Partial melting experiments of peridotite + CO₂ at 3 GPa and genesis of alkalic ocean island basalts

We document compositions of minerals and melts from 3 GPa partial melting experiments on two carbonate-bearing natural lherzolite bulk compositions (PERC: MixKLB-1 + 2.5 wt% CO₂; PERC3: MixKLB-1 + 1 wt% CO₂) and discuss the compositions of partial melts in relation to the genesis of alkalic to highly alkalic ocean island basalts (OIB). Near-solidus (PERC: 1075-1105°C; PERC3: ∼1050deg;C) carbonatitic partial melts with >10 wt% SiO₂ and ∼40 wt% CO₂ evolve continuously to carbonated silicate melts with <25 wt%; SiO₂ and >25 wt% CO₂ between 1325 and 1350°C in the presence of residual olivine, orthopyroxene, clinopyroxene, and garnet. The first appearance of CO₂-bearing silicate melt at 3 GPa is ∼C;150°C cooler than the solidus of CO₂-free peridotite. The compositions of carbonated silicate partial melts between 1350 and 1600°C vary in the range of ∼28-46 wt% SiO₂, 1.6-0.5 wt% TiO₂, 12-10 wt% FeO*, and 19-29 wt% MgO for PERC, and 42-48 wt% SiO₂, 1.9-0.5 wt% TiO₂, ∼10.5-8.4 wt% FeO*, and ∼15-26 wt% MgO for PERC3. The CaO/Al₂ O₃ weight ratio of silicate melts ranges from 2.7 to 1.1 for PERC and from 1.7 to 1.0 for PERC3. The SiO₂ contents of carbonated silicate melts in equilibrium with residual peridotite diminish significantly with increasing dissolved CO₂ in the melt, whereas the CaO contents increase markedly. Equilibrium constants for Fe*-Mg exchange between carbonated silicate liquid and olivine span a range similar to those for CO₂-free liquids at 3 GPa, but diminish slightly with increasing dissolved CO₂ in the melt. The carbonated silicate partial melts of PERC3 at <20% melting and partial melts of PERC at ∼15-33% melting have SiO₂ and Al₂O₃ contents, and CaO/Al₂O₃ values, similar to those of melilititic to basanitic alkali OIB, but compared with the natural lavas they are more enriched in CaO and they lack the strong enrichments in TiO₂ characteristic of highly alkalic OIB. If a primitive mantle source is assumed, the TiO₂ contents of alkalic OIB, combined with bulk peridotite/melt partition coefficients of TiO₂ determined in this study and in volatile-free studies of peridotite partial melting, can be used to estimate that melilitites, nephelinites, and basanites from oceanic islands are produced from 0-6% partial melting. The SiO₂ and CaO contents of such small-degree partial melts of peridotite with small amounts of total CO₂ can be estimated from the SiO₂-CO₂ and CaO-CO₂ correlations observed in our higher-degree partial melting experiments. These suggest that many compositional features of highly alkalic OIB may be produced by ∼1-5% partial melting of a fertile peridotite source with 0.1-0.25 wt% CO₂. Owing to very deep solidi of carbonated mantle lithologies, generation of carbonated silicate melts in OIB source regions probably happens by reaction between peridotite and/or eclogite and migrating carbonatitic melts produced at greater depths.