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�), at temperatures of 1050-1400°C and pressures of 3.5-5.5 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°C at 3.5 GPa and less than 1050°C at 4.5-5.0 GPa. These are similar (3.5 GPa) or lower (4.5-5.0 GPa) temperatures compared with the carbonate solidus (1075-1125°C at 3.5-5.0 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 5.0 GPa. The carbonate solidus is between 1200 and 1225°C (at least 100°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 H 2O/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 ~0.5 wt % H 2O 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 ~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.