The Refractory Nature of Carbonate during Partial Melting of Eclogite: Evidence from High Pressure Experiments and Natural Carbonate-Bearing Eclogites

Abstract

The importance of trace amounts of volatile phases in mantle processes has been well established in recent years. The presence of H20 _+ CO2 can significantly alter melting relations in peridotite and the nature of the primary melts produced. Carbonate-bearing hydrous peridotites are capable of yielding dolomitic carbonatite as the low melting fraction at ~930~ near 2 GPa (Wallace and Green 1988). However, melting relations of carbonate-bearing hydrous marie compositions (quartz-bearing eclogite) have not been determined and may be quite different to peridotite because of the much lower silicate solidus in hydrous, marie compositions (~700~ at 2 GPa). This is of importance as many altered ocean-floor basalts contain calcitic carbonate as a low temperature alteration phase. Subduction of such material has the potential to transport carbonate into a high pressure-temperature regime. The behaviour of carbonate under these conditions is currently constrained by early simple system studies (e.g. Huang et al., 1980) which showed that decarbonation or melting of carbonate would not occur under the PT regime encountered in a slab. This early work did not determine the stability of carbonate in the presence of partial silicate melting. In order to elucidate this, high pressure piston-cylinder experiments were performed using a complex natural composition representative of altered oceanic basalt to which had been added carbonate of varying compositions.