HR: 0800h
AN: V51B-0529    [Abstracts]
TI: Recycling and Mantle Stirring Determined by $^{142}$Nd/144Nd Isotopic Ratios
AU: * Jacobsen, S B
EM: jacobsen@neodymium.harvard.edu
AF: Harvard University, Department of Earth and Planetary Sciences, Cambridge, MA 02138 United States
AU: Ranen, M C
EM: ranen@fas.harvard.edu
AF: Harvard University, Department of Earth and Planetary Sciences, Cambridge, MA 02138 United States
AB: It is now well established that $^{146}$Sm was live in the early solar system with an initial uniform $^{146}$Sm/$^{144}$Sm ratio of ~0.008. Harper and Jacobsen (1992) discovered that a sample from Isua (~3.8 Ga old) had a positive $^{142}$Nd/$^{144}$Nd anomaly of 33 ppm when compared to normal terrestrial and chondritic Nd. Furthermore, Jacobsen and Harper (1996) reported results from other Isua as well as Acasta (~4 Ga old) samples. Three other Isua samples had a possible small range (about -15 to +15), while two Acasta samples had no anomalies (normal to within 5 ppm). The presence of $^{142}$Nd anomalies at Isua has recently been confirmed by two other groups (Boyet et al. 2003; Caro et al. 2003). The available data demonstrate both the existence of early depleted mantle and that the early mantle was isotopically heterogeneous. As discussed by Jacobsen and Harper (1996), the recycling rate can be determined by tracing the decay of the average $^{142}$Nd/$^{144}$Nd value of the depleted mantle. In addition, by using the $^{142}$Nd/$^{144}$Nd heterogeneity in the depleted mantle through time we can determine the stirring rate of the mantle (Kellogg, Jacobsen and O'Connell, 2002) as a function of time. For this project our goal is to obtain a resolution in $^{142}$Nd/$^{144}$Nd measurements of ~1 ppm. We have thus compared results obtained for the Nd isotope composition and $^{142}$Nd enriched standards for three different TIMS instruments: The Finnigan MAT 262 at Harvard, the Isoprobe-T and Finnigan TRITON mass spectrometers in GV Instrument's and Thermo Electron's demo laboratories in Manchester and Bremen, respectively. The Finnigan TRITON was designed in response to a request from the senior author for such an instrument. The results obtained so far demonstrate that all three instruments yield the same $^{142}$Nd/$^{144}$Nd, $^{143}$Nd/$^{144}$Nd and $^{145}$Nd/$^{144}$Nd isotopic ratios to within a few ppm, while $^{148}$Nd/$^{144}$Nd and $^{150}$Nd/$^{144}$Nd ratios agree to within 10-20 ppm, when all ratios are normalized to $^{146}$Nd/$^{144}$Nd using the exponential law. Due to the excellent agreement between results from three different instruments we conclude that other reports that claimed that such measurements could not be reproduced at the 5 ppm level for either our 15 year old MAT262 or the newer instruments must be in error. Acknowledgements: We thank GV Instruments and Thermo Electron Corporation for making measurements of our $^{142}$Nd enriched standards.
DE: 8147 Planetary interiors (5430, 5724)
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
DE: 1010 Chemical evolution
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting