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

tel-00344949, version 1 - 7 Dec 2008
2. Article soumis à EPSL
demonstrated the high mobility of Li during fluid/rock interactions �e.g. Seyfried et al.,
1998; Brenan et al., 1998; Huh et al., 2001�. The elemental partitioning of Li between
mineral and hydrous‐fluids �D Min/Fluid � has been investigated experimentally under a
variety of physical conditions �Berger et al., 1988; Chan et al., 1994; Brenan al., 1998�.
During metamorphism, the Li partition coefficients between crystals and aqueous fluid
�D Min/Fluid � drop from relatively high values for clays at low temperature �0.35 for chlorites
and 1.9 for smectites at 260°C, Berger et al., 1988�, to very low values for the mineralogical
assemblage under high grade metamorphic condition �0.16 for pyroxene and 0.008 for
garnet at 900°C and 2 GPa, Brenan al., 1998�. Because of the evolution of D Min/Fluid with
increasing pressures and temperatures, Li is released into the hydrous fluids during such a
prograde metamorphic evolution. Zack et al. �2003� were the first to model the isotopic
fractionation of Li during the associated dehydration process. In their model, fractionation
of Li isotopes is explained by the preferential affinity of the lighter isotope for the most
highly coordinated site �Nomura et al., 1989�. Because Li often substitutes for Mg, most
silicate minerals contain eight‐coordinated Li while in aqueous fluids, Li is found in four‐
coordinate position �Wenger and Armbruster, 1991�. Throughout the dehydration process,
Li experiences equilibrium exchange between aqueous‐fluids and Mg‐silicates.
Consequently, dehydration during prograde metamorphism leads to a low δ 7 Li in the
remaining granulitic rocks. Zack et al. �2003� have modeled the low δ 7 Li values in some
eclogites by Li isotopic fractionation through open system Rayleigh distillation during
dehydration of clays and chlorite. Nevertheless, this model is based on poorly‐known
parameters, such as the fractionation factor α �α � � 7 Li/ 6 Li�fluids/� 7 Li/ 6 Li�mineral� and the
elemental partition coefficient D Min/Fluid . Recently, experimental studies have investigated
the Li isotope fractionation between different silicates and hydrous fluids of variable
composition at a wide range of temperatures �Wunder et al., 2006, 2007�. The isotopic
fractionation factor becomes negligible at high temperature. Therefore, the isotopic effect
of Li for rocks at temperatures higher than 300 °C is restricted by the limited isotopic
fractionation occurring under these conditions �Marschall et al., 2007�. Based on this
consideration, the light‐δ 7 Li values of high grade metamorphic rocks could have been
generated by kinetic fractionation of the Li isotopes during diffusive influx of Li from the
country rocks into the exhuming metamorphic body �Teng et al., 2006; Marschall et al.,
2007�.
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