We describe the first observation of primary magmatic aragonite in carbonatite and carbonated syenite, occurring as xenoliths in a Pliocene basaltic diatreme located near the Hungary–Slovakia border. The aragonite-hosting matrix consists of disordered P-rich calcite, occasionally associated with trachyte glass. We interpret the aragonite growth as evidence of supra-lithostatic overpressure in the magmatic plumbing system that connected the crustal basaltic reservoir with the partial melting zone of the lithospheric mantle, and the disordered calcite ± trachyte as quenched residual, immiscible melts, generated close to the solidus of the carbonated alkali basalt differentiated in the crustal reservoir. The quenching event was a phreato-magmatic eruption within the stability field of the low-pressure calcite; this was triggered by advective overpressure, caused by expanding gas bubbles in a quasi-incompressible silicate melt system. The high-pressure, pre-eruption origin of aragonite is indicated by enrichment in 13C compared to the associated calcite interpreted as a record of CO2 degassing at T > 500 °C. The oxygen (δ18O ranges of 22.1–24.5‰ V-SMOW in aragonite, 21.6–22.7‰ in calcite) and carbon (δ13C ranges of −4.4 to −5.9‰ V-PDB in aragonite, −11.9 to −12.7‰ in calcite) isotope signatures are consistent with a degassed carbonatite melt primarily derived from a subduction zone.