ND AND SR ISOTOPES FROM DIAMONDIFEROUS ECLOGITES, UDACHNAYA KIMBERLITE PIPE, YAKUTIA, SIBERIA - EVIDENCE OF DIFFERENTIATION IN THE EARLY EARTH

Nd and Sr isotopic data from diamond-bearing eclogites found in the Ud achnaya kimberlite, Yakutia, Siberia, are interpreted as indicating an early (greater-than-or-equal-to 4 Ga) differentiation event, whereby the mantle split into complementary depleted and enriched reservoirs. Reconstructed whole-rock Sr-87/86-Sr ratios (present-day) range from 0 .70151 to 0.70315 and are consistent with a mantle origin for these ro cks. The Nd isotopic evolution lines of four samples (U-5, U-37, U-41 and U-79) converge at 2.2-2.7 Ga. Sample U-5 is unique in exhibiting t he most enriched signature of any of the samples yet analyzed (present -day epsilon(Nd) of - 20), and this sample points unequivocally to an old, enriched component. A complementary depleted mantle component is suggested by two of the eclogite samples, U-86 and U-25, which yield e psilon(Nd) values (at 2.2 Ga) of + 13 and + 7, respectively. The two m antle reservoirs possibly formed prior to 4 Ga and evolved separately until 2.2-2.7 Ga. At that time, the reservoirs were melted forming ecl ogites both as residues (from the enriched reservoir) and as partial m elts of peridotite (from the depleted reservoir), resulting in demonst rably different histories for eclogites from the same locality.