KERGUELEN ARCHIPELAGO REVISITED - GEOCHEMICAL AND ISOTOPIC STUDY OF THE SOUTHEAST PROVINCE LAVAS

In order to constrain an isotopically extreme mantle component involved in the genesis of oceanic basalts, we studied some of the youngest lavas of the Kerguelen Archipelago. According to the model developed by Gautier et al. [1], these young lavas were derived from the Kerguelen mantle plume, with little, if any, depleted MORB component. The alkaline lavas in the Southeast Province of the Kerguelen Archipelago correspond to two main volcanic phases: (a) a 20-22 Ma old lower Miocene series with basalts and trachytes belonging to a mildly alkaline series comparable to the mildly alkaline series of Gautier et al. [1] and (b) a 6.6-10.2 Ma old [2] upper Miocene series with basanites, tephri-phonolites and phonolites, forming a highly alkaline series. The lower Miocene lavas have epsilon(Nd), Sr-87/Sr-86 and Pb isotopic ratios overlapping with those of Kerguelen transitional and mildly alkaline series, i.e. epsilon(Nd) from +2.2 to -0.6 and Sr-87/Sr-86 between 0.70493 and 0.70522. The upper Miocene lavas have higher Sr-87/Sr-86 ratios ( > 0.7054) and lower epsilon(Nd) (much less than 0), i.e. comparable to the highly alkaline series of Gautier et al. [1]. In contrast, compared to other lavas from the Kerguelen Archipelago, the upper Miocene lavas have distinctly less radiogenic Pb-206/Pb-204 ratios. They plot to the left of the Indian OIB field in Pb-Pb diagrams, i.e. in a direction opposite to that expected for contamination with sediment or young continental crust. In both series, the evolved lavas and associated basalts/basanites overlap in isotopic ratios, although incompatible element abundance ratios in the evolved lavas were modified by fractionation of amphibole, Fe-Ti oxides, apatite and sphene. We propose that the isotopic characteristics of the upper Miocene lavas correspond to the pure Kerguelen plume signature, which is: Sr-87/Sr-86 = 0.7054-0.7058, epsilon(Nd) = -0.2 to -2.9, Pb-206/Pb-204 = 18.06-18.27, Pb-207/Pb-204 = 15.54-15.58 and Pb-208/Pb-204 = 38.68-39.16. The Kerguelen plume therefore has an intermediate isotopic composition and corresponds neither to the EM I nor the EM II component [3]. A continuum between EM I and EM II is expected for mantle contaminated by recycled continental crust or continent-derived material.