Knowledge of the temperature, pressure and oxygen fugacity equilibrium conditions, under which diamond-bearing eclogitic rocks were formed in the Earth crust metamorphic processes required to solve the problem of their origin and origin of the diamonds in them.
We can estimate pressure and temperature on the basis of the garnet-clinopyroxene-plagioclase-quarts (Simakov and Shestakova, 1995) and garnet-clinopyroxene (Simakov, 1996; Krogh, 1988) barometers and thermometer. Model of Grt-Cpx-Pl-Qu barometer were developed on the basis of the reaction Gr + Qu = An + Cpx and checked on experimental date of
alkaline basalt (Simakov and Shestakova, 1995). Equilibrium temperatures and pressures were estimated for three diamond-bearing Grt-Cpx-Pl-Qu parageneses from Kokchetav massif (Northern Kazakhstan). They were formed at 650-805 C and 8.6-13.6 kbar. Model of the Gr-Cpx barometer was developed on the basis of the reaction: Gr + Pyr = CaTs + CEn + Dp and checked on experimental data of tonalite, tonalite-peridotite and alkaline basalt at 850-1150 C and 15-18 kb (10% of uncertainty) (Simakov, 1996). This barometer can be applied to garnet-clinopyroxene assemblages with pyrope and jadeite content lower 55% and 30% correspondingly. Equilibrium P-T parameters were estimated for 16 diamond-bearing and 9 diamond-free Gr-Cpx parageneses of Kokchetav massif by this method too. By the calculations diamond-bearing parageneses were formed at 750-1030 C and 15.7-36.5 kbar and diamond-free ones - at 640-1150 C and 12.3-28 kbar. All parageneses correspond to the field of graphite stability, two diamond-bearing ones correspond to the field of coesite stability.
To determine the oxidation state of eclogite formation we used the reaction Andr + CFer + Qu = Hed (Simakov, 1993). The oxidation state of the diamond-bearing and -free Kokchetav Grt-Cpx parageneses was analyzed. Main part of diamond-
free parageneses corresponds to the field of QFM buffer, while diamond-bearing ones - to WM and IW buffers. From the
calculations it follows that oxidized carbon dioxide fluid corresponds to the diamond-free parageneses and more reduced water-methane fluid - to diamond-bearing ones.
Krogh, E.J., Contribs. Mineral. Petrol. 99, 44-48 (1988).
Simakov, S.K., Dokl. Akad. Nauk R. 332, 83-84 (1993).
Simakov, S.K., Dok. Akad. Nauk R. (1996, in press)
Simakov, S.K. & Shestakova, O.E., Dok. Akad. Nauk R. 343, 677-679 (1995).