HR: 14:25h
AN: U33B-04    [Abstracts]
TI: Evolving Mantle Regimes Revealed by $^{143}$Nd Isotopic Compositions of 3850Ma to Modern Mafic rocks
AU: * Bennett, V C
EM: vickie.bennett@anu.edu.au
AF: Research School of Earth Sciences, The Australian National Universitay, Canberra, ACT 0200 Australia
AU: Nutman, A P
AF: Research School of Earth Sciences, The Australian National Universitay, Canberra, ACT 0200 Australia
AU: Norman, M D
AF: Research School of Earth Sciences, The Australian National Universitay, Canberra, ACT 0200 Australia
AU: Friend, C R
AB: The ground truth of mantle models, particularly for the early Earth, relies on accurate determinations of original isotopic compositions. The $^{147}$Sm-$^{143}$Nd isotopic system can provide the most complete record of mantle differentiation processes, when combined with intensive field and geochronologic studies leading to judicious sample selection. Our on-going field and laboratory investigations have identified the islands near Nuuk, southwest Greenland, which include Akilia and Innersuartuut, as a key repository of information on early Earth chemistry. The oldest orthogneisses, with SHRIMP U-Pb zircon dates of 3850-3840Ma now determined at four localities cross-cut and provide minimum ages for varied mafic/ultramafic rock sequences. The initial (at 3850Ma) $^{143}$Nd isotopic compositions determined from 22 samples from the best preserved rocks in low strain areas, all have positive $\epsilon$Nd with a narrow range of initial compositions from +2 to +4 ($\pm$0.5). No negative values have been identified. This isotopic homogeneity at 3850Ma constrasts with the the large range of positive and negative $\epsilon$Hf measured in $>$4.0Ga zircons from Western Australia (Harrison et al., 2004; Goldschmidt abs.). Comparison of the new Nd data with compilations of literature data from younger crust, including 3.0 Ga and 2.7 Ga rocks indicates rapid early mantle depletion, with 3850Ma compositions of ca. +3 followed by little change in $\epsilon$Nd for greater than 1 billion years, from 3.85 to at least 2.7 Ga. This view of the mantle contrasts with the well established near linearly increasing $\epsilon$Nd values throughout much of the Proterozoic and Phanerozoic to values of +10 characterising the MORB source mantle today. The apparent large scale changing pattern of isotopic evolution must record mantle regimes characterised by different mixing times and/or variable (size, number and origin) enriched and depleted reserviors. Early Archean rocks record rapid early depletion of some portions of the upper mantle. The extended period of near constant isotopic compositions records balancing of the effects of crust extraction by either rapid crustal recycling, or mixing with deep mantle enriched domains; post-Archean Nd isotopic compositions reflect the transition to a modern style tectonic regime.
DE: 8125 Evolution of the Earth
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 1010 Chemical evolution
SC: Union [U]
MN: 2004 AGU Fall Meeting