ISSN 1028-334X, Doklady Earth Sciences, 2011, Vol. 437, Part 2, pp. 473–478. © Pleiades Publishing, Ltd., 2011. Original Russian Text © A.V. Korsakov, A.V. Golovin, T. Dieing, J. Toporski, 2011, published in Doklady Akademii Nauk, 2011, Vol. 437, No. 4, pp. 530–535. 473 Crystallization of diamond with participation of COH fluid [1, 2] or the melt [3–7] at PT-parameters of its thermodynamic stability (950–1000°C and 4– 6 GPa) is the dominating point of view on the forma- tion of diamondiferous metamorphic rocks. In spite of lengthy study of diamond-bearing metamorphic rocks of the Kokchetav massif, high-pressure fluid inclu- sions were found only in diamonds [8] and assumed in garnets on the basis of the results of IR-spectroscopic investigations [9]. The presence of potassium-bearing clinopyroxenes in rocks of the Kokchetav massif also provides evidence for the fact that high-potassic liquid occurred in these rocks at the peak of metamorphism [10], but its traces were registered only as inclusions in diamonds [8]. The compositions of the fluid phase and melt at high temperatures and pressures (above the second critical point) become practically indistinguishable. The problem of the position of the second critical point in the most significant petrological systems is of special importance since fluids and melts transport a variable set of elements (see [11] and references therein). The presence of COH-fluid significantly decreases the melting temperature of practically all substrates (metapelites, metabasites, metaperidotites). The appearance of a melt in primary rocks during sub- duction in turn results in disappearance of the free fluid phase, because water solubility in the melt increases with pressure [11]. Because of this, it is important to distinguish the areas of fluid and melt existence, as well as contour the area of coexistence of these phases. In this paper we present original data on the com- position of fluid inclusions in garnets and clinopy- roxenes from high-pressure garnet–clinopyroxene rocks of the Kokchetav massif (Northern Kazakh- stan). The modal composition of garnet–clinopyroxene rocks of the Kokchetav massif varies in broad limits from carbonate (<10 vol % of silicate minerals) up to predominantly silicate (the concentration of carbon- ates is <10 vol %). The portion of garnet and clinopy- roxene in these rocks is usually >85 vol %, whereas the content of carbonates, sulfides, and secondary prod- ucts (amphibole and plagioclase) is ~15 vol %. The compositions of garnet (Alm 10–48 Sps 1.5–3.1 Pyr 0.5–8.9 Grs 33–76 And 5.2–25 ) and potassium-bearing clinopyroxene from garnet– clinopyroxene samples vary in broad limits as well. The significant concentration of trivalent iron in the compositions of garnets (up to 0.47 f. u.) and clinopy- roxenes (up to 0.1 f. u.) is a distinguishing feature of diamond-free garnet–clinopyroxene rocks. In spite of the absence of diamond in these rocks, they are undoubtedly isofacial to surrounding diamondiferous rocks. The presence of K 2 O-bearing clinopyroxenes (K 2 O ~ 1 wt %) is one of the most convincing indica- tors of the high-pressure origin of these rocks. The first reliable data on the presence of a K 2 O admixture in clinopyroxene were reported in [12]. The admixture of K 2 O (0.15 and 0.14 wt %) was established in two chrome diopside inclusions in Yakutian diamond. Inclusion of K-diopside (1.14 wt % K 2 O) was regis- tered in garnet from diamondiferous carbonate rocks of the Kokchetav massif, which indicates the high- pressure rock of this pyroxene [1]. The presence of garnet inclusions in porphyroblasts of potassium- bearing clinopyroxene and inclusions of potassium- bearing clinopyroxenes in garnet porphyroblasts pro- vides evidence for synchronous growth of these miner- als. Lamellae of potassic feldspar, phengite, and phlo- gopite registered in clinopyroxene porphyroblasts pro- vide evidence for the fact that the initial clinopyroxene was enriched in K 2 O and water. The high concentra- Fluid Inclusions in Rock-Forming Minerals of Ultrahigh-Pressure Metamorphic Rocks (Kokchetav Massif, Northern Kazakhstan) A. V. Korsakov a , A. V. Golovin a , T. Dieing b , and J. Toporski b Presented by Academician N.V. Sobolev September 2, 2010 Received October 11, 2010 DOI: 10.1134/S1028334X11040040 a Sobolev Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia b WITec GmbH, Ulm, Germany e-mail: korsakov@uiggm.nsc.ru GEOCHEMISTRY