Géochimie isotopique du lithium dans les basaltes-Géochimie des ...

tel-00344949, version 1 - 7 Dec 2008
1. INTRODUCTION
3. Article soumis à GCA en révision
The � 40‰ range found for Li isotopic compositions in terrestrial samples �Fig. 3.5
and refs therein� makes the Li isotopic system a very promising tool to trace various
geodynamical processes �see reviews by Elliot et al., 2004 and by Tomascak, 2004�.
Because of the large relative mass difference between the two Li isotopes � 6 Li and 7 Li
~15%�, significant isotopic fractionations, either equilibrium or kinetic, are anticipated for
Li during natural physical and chemical processes. Though our understanding of the
systematics of Li isotopic fractionations remains quite rudimentary in comparison with
other "classical" stable isotopes such as O, one major isotopic fractionation of Li has been
identified to take place during Li partitioning between clays and water, water being
enriched in 7 Li relative to clays �Chan et al., 1992; Seyfried et al., 1998; Huh et al., 2001;
Pistiner and Henderson, 2003; Vigier et al., 2007�. In the low temperature environment,
minerals are generally depleted in 7 Li relative to the aqueous fluid with which they
experienced isotopic exchange. This isotopic fractionation is likely at the origin �i� of the
high δ 7 Li of � �32‰ of seawater, a situation quite similar to that of B isotopes �Schwarcz
et al., 1969; Spivack and Edmond, 1987�, and �ii� of the rather systematic 7 Li enrichment of
river waters relative to the original bed‐rock and to the suspended load �Huh et al., 2001;
Kisakurek et al., 2004�. Li isotopes are thus not only a powerful tracer of weathering
processes but they can also be used to study the interaction between deep and swallow
level reservoirs in the Earth. This comes from the fact that, during the alteration of the
oceanic crust, basalts and serpentinized peridotite are enriched in 7 Li by interaction with
seawater �Chan et al., 1992, 2002; Bouman et al., 2004; Decitre et al., 2002, 2004�. During
subduction, the down‐going slab may enrich the overlying mantle in 7 Li during dehydration
processes and partial melting, while a fraction of Li contained in the oceanic crust may be
isotopically fractionated and transferred at greater depths in the mantle �Zack et al., 2003�.
The addition of recycled material �oceanic crust or subduction‐modified mantle� would
therefore
create significant Li isotopic heterogeneities in the mantle �Elliot et al., 2006�.
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