Partial Melting Experiments of Peridotite + CO2 at 3 GPa and Genesis of Alkalic Ocean Island Basalts

Abstract

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: ∼1050°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 ∼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/Al2O3 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.