COGENETIC SILICA-RICH AND CARBONATE-RICH MELTS TRAPPED IN MANTLE MINERALS IN KERGUELEN ULTRAMAFIC XENOLITHS - IMPLICATIONS FOR METASOMATISM IN THE OCEANIC UPPER-MANTLE

In an attempt to characterize metasomatic agents for the oceanic upper mantle, we have undertaken a study of melt and fluid inclusions trapped in metasomatized peridotite nodules (anhydrous spinel lherzolites and harzburgites) from the Kerguelen Islands (southern Indian Ocean). These xenoliths contain three types of genetically related inclusions hosted by olivine, clinopyroxene and orthopyroxene. These are silicate melt inclusions, carbonate-rich inclusions and CO2 fluid inclusions. These inclusions are secondary in nature and form trails along fracture planes in the sheared peridotites. Heating experiments conducted on silicate melt inclusions give an estimation of the entrapment temperatures (almost-equal-to 1250-degrees-C) and indicate that there is no genetic relationship between the inclusions and their host minerals. The chemical composition of the silicate melt inclusions is characterized by normative quartz and feldspar components, with SiO2 almost-equal-to 60wt%, Al2O3 almost-equal-to 20wt%, Na2O and K2O each almost-equal-to 4-5wt%, FeO and MgO < 3 wt%, Cl > 1000 ppm, H2O greater-than-or-equal-to 1.2%, and oversaturation of the melt with CO2. The trace element signature is characterized by LREE enrichment, negative HFSE (Ti and Zr) anomalies and a Ti/ Zr value of 17. The trapped melt has crystallized the following minerals: K-rich amphibole (kaersutite), diopside, rutile, ilmenite and carbonate (magnesite). Carbonate-rich inclusions, interpreted as trapped carbonate melt, have crystallized calcite. The carbonate-rich inclusions are often physically connected with the silicate melt inclusions, indicating the former existence of a homogeneous melt which later unmixed into two separate melts. Cogenetic relationships between CO2 inclusions and both carbonate melt inclusions and silicate melt inclusions yield a minimum trapping pressure for all types of inclusions of 12.5 kbar at 1250-degrees-C, corresponding to upper mantle depths. Based on their daughter mineral types, their chemical composition and high volatile element contents, the silicate-carbonate melt inclusions trapped in the ultrabasic xenoliths of the Kerguelen Islands are interpreted as small amounts of a metasomatic melt phase. These melt inclusions cannot result from melting of the anhydrous peridotite assemblages in which they have been trapped. They, must represent an exotic, migrating metasomatic melt phase in the oceanic lithosphere below the Kerguelen Islands.