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Incompatible trace element and isotopic (D/H) characteristics of amphibole- and phlogopite-bearing ultramafic to mafic xenoliths from Kerguelen Islands (TAAF, South Indian Ocean)

^a Départment de Géologie-Pétrologie-Géochemie, UMR-CNRS 6524, Université Jean Monnet, 23 Rue P. Michelon, 42023 Saint-Etienne cedex 02, France. – e-mail: moineb{at}univ-st-etienne.fr
^b Labaoratoire de Sciences de la Terre, UMR-CNRS 5570, Ecole Normale Supérieure de Lyon, 46 Allée d'Italie, 69366 Lyon cedex 07, France
^c GEMOC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, NSW 2109, Australia

Alkali basalts from the Kerguelen Islands have entrained numerous phlogopite- and amphibole-bearing ultramafic to mafic xenoliths. These are subdivided into mantle harzburgites, dunites and associated composite xenoliths that represent mantle wall-rock (Type-I) and high pressure (10–15 kbar) segregates (Type II). A lamprophyric dyke containing phlogopite megacrysts has been also studied. Chemical compositions of amphiboles and phlogopites from both xenolith types are similar to those recognized in many ultramafic and mafic volatile-bearing xenoliths from kimberlites and alkali basalts and in peridotites and pyroxenites from orogenic lherzolite massifs. Interstitial amphibole and phlogopite in harzburgites and dunites probably formed during diffuse percolation of highly alkaline basic silicate melt within the upper mantle (porous flow). Evidence from composite xenoliths suggest that similar mantle melts migrated through a network of dykes generated by hydraulic fracturing in the Kerguelen upper mantle. The lamprophyre is the surface expression of this highly alkaline magmatic activity. The D values of –92 to –61 SMOW for mica and amphibole of Type I and Type II xenoliths and of the phlogopite megacrysts are within the accepted mantle range. Calculated D-H²O values in equilibrium with amphiboles and micas have a bimodal distribution (- 65 ± 5 and –83 ± 5) indicating that the percolating fluids were isotopically heterogeneous. The ubiquity of the highly alkaline magmatic activity is probably related to the late intraplate activity of the Kerguelen mantle plume.

Key-words: amphibole- and phlogopite-bearing xenoliths, mantle trace elements, stable isotopes, Kerguelen Islands.

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