Constraints from loess on the Hf–Nd isotopic composition of the upper continental crust
Introduction
The formation of the continental crust was certainly one of the most important events that affected the overall composition of the Earth, because the isolation at the surface of a light and enriched layer left behind a depleted residue in the mantle. This observation explains why so many studies concentrate on the processes that produce continental crust and on the timing of crustal growth (e.g., Armstrong, 1968, McCulloch and Wasserburg, 1978, Armstrong, 1981, Allègre and Rousseau, 1984, Goldstein et al., 1984, Taylor and McLennan, 1985, Condie, 1993, Wedepohl, 1995, Gallet et al., 1998, Vervoort et al., 2000, Griffin et al., 2002, Rudnick and Gao, 2003, Harrison et al., 2005, Hawkesworth and Kemp, 2006, Blichert-Toft and Albarède, 2008, Belousova et al., 2010, Hawkesworth et al., 2010, Kemp et al., 2010, Dhuime et al., 2011, Garçon et al., 2011, Iizuka et al., 2013). In this study, we focus on the information provided by two isotopic systems, Sm–Nd and Lu–Hf. The two systems are strongly coupled during magmatic processes leading to the definition of the “Terrestrial Array”, an array along which all terrestrial magmas fall (Vervoort et al., 1999, Vervoort et al., 2011). However, even though crustal magmas fall on the “Terrestrial Array” the location of the average composition of the continental crust remains unknown.
Knowing the position of present-day average crust on the terrestrial array is important because it provides information not only on the processes active during the formation of the crust but also because the two isotopic systems record the past history of crustal materials and can be used to calculate an average extraction age of crust from the mantle. Finally, by using the constraints provided by the Sm–Nd isotopic system and those coming from the Lu–Hf isotopic system, we can evaluate the average crustal 176Lu/177Hf ratio, a very poorly known but crucial parameter necessary to calculate crustal residence ages of zircon populations (e.g., Griffin et al., 2002, Harrison et al., 2005, Kemp et al., 2007, Blichert-Toft and Albarède, 2008, Belousova et al., 2010, Kemp et al., 2010, Condie et al., 2011, Yao et al., 2011, Iizuka et al., 2013).
To address all these issues, we focus on a combined Nd and Hf isotopic study of a variety of recent loess deposits from several continents. Loess deposits result from the local accumulation of windblown dust coming from usually large continental areas. They represent natural mixtures of erosion products of continental surfaces and were therefore used by several previous workers to determine the average composition of the upper continental crust (e.g., Taylor and McLennan, 1981, Taylor et al., 1983, Taylor and McLennan, 1985, Taylor and McLennan, 1995, Barth et al., 2000). We show that they are excellent proxies for the average composition of large areas of upper continental crust for both the Sm–Nd and the Lu–Hf isotopic systems. We then suggest an average value for the upper continental crust for the two systems and we determine an average model age. Finally, the combined Nd and Hf isotopic data allow us to suggest a value for the 176Lu/177Hf ratio of the upper continental crust.
Section snippets
Source of samples
The samples in this study were collected from several locations around the world: Western Europe, Tajikistan, China and Argentina (see Fig. 1). Most were previously studied by Gallet and co-workers (Gallet et al., 1996, Gallet et al., 1998, Jahn et al., 2001) who reported major and trace element data as well as Nd and Sr isotopic compositions for loess samples from China, Western Europe and Argentina. The Tajikistan loess deposits come from the Chashmanigar section and were previously studied
Analytical techniques
All samples were crushed using an agate mortar. Trace element concentrations were obtained after dissolution in a HF–HClO4 mixture using Parr bombs. The complete procedure is that of Chauvel et al. (2011), but the dissolution time was increased to insure complete dissolution of the very resistant minerals, an issue particularly important when using the relationship between REE and Hf concentrations and when measuring Hf isotopes. Both accuracy and reproducibility of the data are estimated as
Results
New trace element data for the loess are provided in the Supplementary Table. Some samples had been previously analyzed for their trace element contents by Gallet et al. (1998) and Jahn et al. (2001) but these were reanalyzed because some key elements (e.g., Hf) were not measured in many of these older studies. The new values reported here are generally similar to those previously published but there are notable exceptions. For example, the new determinations of REE, Th, Zr and Hf
Discussion
Constraining the average composition of the continental crust is a very active field of research (e.g., Armstrong, 1968, McCulloch and Wasserburg, 1978, Armstrong, 1981, Allègre and Rousseau, 1984, Goldstein et al., 1984, Taylor and McLennan, 1985, Condie, 1993, Wedepohl, 1995, Gallet et al., 1998, Vervoort et al., 2000, Griffin et al., 2002, Rudnick and Gao, 2003, Hawkesworth and Kemp, 2006, Belousova et al., 2010, Hawkesworth et al., 2010, Dhuime et al., 2011, Garçon et al., 2011). Several
Conclusions
Our Nd–Hf isotopic study of a large number of loess samples provides new constraints on the average isotopic composition of the upper continental crust. We estimate that present-day upper continental crust has the following characteristics: and . Using the bulk silicate Earth values determined by Bouvier et al. (2008), these isotopic ratios translate into and and fall within error on the terrestrial array of
Acknowledgments
We would like to thank Sylvain Gallet who provided the loess samples that he studied during his PhD. Thanks also to Philippe Telouk in Lyon who always makes sure that the MC-ICP-MS behaves properly when we come to use it. Special thanks to Jeff Vervoort and Nick Arndt who read and made comments on a first draft of this manuscript, to an anonymous reviewer and to Roberta Rudnick who made very constructive comments that helped clarify the discussion and to Mark Harrison for his editorial job.
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