Elsevier

Chemical Geology

Volume 212, Issues 1–2, 26 November 2004, Pages 59-79
Chemical Geology

Lithium inputs to subduction zones

https://doi.org/10.1016/j.chemgeo.2004.08.004Get rights and content

Abstract

We have studied the sedimentary and basaltic inputs of lithium to subduction zones. Various sediments from DSDP and ODP drill cores in front of the Mariana, South Sandwich, Banda, East Sunda and Lesser Antilles island arcs have been analysed and show highly variable Li contents and δ7Li values. The sediment piles in front of the Mariana and South Sandwich arcs largely consist of pelagic sediments (clays and oozes). The pelagic clays have high Li contents (up to 57.3 ppm) and Li isotope compositions ranging from +1.3‰ to +4.1‰. The oozes have lower Li contents (7.3–16 ppm) with δ7Li values of the diatom oozes from the South Sandwich lower (+2.8‰ to +3.2‰) than those of the radiolarian oozes from the Mariana arc (+8.1‰ to +14.5‰). Mariana sediment also contains a significant portion of volcanogenic material, which is characterised by a moderate Li content (14 ppm) and a relatively heavy isotope composition (+6.4‰). Sediments from the Banda and Lesser Antilles contain considerable amounts of continental detritus, and have high Li contents (up to 74.3 ppm) and low δ7Li values (around 0‰), caused by weathering of continental bedrock. East Sunda sediments largely consist of calcareous oozes. These carbonate sediments display intermediate to high Li contents (2.4–41.9 ppm) and highly variable δ7Li values (−1.6‰ to +12.8‰).

Basaltic oceanic crust samples from worldwide DSDP and ODP drill cores are characterised by enrichment of Li compared to fresh MORB (6.6–33.1 vs. 3.6–7.5 ppm, respectively), and show a large range in Li isotope compositions (+1.7‰ to +11.8‰). The elemental and isotopic enrichment of Li in altered basalts is due to the uptake of isotopically heavy seawater Li during weathering. However, old oceanic crust samples from Sites 417/418 exhibit lighter Li isotope compositions compared to young basaltic crust samples from Sites 332B and 504B. This lighter Li isotope signature in old crust is unexpected and further research is needed to explore this issue.

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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