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

Abstract

We have experimentally investigated a K-bearing altered mid-ocean ridge basalt (MORB) composition to which 10% CaCO 3 was added (GA1 10%cc), at temperatures of 1050^14008C and pressures of 35^55 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 11008C at 35 GPa and less than 10508C at 45^50 GPa. These are similar (35 GPa) or lower (45^50 GPa) temperatures compared with the carbonate solidus (1075^11258C at 35^50 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 50 GPa. The carbonate solidus is between 1200 and 12258C (at least 1008C 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 SiO 2 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 H 2 O, 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 2 O/CO 2 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 $05 wt % H 2 O in the bulk composition. Further confirmation that H 2 O plays such a role in the Pt^Gr experiments was provided by an additional experiment performed in a Au^Pd capsule with $10 wt % H 2 O specifically added. In this experiment immiscible carbonate and silicate melts were observed. Carbonate ilicate liquid immiscibility is considered to occur as a result of the H 2 O 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 H 2 O-fluid in the system will act as a 'flux' , depressing the carbonate solidus to much lower temperatures than in