Crystallization of high-Ca chromium garnet upon interaction of serpentine,chromite, and Ca-bearing hydrous fluid     

Chepurov  A. A.  Turkin  A. I.  Pokhilenko  N. P. 《Doklady Earth Sciences》2017,476(2):1229-1232

The results of experimental modeling of the conditions of crystallization of high-Ca chromium garnets in the system serpentine–chromite–Ca-Cr-bearing hydrous fluid at a pressure of 5 GPa and temperature of 1300°С are reported. The mineral association including quantitatively predominant high-Mg olivine and diopside-rich clinopyroxene, bright-green garnet, and newly formed chrome spinel was formed. Garnet mostly crystallized around primary chromite grains and was characterized by a high concentration of CaO and Cr₂O₃. According to the chemical composition, garnets obtained are close to the uvarovite–pyrope varieties, which enter the composition of relatively rare natural paragenesis of garnet wehrlite. The experimental data obtained clearly show that high-Ca chromium garnets are formed in the reaction of chromite-bearing peridotite and Ca-rich fluid at high P–T parameters.

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Pyrope–knorringite garnets: overview of experimental data and natural parageneses     

A.I. Turkin  N.V. Sobolev 《Russian Geology and Geophysics》2009,50(12):1169-1182

This paper gives an analytical overview of the experimental data obtained by different authors at high P and T in the model system MgO–Al₂O₃–SiO₂–Cr₂O₃ (MASCr). A set of four simple polynomial equations is proposed for the temperature and pressure dependence of chromium content in garnet and spinel in the assemblage Gar + Opx + Es and Gar + Fo + Opx + Sp.From the first equation, one can estimate the minimum pressure at a given temperature which is required for the formation of peridotite garnets of uncertain paragenesis with a known knorringite content. A combination of the second and third equations helps estimate P and T from the chromium content of garnet and spinel from assemblages containing both minerals. If the spinel composition is unknown, but there is reason to assign garnet to a spinel-bearing paragenesis, the fourth equation is applicable for estimating pressure at given temperature.Originally, the proposed garnet–spinel geothermobarometer was developed for a harzburgite paragenesis. However, it is applicable to garnets with CaO/Cr₂O₃ < 0.903 (including lherzolitic ones), that is, those within the Pyr–Kn–Uv triangle of the reciprocal quaternary diagram Pyr–Cros–Uv–Kn.Using the above equations and an empirical P_CG geobarometer (Grütter et al., 2006), comparative geothermobarometric estimates were obtained for a set of garnet and garnet–spinel inclusions in diamonds and intergrowths with diamond, as well as garnet inclusions in spinel. If garnet has CaO/Cr₂O₃ = 0.35–0.40, the results are in good accord. For Cr-richest and Ca-poorest garnets, the P_CG barometer shows pressures 10–15% higher compared with our estimates.  相似文献   

Thermal expansion of ZnSiO<Subscript>3</Subscript> high-pressure phases     

V. Galkin  G. Kuznetsov  A. Turkin 《Physics and Chemistry of Minerals》2007,34(6):377-381

Thermal expansion of ZnSiO₃ high-pressure clinopyroxene and ilmenite phases was measured in the temperature range 100–620 K by the X-ray powder diffraction method. Interpolation and extrapolation of experimental data were performed by the procedure based on the Debye-Mie-Gruneisen theory in the range from 50 to 1,500 K. Temperature dependencies of molar volumes and coefficients of bulk thermal expansion of ZnSiO₃ phases were determined.  相似文献