| Реферат: | eng: 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-Cr2O 3 (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 PCG 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 PCG barometer shows pressures 10-15% higher compared with our estimates. © 2009.
|