GEMOC - Department of EPS - Faculty of Science

CARBONATES IN ULTRAMAFIC XENOLITHS FROM THE KERGUELEN ARCHIPELAGO (SOUTHERN INDIAN OCEAN): EVIDENCE FOR CARBONATITE MAGMAS IN THE MANTLE

Bertrand MOINE1, Simon M.F. SHEPPARD2, Sue Y. O'REILLY3, Jean-Yves COTTIN4, Michel GREGOIRE5 , Peter BOWDEN4, and André GIRET4
1. Lab. De Géochimie des Systèmes Volcaniques, ESA 7046 CNRS, IPGP & Université. P.M. Curie, case 109, 4 Pl. Jussieu, F-75005 Paris
2. Dept. des Sciences de la Terre, UMR 5570 CNRS, ENS-Lyon, 46 allée d'Italie, F-69364 Lyon cedex 07
3. GEMOC Macquarie
4. Dept. de Géologie UMR 6524 CNRS, Université J. Monnet, 23 rue Dr. P. Michelon, F-42023 cedex 02Saint-Etienne
5. Dept. of Geological Sciences, University of Cape Town, Rondebosch 7700, South Africa

The ultramafic xenolith collection from Kerguelen archipelago has provided the greatest diversity of xenoliths ever observed in an oceanic setting. Among them, peridotites can be subdivided into mantle harzburgites, dunites and associated composite xenoliths that represent mantle wall-rock. Furthermore, some xenoliths are rich in volatile-bearing minerals (amphiboles, micas, carbonates ? calcite, dolomite, magnesite-) and these have been studied along with phlogopite and amphibole megacrysts from peralkaline silica-undersaturated lavas. This study shows that percolating fluids in peridotites are mantle-derived and related to the latest alkaline magmatic activity which characterizes the Kerguelen plume, with the final stage represented by silicate and carbonate melts, such as those observed within trapped inclusions (Schiano et al., 1994).
Carbonate melts have physical and chemical properties which enhance migration along grain boundaries within mantle peridotites. In contrast, silicate melts seem to be confined to veinlets and the vicinity of the wall-rock (on mm to cm scale). Solid-liquid reactions between silica undersaturated melts and orthopyroxene, can produce Cr-Na-rich clinopyroxene and both alkali-rich silica oversaturated melts and carbonate melts.
Clinopyroxenes, amphiboles, micas, Fe-Ti oxides, Fe-Ni sulfides, carbonates and phosphates are the new mineral phases occurring within magma percolation channels during multi-stage mantle metasomatism. These new mineral phases have preserved their "primary" stable isotopic mantle signatures (C, H) which are relatively homogeneous. The _D values of micas and amphiboles of xenoliths and megacrysts lie within the accepted mantle range, but the _DH2O values calculated in equilibrium with amphiboles and micas at the same temperature, exhibit a bimodal distribution that could indicate the existence of two different mantle fluids.
Carbonate within lavas and peridotites shows mantle isotopic compositions (O, C) and provides no evidence about recycled lithospheric components.
Metasomatic clinopyroxene and amphibole display trace element patterns suggesting that these minerals are in equilibrium with highly alkaline silicate melts and occasionally with carbonate melts. Trace element patterns in carbonates from amphibole-bearing dunites are characterized by low alkali content; high Mg/(Mg+Fe) ratio; high Sr, Ba content, appear unusual in their high LREE enrichment. From microstructural features and chemical composition, it is deduced that carbonate inclusions represent quenched carbonatite liquids rather than crystal cumulates from carbonate-rich melts as observed by Lee et al., (2000) in a different setting. Moreover, trace elements in carbonates (<0.5% in mode) control the trace element signature of the whole-rock. These patterns are similar to those found in carbonated peridotites from Mongolia and Spitsbergen (Ionov et al., 1998).