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首页> 外文期刊>Contributions to Mineralogy and Petrology >A new activity model for Fe-Mg-Al biotites: II-Applications in the K2O-FeO-MgO-Al2O3-SiO2-H2O (KFMASH) system
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A new activity model for Fe-Mg-Al biotites: II-Applications in the K2O-FeO-MgO-Al2O3-SiO2-H2O (KFMASH) system

机译：Fe-Mg-Al Biotites的新型活性模型：II-应用于K2O-Feo-MgO-Al2O3-SiO2-H2O（KFMASH）系统中的应用

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摘要

The new biotite activity model and standard-state thermodynamic properties of Ann, Phl, and Eas presented in part-I were used to make pseudosections of bulk compositions representing experimental Fe-Mg exchange equilibria and (model) pelitic bulk rock compositions in the system K2O-FeO-MgO-Al2O3-SiO2-H2O (KFMASH), using mainly the software Perple_X. These pseudosection calculations (termed 'our calculation(s)' in the following) were compared to analogous ones performed with the solution model of biotite and thermodynamic data cited in White et al. (J Metamorph Geol 32:261-286, 2014, 10.1111/jmg.12071), termed 'W14 calculation'. Our calculations with the experimental bulk composition used by Zhou (Ti-Mg-Fe biotites: formation, substitution, and thermodynamic properties at 650 to 900 degrees C and 1.1 Kb with fO(2) defined by the CH4-graphite buffer. PhD thesis, State University of New York, 1994) in his experimental study of the Fe-Mg exchange between biotite (Bt) and olivine (Ol) confirm that biotite had no or only minimal octahedral Al (Al-VI) in these experiments. The experimental data of Ferry and Spear (-FS78, Contrib Mineral Petrol 66:113-117, 1978, 10.1007/BF00372150) on the Fe-Mg distribution between biotite and garnet (Grt) are well reproduced by our calculations. The computed composition of biotite (X-Fe) in equilibrium with garnet of Alm(90)Py(10) composition and the resulting lnK(D) values as a function of temperature are in good agreement with the experimental brackets. An analogous W14 calculation on the same Fe-rich bulk composition predicts too high X-Fe(Bt) in order of 0.1 mol fraction. The Al-VI contents of biotite of about 0.3-0.45 apfu, as measured by Gessman et al. (Am Mineral 82:1225-1240, 1997, 10.2138/am-1997-11-1218) in similar biotite-garnet exchange experiments performed with Alm(80)Py(20) and Alm(70)Py(30) garnets, are well reproduced by our, as well as by W14 calculations. The extent of Tschermak substitution in biotite in the FS78 experiments, which had Fe-richer bulk compositions, has not been measured. Comparing the FS78 biotites with the ones from Gessman et al. (Am Mineral 82:1225-1240, 1997, 10.2138/am-1997-11-1218), it is very likely that the biotites reported in FS78 contained Al-VI in the same order of ca. 0.3-0.4 apfu. A T-X-Fe (= molar FeO/(FeO + MgO) pseudosection demonstrates the bulk composition dependence of lnK(D) of the Mg/Fe-Grt/Bt exchange reaction in high-variance fields. Further comparisons, demonstrating the application of the new biotite solution model in the KFMASH system, are presented in pseudosections constructed for an average model pelite, as well as for a natural high-T/low-P and a natural high-P metapelite. The pseudosections show that biotite according to our biotite model breaks down at lower temperatures and pressures than predicted from the W14 biotite model in the KFMASH system. This means that KFMASH biotite can break down before the wet solidus is reached, which can explain the existence of dry high-T/low-P metapelites. At higher pressures, biotite according to our calculations breaks down at lower pressures than computed with the W14 biotite model.
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