ND-ISOTOPIC EVIDENCE FOR TRANSIENT, HIGHLY DEPLETED MANTLE RESERVOIRS IN THE EARLY HISTORY OF THE EARTH

3870 Ma to 3760 Ma metadiorites and tonalites and older mafic inclusions contained within these rocks from southern West Greenland, have a range of initial epsilon(Nd) value, from +4.5 to -4.5. The extremely positive initial values determined for nine of the fourteen early Archean samples demonstrate that they were derived from a LREE depleted mantle reservoir which had an epsilon(Nd) value of almost-equal-to +4 prior to 3800 Ma. These rocks provide the best constrained evidence for the existence of highly LREE fractionated mantle reservoirs in the early Earth. In contrast, previous studies of younger 3700 and 3730 Ma tonalites from the same region show that the latter have lower initial epsilon(Nd) values ranging from -4.6 to +2.0. The highest initial epsilon(Nd) Values determined for 3450 Ma komatiites from western Australia and southern Africa are +2 to +4, and for worldwide ca. 2700 Ma greenstone belts are +2 to +5. These values are well below the values for epsilon(Nd)(3450 Ma) (> +8) and epsilon(Nd)(2700 Ma) (> +15) that would be expected for the continued evolution of a highly depleted reservoir with Sm/Nd similar to that which produced the ca. 3.8 Ga Greenland metadiorites and tonalites. Thus, if the source region for the oldest Greenland gneisses was the prevalent upper mantle composition, it must have been an ephemeral feature later modified either by mixing with less-depleted mantle or with recycled LREE enriched, negative epsilon(Nd) crust. The generation of highly positive epsilon(Nd) values by 3.8 Ga requires differentiation of an extremely LREE fractionated reservoir very early in Earth's history. The Archean Nd isotope data may record the isolation, depletion by crustal extraction and subsequent partial rehomogenization of limited portions of the upper mantle, or alternatively may reflect transient large-scale differentiation processes unrelated to crustal extraction such as might occur in a terrestrial magma ocean.