Home – Home – Jornals – Russian Geology and Geophysics 2009 number 12

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 a reason to assign garnet to a spinel-bearing paragenesis, the fourth equation is applicable for estimating pressure at a given temperature.
Originally, the proposed garnet-spinel geothermobarometry was developed for a harzburgite paragenesis. However, it is applicable to garnets with CaO/Cr₂O₃ < 0.90 (including lherzolite ones), i.e., 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, 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.