The solar-system abundances of Nb, Ta, and Y, and the relative abundances of refractory lithophile elements in differentiated planetary bodies

Abstract

Analytical data for Nb, Y, and Ho in 8 carbonaceous chondrites were obtained by spark source mass spectrometry (SSMS). In addition, three carbonaceous chondrites were analyzed for Ta by radiochemical neutron activation analysis (RNAA). From these data and earlier literature data on the C1 -chondrite Orgueil a consistent set of solar-system abundances is derived for Nb, Y, Zr, Ta, Hf and the REE. Ratios among these elements are constant within analytical uncertainties in all groups of carbonaceous chondrites. In particular we do not find a difference in ZrHf ratios between C1 and C2 chondrites.The new abundances for C1-chondrites are: Nb (0.246 ppm), Y (1.57 ppm), Ta (0.014 ppm), or 0.696, 4.64, 0.020 atoms/106 Si atoms, respectively.Based on a large number of analytical data on oceanic basalts, it is argued that the relative abundances of these elements are chondritic in the bulk Earth. Ratios such as ZrHf or NbTa are constant and chondritic in oceanic basalts and agree with estimates of the continental crust. The constant but non-chondritic NbU ratio (47 vs. 30) in oceanic basalts is balanced by a lower NbU ratio (~ 10) in the crust. The bulk Earth ratio may therefore be chondritic.The ZrHf and NbTa ratios are correlated in lunar rocks. Both ratios vary within a factor of two, similar to the variability in terrestrial oceanic basalts. The ZrNb and HfTa ratios, however, are much more constant on the Moon.The available evidence suggests that refractory lithophile elements in the Earth, the Moon and achondritic meteorites occur in the same proportions as in carbonaceous chondrites.Refractory elements have greatly different volatilities. The same pattern of refractory lithophile elements in chondrites and planets therefore indicates that protoplanetary materials have never been subject to high temperature processes that would fractionate refractory elements from each other. The same ratio of ZrNb in the three types of carbonaceous chondrites, in the Earth, the Moon and in differentiated meteorites is a good example, since the condensation temperature for Zr is 177 K higher than that for Nb.