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

Alkali basalts from the Kerguelen Islands have entrained numerous phlogopit e 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) segregat es (Type II). A lamprophyric dyke containing phlogopite megacrysts has been also studied. Chemical compositions of amphiboles and phlogopites from bot h xenolith types are similar to those recognized in many ultramafic and maf ic volatile-bearing xenoliths from kimberlites and alkali basalts and in pe ridotites and pyroxenites from orogenic Iherzolite massifs. Interstitial am phibole and phlogopite in harzburgites and dunites probably formed during d iffuse percolation of highly alkaline basic silicate melt within the upper mantle (porous flow). Evidence from composite xenoliths suggest that simila r mantle melts migrated through a network of dykes generated by hydraulic f racturing in the Kerguelen upper mantle. The lamprophyre is the surface exp ression of this highly alkaline magmatic activity. The delta D values of -9 2 to -61% SMOW for mica and amphibole of Type I and Type II xenoliths and o f the phlogopite megacrysts are within the accepted mantle range. Calculate d delta D-H2O values in equilibrium with amphiboles and micas have a bimoda l distribution (- 65 +/- 5 % and -83 +/- 5 %) indicating that the percolati ng fluids were isotopically heterogeneous. The ubiquity of the highly alkal ine magmatic activity is probably related to the late intraplate activity o f the Kerguelen mantle plume.