Niobium and tantalum in carbonaceous chondrites: Constraints on the solar system and primitive mantle niobium/tantalum, zirconium/niobium, and niobium/uranium ratios

We have determined Nb, Y, and Zr abundances in the carbonaceous chondrites Orgueil (CI), Murray (CM2), Murchison (CM2), Allende (CV3), and Karoonda (C K4), and in the eucrites, Pasamonte and Juvinas, by a recently developed sp ark source mass spectrometric technique using multiple ion counting (MIC-SS MS). The abundance of Ta was determined in the same meteorites by radiochem ical neutron activation analysis (RNAA). Precision of the MIC-SSMS and RNAA techniques is similar to 3% and less than or equal to 5%, respectively. The new abundances for CI chondrites are: Nb = 0.247, Ta = 0.0142, Zr = 3.8 6, Y = 1.56 mu g/g; or 0.699, 0.0202, 11.2, and 4.64 atoms/10(6) Si atoms, respectively. The values agree with earlier compilations, but they are a fa ctor of 2 more precise than earlier analyses. Trace element concentrations in the CM, CV, and CK chondrites are higher than in the CI chondrite Orguei l by about 37, 86, and 120%, respectively, in agreement with the variable a bsolute contents of refractory lithophile elements in different groups of c arbonaceous chondrites. Of particular interest are the chondritic Nb/Ta, Zr/Nb, and Nb/U ratios, be cause these ratios are important tools for interpreting the chemical evolut ion of planetary bodies. We obtained Nb/Ta = 17.4 +/- 0.5 for the carbonace ous chondrites and the Juvinas-type eucrites investigated. Though this valu e is similar to previous estimates, it is much more precise. The same is tr ue for Zr/Nb (15.5 +/- 0.2) and Zr/Y (2.32 +/- 0.12). In combination with r ecently published MIC-SSMS U data for carbonaceous chondrites, we obtained a chondritic Nb/U ratio of 29 +/- 2. Because Nb, Ta, Zr, Y, and U are refractory lithophile elements and presuma bly partitioned into the silicate phase of the Earth during core formation, the elemental ratios may also be used to constrain evolution of the Earth' s primitive mantle and? with the more precise determinations fractionation of Nb and Ta during magmatic processes and mantle-crust interactions, can n ow be interpreted with greater confidence.