GEOCHEMICAL HETEROGENEITY OF THE MAGMATISM OF THE AFANASIJ-NIKITIN RISE, NORTHEASTERN INDIAN-OCEAN

In order to clarify the geochemistry of the source of Afanasij Nikitin Rise rocks, which were formed during the age interval 90-65 Ma ago wi thin the structure of the Southeast Indian Ridge, samples of three ign eous series from the rise were analyzed (tholeiitic subalkalic olivine , porphyritic basalt, and subalkalic trachybasalt-trachyte). The rocks were sampled during a cruise of the RN Akademik Mstislav Keldysh in 1 990, using the manned submersible Mir. The first data on the Rb-Sr, Sm -Nd, and U-Pb systems were obtained by mass spectrometer analysis. The abundances of Pb, U, Sm, Nd, Rb, and Sr in the rocks of the rise were determined by isotope dilution. It was found that (1) the basalts of the tholeiitic series, which are the most common in the rise, are simi lar to the tholeiites of the Ninetyeast Ridge and the Kerguelen Platea u with respect to isotopic signature and incompatible element ratios, which suggests the similar conditions of their formation and similar s ources. (2) The rocks of the subalkalic series differ considerably fro m the tholeiitic rocks. The characteristic ranges of their isotopic ra tios are as follows: Pb-206/Pb-204 = 16.787-17.756, Pb-207/Pb-204 = 15 .456-15.613, Pb-208/Pb-204 = 37.397-38.140, Nd-143/Nd-144 = 0.5121-0.5 124, and Sr-87/Sr-86 = 0.7059-0.7070. The subalkalic olivine porphyrit ic basalts of the lower levels of the Afanasij Nikitin Rise appeared t o have the lowest Pb-206/Pb-204 ratio (16.919) documented in the rocks of the Indian Ocean. The subalkalic series of the rise differ from th e alkali basalts of the Kerguelen Plateau, but are similar to Indian t rap basalts in their high Sr-87/Sr-86 (0.706) and low Pb-206/Pb-204 (1 7.2) values. (3) The wide variations of Pb, Sr, and Nd isotope ratios displayed by the rocks of the Afanasij Nikitin Rise suggest complex co nditions of the mantle melting beneath the Southeastern Indian Ridge a t the initial stages of its opening. Some material enriched in radioge nic Sr and nonradiogenic Pb and Nd isotopes and depleted in Th, Ta, an d Nb was probably involved to a varying degree in the melting process. This material was represented either by crustal granulites or by meta morphosed rocks from the continental lithospheric mantle, the partial melting of the latter during the early stages of Southeast Indian Ridg e opening produced melts, which could be transported into spreading zo nes during the evolution of the Southeast Indian Ridge.