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

We have determined the pressure and temperature of the spinel-garnet transi tion close to the solidus of fertile and depleted peridotite from a combina tion of synthesis, sandwich and garnet-seeded experiments. Garnet is unstab le in MORB-Pyrolite below 2.8 GPa (depths <85 km) at a solidus temperature of 1470 degrees C. In Tinaquillo Lherzolite, which is related to MORB-Pyrol ite by about 7% fractional melting, the transformation occurs at 3.1 GPa at a solidus temperature of 1520 degrees C. In both cases the transformation interval is narrow, approximately 0.1-0.2 GPa. Because the solidus temperat ure increases by 7 degrees C and the spinel-garnet transformation pressure by 0.05 GPa for every 1% melt extracted, extremely high potential temperatu res (Tp > 1450 degrees C) are required to generate the 3-5% melting in the garnet field implied by recent trace element and isotope studies. It appear s likely, therefore, that the 'garnet signature' arises from some mechanism other than anhydrous melting of garnet peridotite. (C) 1998 Elsevier Scien ce B.V. All rights reserved.