THE BEGINNING OF MELTING OF FERTILE AND DEPLETED PERIDOTITE AT 1.5 GPA

We have determined the solidus temperatures and liquid compositions fo r batch melts of fertile and depleted Iherzolite in the 0-15% melting interval at 1.5 GPa. Because establishment of equilibrium is difficult at very low melt fractions we have used an iterative technique in whi ch the liquids from sandwich-type experiments are synthesized and test ed for multiple saturation at the same pressure, temperature condition s as those of the sandwich experiment. Only when all 4 mantle phases a re stable and of correct composition is the result accepted as being a t equilibrium. This technique permits accurate determination of solidu s temperatures to +/- 10 degrees C and provides the compositions of lo w-degree mantle melts. We find that at degrees of batch melting above 3%, liquids produced from a fertile peridotite (MORB-Pyrolite) contain similar to 49% SiO2, have < 4% Na2O and have Fe-Mg partitioning relat ionships with the solid phases which are typical of basalt. As the deg ree of melting drops to 0% in fertile peridotite (MORB-Pyrolite), SiO2 content of the melt increases to 53% and the Na2O content to 8%. The melts remain strongly nepheline-normative and Fe favours the melt rela tive to olivine even more strongly than normal, We ascribe this latter observation to Na-Fe3+ coupling in the melt at high Na2O content. The se high-alkali and high-silica melts are not observed at the solidus o f depleted peridotite (Tinaquillo Lherzolite). These experimental resu lts provide the first direct tests of fractional melting models which invoke the successive extraction of low-degree partial melts. No melti ng model currently available provides a good description of the compos itions of low-degree melts from a fertile peridotite at 1.5 GPa. (C) 1 998 Elsevier Science B.V.