RECALIBRATION OF THE FE-MG EXCHANGE REACTION BETWEEN GARNET AND CORDIERITE AS A THERMOMETER

Abstract

The continuous Fe-Mg exchange reaction, 1/2 Fe-cordierite + 1/3 pyrope = 1/2 Mg-cordierite + 1/3 almandine, between garnet and cordierite is recalibrated as a thermometer. The thermodynamic parameters for this reaction are derived from experimentally determined Fe-Mg fractionation data (n= 177) in the P-T range of 650-1000°C and 4-12 kbar in the FeO-MgO-Al2O3-SiO2-H2O system. The Margules parameters for Fe-Mg binary symmetric mixing in cordierite are also derived from the experimental data by adopting WFeMg and WMgFe interaction parameters of the Fe-Mg-Ca ternary (Mukhopadhyay et al., 1997) and Fe-Mg-Ca-Mn quaternary Berman (1990) mixing models of garnet. The respective values of WFeMg of cordierite (on a one-cation basis) are 2024±248 and 887±108 joule. Based on these mixing models of garnet and derived WFeMg of cordierite, two thermodynamic equations (T1 and T2) are accordingly formulated: T1(K) = [27018 + 0.13(P - 1) - 2024(XFe - XMg)Crd - A]/RlnKD + 12.80 + B), where A = [- 8055 (X MgGt)2 - 7422 (X FeGt)2 + 30954 XFeGt XMgGt + 1073 (XCaGt)2 - 12076 XCaGt X FeGt - 43454 XCaGt XMgGT + 10464 XCaGT (XMg - XFe)GT]; B = [7.36 (XMgGT)2 + 4.13 (XFeGT)2 - 22.98 XFeGT XMgGT - 4.01 (XCaGT)2 + 10.34 XCaGT XFeGT + 13.88 XCaGT XMgGT - 2.11 XCaGT (XMg - XFe)Gt], and KD = (XFe/XMg)Gt · (XMg/XFe)Crd; T2(K) = [26932 + 0.13(P - 1) - 887(XFe - XMg)Crd- C]/(RlnKD + 12.36 + D), where C = [77(XMgGT)2 - 1240 (XFeGT)2 + 2326 XFeGt XMgGT - 414 (XCaGT)2 - 17862 XCaGT XFeGT - 26526 XCaGT XMgGT + 19608 XCaGT (XMg - XFe)Gt + 658 XMnGT (XMg - XFe)Gt - 11304 XCaGT XMnGt], and D = [4.57 (XCaGt)2 + 4.58 XCaGt XFeGt + 4.56 XCaGt XMgGt - 7.96 XCaGt (XMg - XFe)Gt + 4.57 XCaGt XMnGt] Application of these formulations to experimental and natural data provides better temperature estimates compared to the previous calibrations for coexisting garnet-cordierite pairs.