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] +++++++++++++++++++++++++++++++++++++++++++++++++