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% CO 2 ; PERC3: MixKLB-1 + 1 wt% CO 2 ) 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% SiO 2 and ∼40 wt% CO 2 evolve continuously to carbonated silicate melts with >25 wt% SiO 2 and <25 wt% CO 2 between 1325 and 1350°C in the presence of residual olivine, orthopyroxene, clinopyroxene, and garnet. The first appearance of CO 2 -bearing silicate melt at 3 GPa is ∼150°C cooler than the solidus of CO 2 -free peridotite. The compositions of carbonated silicate partial melts between 1350 and 1600°C vary in the range of ∼28–46 wt% SiO 2 , 1·6–0·5 wt% TiO 2 , 12–10 wt% FeO*, and 19–29 wt% MgO for PERC, and 42–48 wt% SiO 2 , 1·9–0·5 wt% TiO 2 , ∼10·5–8·4 wt% FeO*, and ∼15–26 wt% MgO for PERC3. The CaO/Al 2 O 3 weight ratio of silicate melts ranges from 2·7 to 1·1 for PERC and from 1·7 to 1·0 for PERC3. The SiO 2 contents of carbonated silicate melts in equilibrium with residual peridotite diminish significantly with increasing dissolved CO 2 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 CO 2 -free liquids at 3 GPa, but diminish slightly with increasing dissolved CO 2 in the melt. The carbonated silicate partial melts of PERC3 at <20% melting and partial melts of PERC at ∼15–33% melting have SiO 2 and Al 2 O 3 contents, and CaO/Al 2 O 3 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 TiO 2 characteristic of highly alkalic OIB.