Lithium inputs to subduction zones
Introduction
The use of Li and its isotopes in studying island-arc processes is a relatively new tool in geochemistry. Until now, most studies have focused on the volcanic output of Li at subduction zones (Moriguti and Nakamura, 1998a, Benton and Tera, 2000, Chan et al., 2001, Chan et al., 2002a, Tomascak et al., 2002), rather than on the slab input of Li, i.e. marine sediments and the altered portion of the downgoing oceanic crust. Although there is a fairly large dataset of Li in altered basalts from the oceanic crust (e.g., Seyfried et al., 1984, Ryan and Langmuir, 1987, Chan and Edmond, 1988, Chan et al., 1992, Chan et al., 2002b, Decitre et al., 2002), the sedimentary Li input is not well characterised. Chan and co-workers (Chan et al., 1994, Chan et al., 1999, You et al., 1995, You and Chan, 1996, Chan and Kastner, 2000) have provided much Li (isotope) data on various sediment types, but no systematic investigations of marine sediments at various subduction zones have been carried out so far.
The aim of this study is to gain more information on the Li isotope geochemistry of the two major inputs at subduction zones, deep-sea sediments and altered oceanic crust. For this purpose, a variety of sediment lithologies from drill cores in front of five different island arcs have been analysed. In addition to the sediments that were the primary aim of this study, we also analysed some altered basalt samples from several locations in the Atlantic, Pacific and Antarctic. The samples we analysed widen the geographical spread and age of the already considerable dataset on altered mafic oceanic crust.
Section snippets
Sediments
The sediment samples come from drill cores in front of five geologically and geochemically diverse island arcs (Fig. 1). A variety of sediments from ODP Site 801 (Mariana), ODP Site 701 (South Sandwich), DSDP Site 262 (East Sunda) and DSDP Sites 27 and 543 (Lesser Antilles) were selected for this study. For the Banda arc, a selection was made from the large set of box- and piston cores taken during the Snellius II expedition (1984–1985, Vroon et al., 1995).
An overview of the sedimentary
Sample dissolution
Powders of sediment (20 mg) and altered basalt (50 mg) were digested in cleaned PFA beakers using a concentrated HNO3/HF/HClO4 mixture. Perchloric acid was added to avoid the formation of insoluble Li fluorides (Ryan and Langmuir, 1987). After at least 2 days on a hotplate, the solutions were dried down, chlorinated, dried down again and subsequently dissolved in a known amount of 5 N HNO3. This was then diluted with four times the volume of distilled methanol in preparation for the column
Sediments
Li concentrations and isotope compositions of the sediment samples are presented in Table 6.
Sediments from ODP Site 801 (Mariana) have Li concentrations between 7.3 and 50.8 ppm. The highest Li content is found in the pelagic clay. Cherts, radiolarite-rich and volcaniclastic material show much lower concentrations, ranging from 7.3 to 21.8 ppm. The sediments show heavy Li isotope signatures (+4.1‰ to +14.5‰). The brown radiolarite is most enriched in 7Li (δ7Li=+14.5‰).
The two lithologic units
Variations between Li, δ7Li and CaCO3
Fig. 6A shows the effects of calcium carbonate and silica dilution on the Li contents of the sediments. The effect of calcium carbonate dilution is well displayed by sediments from Indonesia (Snellius and DSDP Site 262). With increasing calcium carbonate from 0 to 100 wt.%, Li content decreases from ∼75 to ∼2 ppm. Significant variations also exist at low CaCO3 contents (<3 wt.%). Li concentrations of sediments from ODP Sites 801 (Mariana), 701 (South Sandwich) and DSDP Sites 27 and 543 (Lesser
Conclusions
Marine sediments and altered basalts from several drill cores around the world show large variations in Li isotope geochemistry.
Among the sediments, we recognised four groups with different Li (isotope) systematics:
- (1)
Pelagic sediments (ODP Sites 701 and 801) consisting of pelagic clays and ooze-rich material. Both have distinctly low Li contents, most likely caused by silica dilution. Sediments from ODP Site 801 have heavier Li (+4.1‰ to +14.5‰) compared to sediments from ODP Site 701 (+1.3‰ to
Acknowledgements
We would like to thank Jurian Hoogewerff, Terry Plank, Thijs van Soest and Hubert Staudigel for providing samples. Coos van Belle, Richard Smeets and Bas van der Wagt are thanked for their valuable expertise with analytical procedures at the Vrije Universiteit (Amsterdam). Alistair Jeffcoate, Carolyn Taylor and Alex Thomas are thanked for their assistance at the Department of Earth Sciences in Bristol. Roberta Rudnick, Lui-Heung Chan and Jeff Ryan are thanked for their thorough reviews [RR].
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