HR: 0800h
AN: U11A-0020    [Abstracts]
TI: Nd Isotope Systematics for the 3.8 Ga Nuvvuagittuq Greenstone Belt: Implications for the Degree of Depletion of Early Earth's Mantle
AU: * O'Neil, J
EM: oneil_jo@eps.mcgill.ca
AF: Earth & Planetary Sciences, McGill University, 3450 University St., Montreal, Qc H3A 2A7, Canada
AU: Francis, D
EM: donf@eps.mcgill.ca
AF: Earth & Planetary Sciences, McGill University, 3450 University St., Montreal, Qc H3A 2A7, Canada
AU: Stevenson, R
EM: stevenson.ross@uqam.ca
AF: GEOTOP-UQAM-McGill, PO Box 8888, St. Centre-ville, Montreal, Qc H3C 3P8, Canada
AB: Rare occurrences of Eoarchean mantle-derived crust provide the only compositional and isotopic constraints on the early crust-mantle differentiation of the Earth. The relatively high positive initial εNd values obtained on Eoarchean rocks (Acasta Gneisses, Slave Province, Canada; Nulliak Assemblage, Labrador, Canada; Amītsok Gneisses, Akilia and Isua supracrustal assemblages, SW Greenland) are interpreted to indicate derivation from a mantle source already strongly depleted in the Eoarchean, implying that significant volumes of continental crust had formed early in the Earth's evolution. Here we present the Nd isotope data of the newly discovered ca. 3.8 Ga Nuvvuagittuq greenstone belt (Northern Québec, Canada), and discuss their implications for early mantle depletion. The Nuvvuagittuq greenstone belt is a volcano-sedimentary sequence mainly composed of cummingtonite- amphibolites, numerous ultramafic and gabbroic sills, and chemical sedimentary rocks. Most mantle-derived samples display positive initial εNd values ranging from -2 to +4, with an average of +1.2 ± 0.2 for 44 samples. Isotopic data for all the samples that have been analysed yield a Sm-Nd regression age of 3.81 ± 0.13 Ga (MSWD = 8.5), and a subset of samples from the gabbroic sills gives an isochron age of 3.83 ± 0.39 Ga (MSWD = 0.39), with initial εNd values of +1.2 and +2 respectively. Sm-Nd isotopic data for the Nuvvuagittuq mantle-derived rocks support an age of 3.8 Ga, consistent with previous geochronology, and the low MSWD for the Sm-Nd regression lines suggest minimal disturbance of the Sm-Nd isotopic system. The few samples that yield εNd_(3.8Ga) values greater than +3 suggest that they may have been derived from a mantle source that had already experienced even more extensive trace element depletion than the present-day MORB source by 3.8 Ga. Such a degree of trace element depletion, however, is not supported by the flat to slightly LREE-depleted profiles of the gabbros. Nuvvuagittuq samples which yield εNd_(3.8Ga) values greater than +3 have Sm/Nd ratios which are too low to be consistent with melting of a mantle source having an εNd value of +3 or greater at 3.8 Ga. This implies that the trace element depletion is not consistent with the isotopic depletion seen in these most isotopically depleted samples. Consequently, εNd_(3.8Ga) values of ~+2 for the Nuvvuagittuq mantle-derived rocks are most plausible, indicating derivation from mantle already depleted in the Early Archean, but depleted to a lesser extent than that which is seen in the 3.8 Ga SW Greenland rocks.
DE: 1025 Composition of the mantle
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
SC: Union [U]
MN: 2007 Fall Meeting