We have determined melting phase relationships on a synthetic olivine lamproite composition with H2O contents from 6 to 12 wt % in the pressure range of 3.5 to 8 GPa and at temperatures of 1100 to 1500 ¿C. The composition with 6 wt % H2O is multiply saturated with olivine, orthopyroxene, and garnet at 5.3 GPa and 1370 ¿C, indicating that olivine lamproite can coexist with garnet harzburgite. The Mg # (molar 100Mg/Mg+Fe) of olivine, orthopyroxene, and garnet in experiments that closely bracket the multiple saturation point are 94.3, 93.7, and 88.9, respectively, and the garnet contains 3 wt % CaO. This residual mineral assemblage corresponds to low-Ca garnet harzburgite. Phlogopite crystallized at a temperature 150 ¿C below the multiple saturation point, indicating that all phlogopite in the source is consumed during partial melting prior to magma segregation. If the conditions of multiple saturation represent those of magma segregation, then the source region of the olivine lamproite exists near the base of continental lithosphere. Metasomatized, depleted harzburgite with veins containing centimeter-sized phlogopite grains have been reported to occur in mantle xenoliths from kimberlites. We propose that olivine lamproite magma is formed by preferential melting of phlogopite veins, with subsequent equilibration of the magma with surrounding garnet harzburgite.¿ 1997 American Geophysical Union