An Experimental Study of Carbonated Eclogite at 3•5-5•5 GPa-Implications for Silicate and Carbonate Metasomatism in the Cratonic Mantle

We have experimentally investigated a K-bearing altered mid-ocean ridge basalt (MORB) composition to which 10% CaCO3 was added (GA1 + 10%cc), at temperatures of 1050-1400 degrees C and pressures of 3 center dot 5-5 center dot 5 GPa. Experiments were conducted in piston-cylinder apparatus in Pt-Gr (Pt with inner graphite) and Au-Pd capsules. Sub-solidus assemblages for both sets of experiments contain clinopyroxene, garnet, carbonate, rutile, coesite and K-feldspar. Apatite was observed only in the Pt-Gr experiments. Melting behaviour in experiments using different capsule materials contrasted markedly. Experiments in Pt-Gr capsules showed the silicate solidus to be at temperatures less than 1100 degrees C at 3 center dot 5 GPa and less than 1050 degrees C at 4 center dot 5-5 center dot 0 GPa. These are similar (3 center dot 5 GPa) or lower (4 center dot 5-5 center dot 0 GPa) temperatures compared with the carbonate solidus (1075-1125 degrees C at 3 center dot 5-5 center dot 0 GPa). Melts in the Pt-Gr runs evolve with increasing degree of melting from K-rich silicate melts at the lowest degree of melting to carbonate-silicate immiscible liquids and silicate-carbonate melts at intermediate degrees of melting, and finally to silicate melts at the highest degrees of melting. Experiments in Au-Pd capsules were performed only at 5 center dot 0 GPa. The carbonate solidus is between 1200 and 1225 degrees C (at least 100 degrees C higher than in the experiments in Pt-Gr capsules at the same pressure-temperature conditions). The first melts to be produced are carbonatitic and exhibit increasing SiO2 content with increasing temperature. This contrast in melting behaviour is explained by the relatively rapid diffusion of H through the Pt-Gr capsules, resulting in formation of H2O, and thus dramatically depressing both the silicate and the carbonate solidi in the Pt-Gr experiments compared with those in the Au-Pd experiments. This presumably reflects the lower permeability of Au-Pd to H, resulting in a much lower H2O/CO2 ratio in the Au-Pd encapsulated experiments. The presence of water in the melt was demonstrated by Fourier transform infrared (FTIR) spectroscopic analysis of one Pt-Gr experiment, indicating similar to 0 center dot 5 wt % H2O in the bulk composition. Further confirmation that H2O plays such a role in the Pt-Gr experiments was provided by an additional experiment performed in a Au-Pd capsule with similar to 10 wt % H2O specifically added. In this experiment immiscible carbonate and silicate melts were observed. Carbonate-silicate liquid immiscibility is considered to occur as a result of the H2O present in the system. These results can be applied to natural systems in several ways. First, the presence of a small amount of either silicate melt or H2O-fluid in the system will act as a 'flux', depressing the carbonate solidus to much lower temperatures than in anhydrous systems. Second, the full trend in melt evolution from silicate-rich to carbonate-rich melts, which is also observed in inclusions in diamonds, can be explained by melting of K- and CO2-bearing, water-undersaturated MORB compositions. In cratonic environments low-degree silicate and immiscible silicate and carbonate melts will metasomatize the overlying mantle in different ways, producing, in the first instance, Si enrichment and crystallization of additional orthopyroxene, phlogopite, pyrope-rich garnet and consuming olivine, and, in the second case, carbonate metasomatism, with additional magnesite-dolomite, clinopyroxene and apatite. Both metasomatic styles have been described in natural peridotite xenoliths from the cratonic lithosphere.