『Abstract
The combined use of Lu-Hf and Sm-Nd isotope systems potentially
offers a unique perspective for investigating continental erosion,
but little is known about whether, and to what extent, the Hf-Nd
isotope composition of sediments is related to silicate weathering
intensity. In this study, Hf and Nd elemental and isotope data
are reported for marine muds, leached Fe-oxide fractions and zircon-rich
turbidite sands collected off the Congo River mouth, and from
other parts of the SE Atlantic Ocean. All studied samples from
the Congo fan (muds, Fe-hydroxides, sands) exhibit indistinguishable
Nd isotopic composition (εNd〜-16), indicating
that Fe-hydroxides leached from these sediments correspond to
continental oxides precipitated within the Congo basin. In marked
contrast, Hf isotope compositions for the same samples exhibit
significant variations. Leached -hydroxide fractions are characterized
byεHf values (from -1.1 to +1.3) far more
radiogenic than associated sediments (from -7.1 to -12.0) and
turbidite sands (from -27.2 to -31.6). εHf
values for Congo fan sediments correlate very well with Al/K (i.e.
a well-known index for the intensity of chemical weathering in
Central Africa). Taken together, these results indicate that (1)
silicate weathering on continents leads to erosion products having
very distinctive Hf isotope signatures, and (2) a direct relationship
exists between εHf of secondary clay minerals
and chemical weathering intensity.
These results combined with data from the literature have global
implications for understanding the Hf-Nd isotope variability in
marine precipitates and sediments. Leached Fe-hydroxides from
Congo fan sediments plot remarkably well on an extension of the
‘seawater array’(i.e. the correlation defined by deep-sea Fe-Mn
precipitates), providing additional support to the suggestion
that the ocean Hf budget is dominated by continental inputs. Fine-grained
sediments define a diffuse trend, between that for igneous rocks
and the ‘seawater array’, which we refer to as the ‘zircon-free
sediment array’(εHf =0.91 εNd+3.10).
Finally, we show that the Hf-Nd arrays for seawater, unweathered
igneous rocks, zircon-free and zircon-bearing sediments (εHf =1.80 εNd+2.35) can all
be reconciled, using Monte Carlo simulations, with a simple weathering
model of the continental crust.
Keywords: neodymium isotopes; hafnium isotopes; silicate weathering;
continental erosion; seawater array; Congo fan』
1. Introduction
2. Samples and methods
3. Results
4. Discussion
4.1. Leached Fe-oxide fractions and the ‘seawater array’
4.2. Marine muds and the ‘zircon-free sediment array’
4.3. Turbiditic sands and the ‘zircon-bearing’ sediment array
4.4. Constraints on global silicate weathering
5. Concluding remarks
Acknowledgements
References