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

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 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.