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

In order to constrain an isotopically extreme mantle component involve d in the genesis of oceanic basalts, we studied some of the youngest l avas 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 alkal ine lavas in the Southeast Province of the Kerguelen Archipelago corre spond to two main volcanic phases: (a) a 20-22 Ma old lower Miocene se ries 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-pho nolites and phonolites, forming a highly alkaline series. The lower Mi ocene lavas have epsilon(Nd), Sr-87/Sr-86 and Pb isotopic ratios overl apping 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 a nd 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, compare d to other lavas from the Kerguelen Archipelago, the upper Miocene lav as 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 directio n opposite to that expected for contamination with sediment or young c ontinental crust. In both series, the evolved lavas and associated bas alts/basanites overlap in isotopic ratios, although incompatible eleme nt abundance ratios in the evolved lavas were modified by fractionatio n 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-20 4 = 15.54-15.58 and Pb-208/Pb-204 = 38.68-39.16. The Kerguelen plume t herefore has an intermediate isotopic composition and corresponds neit her 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.