Simon,
Your point (b) is the important issue that I noted in my e-mail.
Has anyone addressed the issue of variable cation order on cordierite
stability?
eric
>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]
>+++++++++++++++++++++++++++++++++++++++++++++++++
Eric Essene
Professor of Geology
Department of Geological Sciences
2534 C.C. Little Bldg.
425 E. University Ave.
University of Michigan
Ann Arbor MI 48109-1063 USA
fx: 734-763-4690
ph: 734-764-8243
