Letter. High-pressure aragonite phenocrysts in carbonatite and carbonated syenite xenoliths within an alkali basalt

Abstract

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 quasiincompressible 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.