
CO2 per unit formula (18 oxygens volatile free crd).

The H2O model has, somewhat conveniently, just come
out in JPetrol:

The Distribution of H2O between Cordierite and Granitic
Melt: H2O Incorporation in Cordierite and its Application
to High-grade Metamorphism and Crustal Anatexis
         S. L. HARLEY and D. P. CARRINGTON
         J. Petrology 2001;42 1595-1620.

whilst the CO2 model and discussion will appear in JMG
soon:
Cordierite as a sensor of fluid conditions in high-grade
metamorphism and crustal anatexis
S.L. Harley, P. Thompson, B.J. Hensen and I.S. Buick

and will be fully described (with the experimental basis
etc) in our papers soon to appear in CMP and later one to
go to JoP.

The H2O expression you need (from H&C and in H et al)
is:
aH2O = [n/(1-n)]{exp[(4203 (320))/T -11.75 (0.33)]/fH2O}

The CO2 expression you need (in H et al) is:
aCO2 = [m/(1-m)]{exp[(3073 (200)- (0.3023P))/T -
13.1(1.2)]/fCO2}

from these you can calculate the activities using
measured H2O and CO2 contents in the cordierite, which
we do using SIMS at Edinburgh (see the website at
www.glg.ed.ac.uk and tunnel down into the SIMS
section).

In response to Erics comment that non-one has looked
into this, the above information demonstrates that it is
very much being looked at. If you want to go back further
then you can read Kurepins paper:

Kurepin, V. A., 1984. H2O and CO2 contents of cordierite
as an indicator of thermodynamical conditions of
formation. Geokhimiya, 8, 1125-1134.

However, as explained in our work, Kurepins models
involving n and m actually contravene the phase rule in
assuming (implicitly) a coupling between H2O and CO2
contents in cordierite that ends up being independent of
either one of aH2O or aCO2... the devil is in the details.
As a consequence one cannot calculate aCO2 or aH2O
correctly using the Kurepin model, and we (of course !)
recommend our new models as briefly given above.

Cheers,

Simon Harley


================================================
Simon Harley FRSE
Professor of Lower Crustal Processes
Department of Geology & Geophysics
University of Edinburgh
Kings Buildings, West Mains Road
EDINBURGH, EH9 3JW, SCOTLAND

phone: (44) 0131 650 8547
fax: (44) 0131 668 3184
email: [log in to unmask]
+++++++++++++++++++++++++++++++++++++++++++++++++

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