The effect of bulk composition on the solidus of carbonated eclogite from partial melting experiments at 3 GPa

To explore the effect of bulk composition on the solidus of carbonated eclogite, we determined near-solidus phase relations at 3 GPa for four different nominally anhydrous, carbonated eclogites. Starting materials (SLEC1, SLEC2, SLEC3, and SLEC4) were prepared by adding variable proportions and compositions of carbonate to a natural eclogite xenolith (66039B) from Salt Lake crater, Hawaii. Near-solidus partial melts for all bulk compositions are Fe-Na calcio-dolomitic and coexist with garnet + clinopyroxene + ilmenite +/- calcio-dolomitic solid solution. The solidus for SLEC1 (Ca#=100 x molar Ca/(Ca + Mg + Fe-T)=32, 1.63 wt% Na2O, and 5 wt% CO2) is bracketed between 1,050 degrees C and 1,075 degrees C (Dasgupta et al. in Earth Planet Sci Lett 227:73-85, 2004), whereas initial melting for SLEC3 (Ca# 41, 1.4 wt% Na2O, and 4.4 wt% CO2) is between 1,175 degrees C and 1,200 degrees C. The solidus for SLEC2 (Ca# 33, 1.75 wt% Na2O, and 15 wt% CO2) is estimated to be near 1,100 degrees C and the solidus for SLEC3 (Ca# 37, 1.47 wt% Na2O, and 2.2 wt% CO2) is between 1,100 degrees C and 1,125 degrees C. Solidus temperatures increase with increasing Ca# of the bulk, owing to the strong influence of the calcite-magnesite binary solidus-minimum on the solidus of carbonate bearing eclogite. Bulk compositions that produce near-solidus crystalline carbonate closer in composition to the minimum along the CaCO3-MgCO3 join have lower solidus temperatures. Variations in total CO2 have significant effect on the solidus if CO2 is added as CaCO3, but not if CO2 is added as a complex mixture that maintains the cationic ratios of the bulk-rock. Thus, as partial melting experiments necessarily have more CO2 than that likely to be found in natural carbonated eclogites, care must be taken to assure that the compositional shifts associated with excess CO2 do not unduly influence melting behavior. Near-solidus dolomite and calcite solid solutions have higher Ca/(Ca + Mg) than bulk eclogite compositions, owing to Ca-Mg exchange equilibrium between carbonates and silicates. Carbonates in natural mantle eclogite, which have low bulk CO2 concentration, will have Ca/Mg buffered by reactions with silicates. Consequently, experiments with high bulk CO2 may not mimic natural carbonated eclogite phase equilibria unless care is taken to ensure that CO2 enrichment does not result in inappropriate equilibrium carbonate compositions. Compositions of eclogite-derived carbonate melt span the range of natural carbonatites from oceanic and continental settings. Ca#s of carbonatitic partial melts of eclogite vary significantly and overlap those of partial melts of carbonated lherzolite, however, for a constant Ca-content, Mg# of carbonatites derived from eclogitic sources are likely to be lower than the Mg# of those generated from peridotite.