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Rasmus Andreasen

High Precision Neodymium Isotope Measurements - Reconciling Chondrite Meteorite and Terrestrial Flood Basalt Data

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  • R. Andreasen
  • M. Sharma, Department of Earth Sciences, Dartmouth College, 6105 Sherman Fairchild Hall, Hanover, NH 03755, USA Mukul.Sharma@Dartmouth.edu
Recent high-precision static-mode neodymium isotope measurements of chondrites give an average of -20 mu142 (= ppm deviations from a terrestrial std) indicating an early terrestrial differentiation event (Boyet and Carlson, 2005). This also suggests that a complementary low Sm/Nd reservoir should also be present somewhere on earth with negative m142 values. In order to investigate this issue further we have obtained high precision Nd isotopes data for Allende (CV3), Franconia (H5), NWA 800 (R4) and NWA869 (L4) using the Dartmouth Triton. The data were obtained in both static and dynamic modes. In dynamic mode the CIT-Nd-β gives 142Nd/144Nd =1.141832 with a long term external reproducibility of 9 m (2s). For Allende we obtain mu142= -4 relative to CIT-Nd-β, the three other meteorites all give an average mu142 of -17. We get a present day e143 for Allende of +2.1, -3.7 for Franconia, -0.5 for NWA800 and +0.3 for NWA869. It is intriguing that the Allende sample does not give a negative anomaly as observed by Boyet and Carlson (2005). Meteorite fragments with variable Sm/Nd ratios can potentially display this pattern. If so, they should fall on a line in e143-mu142space provided there is no later disturbance of the Sm/Nd ratio; however the observed linear trends are hampered by relatively large analytical uncertainties and are not easily resolvable with the isotope abundances and decay constants. Further high-accuracy and high-precision analyses are needed in order to quantify and explain the e143-mu142 variations seen in chondrites. Measurements of primitive terrestrial basalts from mantle plumes (Deccan and Iceland), including samples with a solar He and Ne component, presumably originating from the D''-layer do not have resolvable 142Nd anomalies in dynamic mode. Extensive experimentation with measurements in multi-collector static and dynamic modes with normal and enriched Nd standards interleaved with samples shows that dynamic mode is much preferable in order to obtain results that are both accurate and precise. We find that even with utmost care the 142Nd/144Nd measured in static mode is at times significantly lower than dynamic mode.
OriginalsprogEngelsk
TidsskriftAmerican Geophysical Union, Fall Meeting 2005
Vol/bind33
StatusUdgivet - 1 dec. 2005
Eksternt udgivetJa

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