HR: 0800h
AN: V51B-0570    [Abstracts]
TI: An Improved Method for TIMS High Precision Nd Isotopic Analysis of Very Small Aliquots (1- 10ng) With Example Application in Garnet Sm/Nd Geochronology
AU: * Baxter, E F
EM: efb@bu.edu
AF: Boston University, Earth Sciences, 685 Commonwealth Avenue, Boston, MA 02215, United States
AU: Harvey, J
EM: harveyj@bu.edu
AF: Boston University, Earth Sciences, 685 Commonwealth Avenue, Boston, MA 02215, United States
AU: Mehl, L Y
EM: lmehl@bu.edu
AF: Boston University, Earth Sciences, 685 Commonwealth Avenue, Boston, MA 02215, United States
AU: Peterman, E M
EM: epeterman@umail.ucsb.edu
AF: Earth Science, University of California, Santa Barbara, CA 93106, United States
AB: Technological and scientific developments have demonstrated both the attainability and the utility of very high precision (i.e. 5-20ppm 2 σ) Nd isotopic measurements with TIMS. However such high precision has been limited to relatively large aliquots of Nd, on the order of several hundred nanograms. Several potential applications of precise Nd isotopic measurements, including garnet Sm/Nd geochronology, do not always permit such large samples, instead yielding only a few nanograms of Nd. We have explored and tested an improved method for Nd isotopic analysis of such small (1-10ng) aliquots of Nd using the NdO+ method with a Triton TIMS at Boston University. Analyzing Nd isotopes as the oxide is a well known technique, frequently involving an oxygen bleed valve. Instead, we forego the bleed valve and load samples with a TaO slurry which provides the oxygen source. Using an in-house Nd isotopic standard solution, 4ng loads easily yield stable 2.0-2.5 volt beams resulting in internal precisions of 10ppm 2 σ RSE. Within barrel external precision of 4ng loads of the Nd standard is 13ppm 2 σ RSD (n=20). Long term (6 months, six analysts) external precision of 4ng loads of the standard is currently 23ppm 2 σ RSD (n=55) suggesting that further improvements are possible. As a further test of this method, we dissolved a natural rock sample (a metapelite), separated the Nd using TRU- spec and MLA column chemistry, and loaded nineteen 4ng loads in one barrel. Within barrel external precision was 21ppm 2 σ RSD (n=18). This precision represents a significant advance over previous NdO+ analyses of small samples using an oxygen bleed valve. The TaO loading method for small Nd aliquots is useful in Sm/Nd garnet geochronology as exemplified by two case studies. Garnets from eclogite facies gneisses from Norway ran very well with 2.4-18ng loads and yielded age precision as good as 0.8 million years 2 σ. Conversely, garnets from blueschist facies rocks from Sifnos, Greece, ran poorly with similarly sized 1-17ng loads and consequently yielded generally poorer age precision. Differences between the two garnet sample suites must relate to the garnets themselves (notably including much lower Nd concentration in Sifnos garnets), not the identical column chemistry nor the TaO loading method. Additional procedures may be required to cleanly separate Nd from samples where Nd concentrations are very low (≪1ppm). As always, clean separation and column chemistry represents an unavoidable limiting factor in achieving precise isotopic measurements.
DE: 1040 Radiogenic isotope geochemistry
DE: 1094 Instruments and techniques
DE: 1115 Radioisotope geochronology
DE: 1194 Instruments and techniques
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting