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

Abundant exsolved quartz rods occur in matrix clinopyroxene of eclogite fro m the Kokchetav massif, Kazakhstan. These rocks are diamond-grade, ultrahig h-pressure (UHP) metamorphic rocks that recrystallized at P > 6 GPa and T > 1000 degrees C. Zircon is an excellent container, which effectively protec ts peak UHP metamorphic phases from retrogression. Therefore, to ascertain the pre-exsolution composition of the clinopyroxene, we analyzed clinopyrox ene inclusions in zircon of the eclogite and a diamond-bearing biotite gnei ss. Clinopyroxene in zircon has an excess of Fe3+ + Al-VI over Al-IV + Na K, and calculated cation totals significantly less than 4.0 per six O atom s. The stoichiometry of these pyroxenes can be reconciled if the Ca-Eskola end-member (Ca(0.5)square(0.5)AlSi(2)O(6)) is considered. The zircon-hosted clinopyroxene in the eclogite contains up to 9.6 mol% of the Ca-Eskola com ponent, and in the biotite gneiss up to 18.2 mol%, whereas the matrix clino pyroxene contains much less (1.3 mol%, on average). Recalculation of the co mposition of the matrix clinopyroxene prior to exsolution of quartz rods yi elds 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 Ca(0.5)square(0.5)AlSi(2)O(6) --> CaAl2SiO6 + 3 SiO2 resulting in exsolution of the quartz rods in the matrix clinopyroxene. Subducted crustal and supracrustal rocks form the Ca-Eskola clinopyroxene a t high pressures and temperatures. The vacancy-containing clinopyroxene may have an important bearing on the physico-chemical properties of the subduc ted slab at upper mantle depth.