Geochemistry of hydrous ultrabasic-basic xenoliths: Fluids in the Kerguelen plume.

Moine B.N.1 (bertrand.moine@univ-st-etienne.fr), Cottin J-Y.1 (cottin@univ-st-etienne.fr), Sheppard S.M.F.2 (Simon.Sheppard@ens.ens-lyon.fr), Gregoire M.3 (mgregoir@laurel.ocs.mq.edu.au), Giret A.1 (giret@univ-st-etienne.fr)

1 Departement de Geologie-Petrologie-Geochimie, Universite Jean Monnet,23

rue Dr P. Michelon, 42023 Saint-Etienne, FRANCE

2 ENS-Lyon, 46 all e d'Italie, 69364 Lyon cedex 07, FRANCE

3 GEMOC, Macquarie University, NSW 2109, AUSTRALIA

Alkali basalts from the Kerguelen Islands have entrained many mantle peridotites (harzburgites and dunites) in addition to various ultramafic-mafic xenoliths (deep tholeiitic- transitional and alkali segregates). Numerous peridotites contain disseminated and/or in vein Ti-pargasite and/or Ti-phlogopite. The interstitial hydrous phases are systematically associated with Na-Cr rich poecilitic augite, Cr-spinel enriched in Ti and Fe3+; serpentine and carbonates may be present. In the alkaline segregates (amphibole and/or phlogopite bearing clinopyroxenites, hornblendites, glimmerites) the major hydrous minerals are <95 vol%.

The Cl and F contents of amphiboles and phlogopites, analysed by electron microprobe, display similarities with hydrous phases of kimberlites (Mica:Cl: 0-0.11 wt% and F: 0.1-0.75wt% ; amphibole: Cl: 0.018-0.095wt% and F: 0.05-0.22wt%).

The calculated liquids in equilibrium with amphiboles may be related either to an alkaline type (La: 200x prim.mantle and La/Yb=9) or to a carbonatitic one (La: 1000x prim.mantle and La/Yb=116). As in subduction zones, the amphibole plays the role of a Nb and Ta reservoir. Trace elements and REE spider-diagrams show a large LILE enrichment with negative Rb, Ba, K, Sr, Hf and Ti anomalies which fit the positive ones observed in phlogopite. Such anomalies may be explained by early phlogopite fractionation of the percolating fluids.

Both amphiboles and micas show a systematic deficit in the filling of the OH site (30 to 75%) which cannot be explained by their halogen contents (0.8wt% max).The dD value of -75 to -90o/oo SMOW are within the accepted mantle range. With no evidence for late loss of hydrogen, the infiltrating fluids were relatively homogeneous. Hydrogen deficiency could thus be a characteristic of mantle minerals.

Back to the GEMOC Abstract Titles Page