Niobium and Ta in carbonaceous chondrites: constraints on the solar-system and primitive mantle Nb/Ta, Zr/Nb and Nb/U ratios

Details Niobium and Ta in carbonaceous chondrites: constraints on the solar-system and primitive mantle Nb/Ta, Zr/Nb and Nb/U ratios Niobium and Ta in carbonaceous chondrites: constraints on the solar-system and primitive mantle Nb/Ta, Zr/Nb and Nb/U ratios

Mar 2000

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000m%26ps...35..229j&link_type=abstract

Meteoritics & Planetary Science, vol. 35, no. 2, pp. 229-235 (2000).

Computer Science

6

Scientific paper

We have determined Nb, Y, and Zr abundances in the carbonaceous chondrites Orgueil (CI), Murray (CM2), Murchison (CM2), Allende (CV3), and Karoonda (CK4) and in the eucrites Pasamonte, and Juvinas by a recently developed spark source mass spectrometric technique using multiple ion counting (MIC-SSMS). The abundance of Ta was determined in the same meteorites by radiochemical neutron activation analysis (RNAA). Precision of the MIC-SSMS and RNAA techniques is about 3% and = 5%, respectively. The new abundances for CI chondrites are: Nb=0.247, Ta=0.0142, Zr=3.86, Y=1.56 μg/g, or 0.699, 0.0202, 11.2, 4.64 atoms/106 Si atoms, respectively. The values agree with earlier compilations, but they are a factor of 2 more precise than earlier analyses. Trace element concentrations in the CM, CV, and CK chondrites are higher than in the CI chondrite Orgueil by about 37, 86, and 120%, respectively, in agreement with the variable absolute contents of refractory lithophile elements in different groups of carbonaceous chondrites. Of particular interest are the chondritic Nb/Ta, Zr/Nb, and Nb/U ratios, because these ratios are important tools to interpret the chemical evolution of planetary bodies. We obtained Nb/Ta=17.4±0.5 for the carbonaceous chondrites and the Juvinas-type eucrites investigated. This value is similar to, however much more precise than previous estimates. The same is true for Zr/Nb (15.5±0.2) and Zr/Y (2.32±0.12). In combination with recently 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 presumably partitioned into the silicate phase of the Earth during core formation, the element 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 now be interpreted with greater confidence.

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