We determined the fluid-present and fluid-absent near-solidus melting of an Al-poor carbonated pelite at 3–7 GPa, to constrain the possible influence of sediment melt in subduction zones. Hydrous silicate melt is produced at the solidi at 3–4 GPa whereas Na-K-rich carbonatite is produced at the solidi at ≥5 GPa for both starting compositions. At ≥5 GPa and 1050°C, immiscible carbonate and silicate melts appear with carbonate melt forming isolated pockets embedded in silicate melt. Application of our data to Nicaraguan slab suggests that sediment melting may not occur at sub-arc depth (∼170 km) but carbonatite production can occur atop slab or by diapiric rise of carbonated-silicate mélange zone to the mantle wedge at ∼200–250 km depth. Flux of carbonatite to shallower arc-source can explain the geochemistry of Nicaraguan primary magma (low SiO2and high CaO, Ba/La). Comparison of carbonate-silicate melt immiscibility field with mantle wedge thermal structure suggests that carbonatite might temporally be trapped in viscous silicate melt, and contribute to seismic low-velocity zone at deep mantle wedge of Nicaragua.