PHASE RELATIONS OF CARBONATE-BEARING ECLOGITE ASSEMBLAGES FROM 2.5 TO 5.5 GPA: IMPLICATIONS FOR PETROGENESIS OF CARBONATITES

Abstract

We have experimentally investigated the phase and melting relations of garnet + clinopyroxene + carbonate assemblages at 2.5–5.5GPa, to assess the feasibility of carbonated eclogite as a source for some crustally emplaced carbonatites. The solidus of our composition was at 1,125C at 2.5GPa, 1,225C at 3.5GPa and 1,310C at 5.0GPa. Melts were sodic calcio-dolomitic carbonatites, and were markedly more calcic than the dolomitic melts produced by partial melting of carbonated peridotite. Na contents of the experimental carbonatites decreased with increasing pressure when compared at similar degrees of melting, and SiO2 contents increased with degree of melting. Experiments on a second composition with enhanced Na2O demonstrated its strong effect in lowering melting temperatures in carbonate eclogite. Natural carbonated eclogite bodies in the peridotitic upper mantle will have a range of solidus temperatures. In many cases, carbonate will be molten in the upper 250km. Carbonate melt would segregate from its source eclogite at very low melt fractions and infiltrate surrounding peridotitic wall rock. This would result in metasomatic enrichment of the peridotitic wall rock, but its exact nature will depend on the relative P–T positions of the eclogite + CO2 and peridotite + CO2 solidii. As a result of these inevitable metasomatic interactions, it is considered unlikely that carbonatite melts derived from carbonated eclogite in the upper mantle could be emplaced into the crust unmodified. However, they may have a role in metasomatically enriching and carbonating parts of the upper mantle, producing sources suitable for subsequent production of silica undersaturated silicate liquids and carbonatites ultimately emplaced in the crust.