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 <85 km) at a solidus temperature of 1470°C. In Tinaquillo Lherzolite, which is related to MORB-Pyrolite by about 7% fractional melting, the transformation occurs at 3.1 GPa at a solidus temperature of 1520°C. In both cases the transformation interval is narrow, approximately 0.1–0.2 GPa. Because the solidus temperature increases by 7°C and the spinel–garnet transformation pressure by 0.05 GPa for every 1% melt extracted, extremely high potential temperatures (Tp > 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 signatureʹ arises from some mechanism other than anhydrous melting of garnet peridotite.