TRACKING THE BUDGET OF NB AND TA IN THE CONTINENTAL CRUST

Abstract

High precision trace element data are reported for representative samples of the upper continental crust: 11 loess samples and 22 shale samples (PAAS) previously used by Taylor and McLennan to define the rare earth element (REE) content of the upper crust. Our results confirm the REE concentrations of Taylor and McLennan's [Taylor, S.R., McLennan, S.M., 1985. The continental crust: its composition and evolution. Blackwell, Oxford, 312 pp.] estimate of the upper continental crust but suggest substantial revisions for Nb and Ta, in agreement with recent work of Plank and Langmuir [Plank, T., Langmuir, C.H., 1998. The chemical composition of subducting sediment and its consequences for the crust and mantle. Chem. Geol. 145, 325-394.]. From our data, the upper continental crust has average Nb=11.5+/-2.6 ppm (instead of 25 ppm) and Ta=0.92+/-0.12 ppm (instead of 2.2 ppm), which translates into a bulk crust Nb=8 ppm, Ta=0.7 ppm, La/Nb=2.2, and Nb/Ta=12-13. These revisions confirm the crustal subchondritic Nb/Ta and superchondritic La/Nb ratios and reinforce the observation that the continental crust and the Depleted Mantle are not strictly complementary: an additional Nb- and Ta-rich reservoir having superchondritic Nb/Ta is required to balance the Silicate Earth. Using the continental crust's La/Nb ratio to estimate the proportions of crustal growth in convergent margin and intraplate settings, we suggest a plume contribution of only between 5 and 20% to the continents, even lower than previous estimates.