Melting phase relations of a model chloride- and carbonate-bearing eclogite have been studied at 7.0–10.5 GPa and 1200–1675 °C. The mineral assemblage coexisting with partial melts is garnet, omphacite, kyanite, and coesite or stishovite. At temperatures of 1200–1400 °C, the partial melt has an SiO2-poor, Cl-bearing carbonatite composition. With increasing temperature, it becomes progressively SiO2 rich, and at temperatures of 1500–1700 °C contains as much as 53 wt% SiO2. The compositions of the melts are comparable with those of silicic end members of inclusions in fibrous and/or cloudy diamonds worldwide, implying that they may be produced via chemical reactions of alkalic chloride-carbonate liquids with mantle eclogites. Our experiments reproduced the trends of the compositional variations of the fluid or melt inclusions within eclogitic diamonds, and thus suggest a reliable model for their origin.