HR: 0800h
AN: U41A-0723    [Abstracts]
TI: New Search for 142Nd Anomaly in Kimberlites and Carbonatites
AU: * Boyet, M
EM: boyet@dtm.ciw.edu
AF: Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Rd., N.W., Washington, DC 20015 United States
AU: Carlson, R W
EM: carlson@dtm.ciw.edu
AF: Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Rd., N.W., Washington, DC 20015 United States
AB: The short-lived chronometer 146Sm-142Nd (T1/2 = 103 Ma) has been successfully used to improve our understanding of early planetary differentiation on asteroids, the Moon, Mars and Earth. On the Earth, all reported 142Nd excesses (8 to 30 ppm) are in early-Archean samples from SW Greenland (Harper and Jacobsen, Nature 1992; Sharma et al., GCA 1996; Boyet et al., EPSL 2003; Caro et al., Nature 2003). This evidence for silicate differentiation during the first hundred Myr of Earth's history is now robust, but traces of this major fractionation event are still very scarce and only the light-REE depleted reservoir has been identified so far. To determine whether the chemically distinct reservoirs created in this early differentiation still exist, we examined the 146Sm-142Nd system in kimberlites and carbonatites, the most incompatible element rich melts on Earth, some of which are believed to be of deep mantle origin. Nd was measured with the Finnigan TRITON installed at DTM in April 2004. The two standards analyzed, La Jolla (n=41) and JNdi-1 (n=20) give an external reproducibility for the 142Nd/144Nd ratio of 5 ppm, 2-sigma. Our data show no correlation between 150Nd/144Nd and 142Nd/144Nd, hence a second-order fractionation correction (Caro et al., 2003) has not been applied. JNdi-1 appears to have a 142Nd/144Nd ratio 6 ppm higher than the La Jolla standard. Though requiring more investigation, this result raises the question of which, if either, of these standards best represents the chondritic, and hence bulk-earth, 142Nd/144Nd. Repeat analyses of terrestrial samples show 142Nd/144Nd reproducibility of better than 8 ppm 2-sigma. We confirm previously reported (Boyet et al., 2003) positive anomalies (up to 20 ppm) in some metabasalts from Isua. However, no 142Nd anomalies were detected in 20 samples of Group I kimberlite from Siberia, Brazil, SW Greenland, Montana, Colorado and S. Africa, nor in two mid-ocean ridge basalts (all of which lie within the range -8 to 7 ppm of the average 142Nd/144Nd in La Jolla Nd). Among the three carbonatites from SW Greenland analyzed, only one is characterized by a very small, and still inadequately resolved, positive 142Nd excess of 11 ppm. This new investigation resolves no trace of the early silicate differentiation in the source of these rocks. Either the effects of this differentiation were mixed away by early Earth convection, or the distinct reservoirs created in this event do not produce magmas that reach the surface.
DE: 8125 Evolution of the Earth
DE: 8147 Planetary interiors (5430, 5724)
DE: 1025 Composition of the mantle
DE: 1010 Chemical evolution
SC: Union [U]
MN: 2004 AGU Fall Meeting