The early history of the Earth's mantle: Insights from 146Sm-142Nd chronometry

Details The early history of the Earth's mantle: Insights from 146Sm-142Nd chronometry The early history of the Earth's mantle: Insights from 146Sm-142Nd chronometry

Apr 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja.....5991c&link_type=abstract

EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #5991

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The 146Sm-142Nd chronometer (T1/2 = 103 Ma) can provide precise age constraints on the very early (> 4.2 Ga) history of the Earth's mantle. However, due to its low initial abundance [1], the decay of now extinct 146Sm in mantle reservoirs with fractionated Sm/Nd is unlikely to have produced 142Nd anomalies larger than 30-40 ppm. Because until recently the precision of Nd isotope ratios was limited to 15-20 ppm using the most precise instruments, previous attempts to decipher the terrestrial record of 146Sm yielded only controversial hints of the presence of 142Nd anomalies in early Archean rocks [2,3]. We have reconsidered this issue using the new generation TIMS TRITON. From repeated measurements of the Nd standard AMES, we estimate that a long-term reproducibility of 5 ppm (2σ) can be achieved for the 142Nd/144Nd ratio, thereby allowing confident resolution of 142Nd anomalies as small as 10 ppm. Our measurements of 7 metasediment samples from Isua (Greenland, 3.75 Ga) yielded well-resolved 142Nd anomalies averaging 15 ± 4 ppm (2σ). This probably reflects differentiation of the Earth's mantle ca. 100 Ma after the oldest meteorites formed. Nevertheless, several questions remain unanswered. Firstly, it is not known whether measured anomalies are representative of the early Archaean mantle or, alternatively, reflect a more complex geological history of the metasediment source region. Secondly, it is still uncertain whether the present-day MORB source is chondritic and can be used as an estimate for the Bulk Earth. In an attempt to answer these questions, we will present new ultrahigh-precision 142Nd data for various types of supracrustal rocks from Isua and South Africa (3.4 Ga), as well as plutonic gneisses from West Greenland (3.65 Ga and 3.81 Ga) and Acasta (4.0 Ga). Additionally, the present-day 142Nd signature of mantle reservoirs will be discussed based on measurements of different OIB lavas. References [1] Prinzhofer, A., Papanastassiou, D.A. and Wasserburg, G.J., (1992) Geochim. Cosmochim. Acta 56, 797-815. [2] Harper C.L. and Jacobsen S.B., (1992), Nature 360, 728-732. [3] Sharma M., Papanastassiou D.A. and Wasserburg G.J., (1996), Geochim. Cosmochim. Acta 60, 2037-2047.

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