Nd isotopic evidence for transient, highly depleted mantle reservoirs in the early history of the Earth

Details Nd isotopic evidence for transient, highly depleted mantle reservoirs in the early history of the Earth Nd isotopic evidence for transient, highly depleted mantle reservoirs in the early history of the Earth

Sep 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993e%26psl.119..299b&link_type=abstract

Earth and Planetary Science Letters (ISSN 0012-821X), vol. 119, no. 3, p. 299-317

Physics

Geophysics

29

Earth (Planet), Earth Mantle, Geology, Geophysics, Inclusions, Neodymium Isotopes, Planetary Evolution, Chemical Reactions, Chondrites, Depletion, Gneiss, Greenland

Scientific paper

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(sub Nd) values 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(sub Nd) value of approximately 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. The highest initial epsilon(sub 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(sub Nd)(3450 Ma)(greater than +8) and epsilon(sub Nd)(2700 Ma)(greater than +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(sub Nd) crust. 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.

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