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Unformatted text preview: American Mineralogist, Volume 73, pages 338-344, 1988 Fe-Mg mixing in cordierite: Constraints from natural data and implications for cordierite-garnet geothermometry in granulites A. Bnarr,lcHARyA, A. C. M,q.zuMDAR, S. K. SBN Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur, '121 3O2,lndia Anstnacr From chemicaldata on 108 pairs of naturally occurring garnet and cordierite, the in- terchange energy of Fe-Mg mixing in cordierite (Wo*-") and the free-energy change of the exchange equilibrium %FerAlrSirO,, -t t/zMgrAIoSirO,, : Y:MgrAlrSirO,, * YzFerAloSirO,, in the temperature range of 700-850 'C have been extracted using standard methods of linear regression analysis. The input thermochemical parameters for multicomponent Ca- Mg-Fe-Mn garnet solid solutions are based on the updated and revised values after Gan- guly and Saxena (1984), e.g., Wr"_.": 2500 cal, W*: W*": 3000 cal calculated on the basis of one-third formula unit. On the basis of the derived values at 710, 760, 800, and 850 "C, Fe-Mg mixing in cordierite has been approximated to a symmetric regular solution. The Fe-Mg substitution shows positive deviation from ideality in cordierite, with tzi:1., : 9.34 + 4.54kJ/mol of divalent cation. The values derived for the free energy of mixing, at Xi:d : Xi;ds : 0.5, are -3.26 kJ at 700 'C and -4.39 kl at 900 "C. The AG,.. value (in joules) for the above equilibrium can be expressed as 15079 - 8.552. Incorporating these parameters, the geothermometric expression for the exchange equilibrium is-_ l8l4 + 0.0152P + rr22(xffi - xFf) - r258(xffie- xt:) + 1510(xg" + xf;i") 1.028 - ln(Xr./ X*)"d / (XF./ XM)e, where I is in kelvins and P is in bars. When applied to natural assemblages, the predicted temperatures, over the wide compositional range 0.38 < X*" I 0.92, are found to be from 620 "C to 860 oC-a reasonable span for granulite-facies rocks. Although the "accuracy" of the computed Z values is of the order of +65 'C, the "precision" of the proposed geothermometer is higher by 300/o to 600/o than for earlier calibrations. The refinement in the mixing models of cordierite and garnet results in temperatures that are more reliable than those from the earlier formulations postulating ideal mixing in both phases. INtnorucrroN to refine the cordierite-garnet thermometer, we were 'the T-X dependence of Mg-Fe partitioning between prompted to examine compositional data from naturally coexisting garnet and cordierite has been a subject ofmuch occurring cordierite-garnet pairs in search ofevidence on debate. Experimental studies on the exchange equilibri- the nature of Fe-Mg mixing in cordierite. The present um yield contradictory results regarding the naturl of the communication is directed toward an analysis of the en- Zvs. ln Ko slope (Currie, 197 1,1974;He.rsen and Green, ergetic_s of Fe-Mg mixing in cordierite with the ultimate 1973; Holdaway,1976;perchuk and Lavrent'eva, l9g3)....
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This note was uploaded on 03/19/2012 for the course GEOLOGY 6 taught by Professor Rm during the Spring '12 term at UNAM MX.
- Spring '12