Supersilicic clinopyroxene and silica exsolution in UHPM eclogite and pelitic gneiss from the Kokchetav massif, Kazakhstan

I. Katayama, C. D. Parkinson, K. Okamoto, Y. Nakajima, S. Maruyama

    Research output: Contribution to journalArticlepeer-review

    144 Citations (Scopus)


    Abundant exsolved quartz rods occur in matrix clinopyroxene of eclogite from the Kokchetav massif, Kazakhstan. These rocks are diamond-grade, ultrahigh-pressure (UHP) metamorphic rocks that recrystallized at P > 6 GPa and T > 1000 °C, Zircon is an excellent container, which effectively protects peak UHP metamorphic phases from retrogression. Therefore, to ascertain the pre-exsolution composition of the clinopyroxene, we analyzed clinopyroxene inclusions in zircon of the eclogite and a diamond-bearing biotite gneiss. Clinopyroxene in zircon has an excess of Fe3+ + (VI)Al over (IV)Al + Na + K, and calculated cation totals significantly less than 4.0 per six O atoms. The stoichiometry of these pyroxenes can be reconciled if the Ca-Eskola end-member (Ca0.5[ ]0.5AlSi2O6) is considered. The zircon-hosted clinopyroxene in the eclogite contains up to 9.6 mol% of the Ca-Eskola component, and in the biotite gneiss up to 18.2 mol%, whereas the matrix clinopyroxene contains much less (1.3 mol%, on average). Recalculation of the composition of the matrix clinopyroxene prior to exsolution of quartz rods yields 6.8 mol% Ca-Eskola component, which is consistent with the composition of the clinopyroxene inclusions in zircon. We conclude that the Ca-Eskola component in the peak metamorphic clinopyroxenes broke down by a retrograde reaction: 2 Ca0.5[ ]0.5AlSi2O6 → CaAl2SiO6 + 3 SiO2 resulting in exsolution of the quartz rods in the matrix clinopyroxene. Subducted crustal and supracrustal rocks from the Ca-Eskola clinopyroxene at high pressures and temperatures. The vacancy-containing clinopyroxene may have an important bearing on the physico-chemical properties of the subducted slab at upper mantle depth.

    Original languageEnglish
    Pages (from-to)1368-1374
    Number of pages7
    JournalAmerican Mineralogist
    Issue number10
    Publication statusPublished - 2000

    ASJC Scopus subject areas

    • Geophysics
    • Geochemistry and Petrology


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