The interaction of basalt melt with mantle harzburgite at low pressure might result in silica enrichment of the melt by assimilation of orthopyroxene. Experimental tests of this hypothesis show that silica-rich liquids (56 wt %) are produced by melt-orthopyroxene reaction at 1200¿ to 1250¿C. These silica-rich liquids are enriched in Na2O and K2O over that found in the starting basalt melt which indicates that the alkalis diffused against a concentration gradient into the silica-rich liquid. The liquids produced in this way are chemically similar to high-magnesium andesites or boninites.