Abstract
The pseudo-binary system CaMgSi2O6-KAlSi2O6, modeling the potassium-bearing clinopyroxene (KCpx) solid solution, has been studied at 7 GPa and 1,100–1,650 °C. The KCpx is a liquidus phase of the system up to 60 mol% of KAlSi2O6. At higher content of KAlSi2O6 in the system, grossular-rich garnet becomes a liquidus phase. Above 75 mol% of KAlSi2O6 in the system, KCpx is unstable at the solidus as well, and garnet coexists with kalsilite, Si-wadeite and kyanite. No coexistence of KCpx with kyanite was observed. Above the solidus, KAlSi2O6 content of the KCpx coexisting with melt increases with decreasing temperature. Near the solidus of the system (about 1,250 °C) KCpx contains up to 5.6 wt% of K2O, i.e. about 22–26 mol% of KAlSi2O6. Such high concentration of potassium in KCpx is presumably the maximal content of KAlSi2O6 in the Fe-free clinopyroxene at 7 GPa. In addition to the major substitution MgM1CаМ2⇔AlМ1KМ2, the KCpx solid solution contains Ca-Eskola and only minor Ca-Tschermack components. Our experimental results indicate that the natural assemblage KCpx+grossular-rich garnet might be a product of crystallization of the ultra-potassic SiO2-rich alumino-silicate mantle melts (>200 km).
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Acknowledgements
Constructive personal discussions with George Harlow and Andrey Girnis gave some new ideas to the interpretation of the experiments and phase diagram. Comments by Robert Luth, Roland Stalder, and Mario Tribaudino improved the text of the paper. Authors especially thank J. Hoefs and J. Touret for quick organization the paper review. The authors are very grateful to Ludmila P. Red'kina (Institute of Experimental Mineralogy) for the preparation of starting materials and mixtures. The facilities for electron microprobe analyses were provided by both the Department of Petrology (analysts Elena V. Guseva and Natalia N. Korotaeva) and the Department of Mineralogy (analyst Dmitriy A. Varlamov) of the Moscow State University. This study is supported by the Russian Foundation for Basic Research (projects 01-05-64775, 03-05-06289 to OGS and 02-05-64684 to Y.A.L.), the program for young scientists of the Russian Academy of Science (project no. 323 to O.G.S.), the Science Support Foundation (program for young scientists), the Russian State Leading Scientific Schools Program (project no. 16452003.5 to L.L.P.), and by M.U.R.S.T., cofinanziamento 2001, project "Structural complexity and properties of minerals: microstructures, modularities, modulations" (to L.B. and S.M.).
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Abbreviations
Abbreviations
Mineral abbreviations
- Ca-Esk:
-
Ca-Eskolaite (Ca0.5AlSi2O6)
- Ca-Ts:
-
Ca-Tschermackite (CaAl2SiO6)
- Cpx:
-
potassium-free clinopyroxene solid solution
- Di:
-
diopside (CaMgSi2O6)
- Fo:
-
forsterite (Mg2SiO4)
- GS :
-
alumino-silicate glass
- Grs:
-
grossular (Ca3Al2Si3O12)
- Grt:
-
garnet solid solution
- KCpx:
-
potassium-bearing clinopyroxene solid solution
- Kfs:
-
K-feldspar (KAlSi3O8)
- KJd:
-
potassium jadeite (KAlSi2O6)
- Ks:
-
kalsilite (K8Al8Si8O32, the formula KAlSiO4 is used in the reactions in the text for simplicity)
- Ky:
-
kyanite (Al2SiO5)
- L:
-
alumino-silicate melt
- Lc:
-
leucite (KAlSi2O6)
- Mc:
-
mica-like phases, quench products of the alumino-silicate melt
- Prp:
-
pyrope (Mg3Al2Si3O12)
- Q:
-
quench products of the alumino-silicate melt
- SWd:
-
Si-wadeite (K2Si4O9)
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Safonov, O.G., Litvin, Y.A., Perchuk, L.L. et al. Phase relations of potassium-bearing clinopyroxene in the system CaMgSi2O6-KAlSi2O6 at 7 GPa. Contrib Mineral Petrol 146, 120–133 (2003). https://doi.org/10.1007/s00410-003-0491-9
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DOI: https://doi.org/10.1007/s00410-003-0491-9