Olivine melilitites and nephelinites are ultramafic magmas predominantly found in within-plate settings. In the Alaotra Lake district (Madagascar), olivine melilitites exhibit high Mg# values (68–77), suggesting compositions close to mantle-derived melts. Similar to many global occurrences, including plutonic equivalents, these low-silica magmas lack primary amphibole, with F-phlogopite as the main hydrous phase. This indicates a limited role of H2O during differentiation. Experimental studies have reproduced melilititic melts at ∼3 GPa under high melt fractions in the CaO-MgO-Al2O3-SiO2-CO2 system and through reactions between carbonated MORB-eclogite partial melts and fertile peridotite. At lower pressures (∼1.5 GPa), melting of amphibole-bearing lithosphere has been proposed as an alternative mechanism. Liquidus phase relations of carbonated melilitites indicate stability of clinopyroxene alongside olivine or garnet.

Based on previous experimental work on alkaline melts, we investigate the melting behavior of clinopyroxene-rich carbonated peridotites at 3.2 GPa, above the “solidus ledge”. Peridotite compositions with variable silica and clinopyroxene/garnet ratios were prepared. Samples were loaded in graphite capsules within sealed platinum capsules, with vitreous carbon spheres used as liquid traps. Preliminary experiments (3.2 GPa, 950–1450 °C) indicate a solidus temperature around 1050 °C for a model wehrlite composition. In all runs, an orthopyroxene-rich layer with polygonal microstructure formed at the interface between aggregates and quenched liquids. Liquid compositions at 1150–1200 °C are melilititic, with ∼2.5 wt.% TiO2 on a volatile-free basis. A second set of sandwich experiments explores the equilibration of olivine melilitite and nephelinite from Ankaratra (Madagascar) with peridotite compositions. At 3.2 GPa and 1150 °C, a larnite-normative liquid, compositionally intermediate between melilitite and nephelinite, was generated, in equilibrium with olivine, clinopyroxene, minor garnet, and orthopyroxene. These preliminary results suggest that the generation of low-silica magmas is controlled by the garnet/clinopyroxene ratio within the forsterite-enstatite-nepheline-larnite volume in the presence of CO2.