Bayon,G., Burton,K.W., Soulet,G., Vigier,N., Dennielou,B., Etoubleau,J., Ponzevera,E., German,C.R. and Nesbitt,R.W.(2009): Hf and Nd isotopes in marine sediments: Constraints on global silicate weathering. Earth and Planetary Science Letters, 277, 318-326.

『海洋堆積物中のハフニウムとニオジム同位体:世界の珪酸塩風化に対する制約』


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


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