The deep carbon cycle and melting in Earth's interior

We document compositions of minerals and melts from 3 GPa partial melting experiments on two carbonate-bearing natural lherzolite bulk compositions (PERC: MixKLB-1þ2 5 wt% CO2; PERC3: MixKLB-1þ1wt% 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: 1075^11058C; PERC3: 10508C) carbonatitic partial melts with 510 wt% SiO2 and 40 wt% CO2 evolve continuously to carbonated silicate melts with 425 wt% SiO2 and525 wt%CO2 between 1325 and 13508C in the presence of residual olivine, orthopyroxene, clinopyroxene, and garnet. The first appearance of CO2-bearing silicate melt at 3 GPa is 1508C cooler than the solidus of CO2-free peridotite.The compositions of carbonated silicate partial melts between 1350 and 16008C 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%MgOforPERC3.TheCaO/Al2O3weight ratio ofsilicate 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 at520% melting and partial melts of PERC at 15^33% melting have SiO2 andAl2O3 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 CaOand they lack the strong enrichments inTiO2 characteristic of highly alkalic OIB. If a primitive mantle source is assumed, theTiO2 contentsofalkalicOIB, combinedwith bulkperidotite/melt partition coefficients ofTiO2 determined in this study and in volatile-free studiesofperidotite partialmelting, canbe used to estimate that melilitites, nephelinites, and basanites from oceanic islands are produced from 0^6% partial melting.The SiO2 and CaOcontents of such small-degree partial melts of peridotite with small amounts of total CO2 can be estimated from the SiO2^CO2 and CaO^CO2 correlationsobserved inourhigher-degreepartialmeltingexperiments.These suggest that many compositional features of highly alkalic OIBmay 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, generationofcarbonated silicatemelts inOIBsource regions probably happens by reaction between peridotite and/or eclogite and migrating carbonatitic melts produced atgreaterdepths.

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M. Javoy

Papers

A melt and fluid inclusion study of the gas phase at Piton de la Fournaise volcano (Réunion Island)