Thermodynamic modelling of Cr-bearing garnets with implications for diamond inclusions and peridotite xenoliths

A new approach is presented for modelling Cr-rich peridotite compositions and garnet-spinel compositions found in diamonds and xenoliths at conditions relevant to the deep continental Earth. Using recent experimental data, it is now possible to calculate phase relations and mineral compositions relevant to pressures, temperatures, and compositions of the deep lithospheric Earth using free energy minimization techniques. Here we present calculated phase relations in Cr-rich mantle compositions from pressures of 20-60 kbar, and temperatures 800-1400 °C. The model is successful at modelling a wide range of natural mineral compositions which are found as xenoliths in diamond-bearing kimberlites from South Africa, and is illustrated using suites of Cr-rich xenoliths from near Kimberley, South Africa. The model can explain and quantify instances of garnet zonation in naturally occurring mantle rocks as a consequence of pressure-temperature re-equilibration; and shows that not all garnet zonations result from metasomatic processes. This sheds further light on peridotitic diamond inclusions and their abundances, and allows further quantitative constraints to be applied to diamond-indicator mineral chemistry.