The depth of the spinel to garnet transition at the peridotite solidus

We have determined the pressure and temperature of the spinel-garnet transition close to the solidus of fertile and depleted peridotite from a combination of synthesis, sandwich and garnet-seeded experiments. Garnet is unstable in MORB-Pyrolite below 2.8 GPa (depths 1450°C) are required to generate the 3-5% melting in the garnet field implied by recent trace element and isotope studies. It appears likely, therefore, that the 'garnet signature' arises from some mechanism other than anhydrous melting of garnet peridotite