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

• Published in: Russian geology and geophysics Vol. 50; no. 12; pp. 1169 - 1182
• Main Authors: Turkin, A.I., Sobolev, N.V.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.12.2009; Allerton Press
• Subjects: chromite; diamond; experimental studies; garnet; garnet group; geologic barometry; geologic thermometry; Geophysics; geothermobarometry; high pressure; inclusions; knorringite; mineral assemblages; mineral inclusions; Mineralogy; native elements; nesosilicates; orthosilicates; oxides; P-T conditions; paragenesis; phase equilibria; pressure; pyrope; related to mantle; silicates; spinel; stability; garnet; chromite; knorringite; geothermobarometry
• ISSN: 1068-7971; 1878-030X
• DOI: 10.1016/j.rgg.2009.11.015

This paper gives an analytical overview of the experimental data obtained by different authors at high P and T in the model system MgO–Al 2O 3–SiO 2–Cr 2O 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/Cr 2O 3 < 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/Cr 2O 3 = 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.