Garnet-peridotite stability and occurrence in crust and mantle

The boundary between the spinel- and garnet-lherzolite mineral facies is strongly curved between 1300° C and 1450° C; below 1200° C it lies almost parallel to the temperature axis of the T-P diagram.Pressure of at least 12 kb is required to stabilise garnet-peridotite on the geothermal gradient and the depth at which the boundary is encountered is not sensitive to variations in the geotherm.Garnet-peridotite is metastable with respect to spinel-peridotite in normal continental crust. Natural occurrences are mantle derived and have either suffered rapid upward transit in diatremes or, in orogenic zones, either they were emplaced after upward tectonic transport through tens of kilometres, or have originated by in situ metamorphism of pre-existing crustal peridotite in an orogenic root downfolded to depths of at least 40 km.