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

tel-00344949, version 1 - 7 Dec 2008
3. Article soumis à GCA en révision
possible values for the β parameter is unchanged. Using these sets of values for Ea, D0 and β
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The calculations show that there is in fact a range of values for these parameters �D0,
Ea and β � consistent with the Li systematic observed in the present olivines. For the two
olivine crystals having different cooling histories, the diffusion equation �1� allows
calculation of the minimal change in δ 7 Li which can be produced during diffusion for given
values of D and β. Because D is a function of D0 and Ea, the calculations were made with two
different activation energies �Ea�: either the value found for pyroxene �Ea � 258 kJ, Coogan
et al., 2005� or an activation energy 30 % lower �Ea � 180 kJ�. The ranges of D0 and β which
allow to match the lowest δ 7 Li values observed in the two olivines, having different cooling
rates, corresponds to the intersection of the two fields in figure 3.10. This range is rather
limited for Ea � 258 kJ �Fig. 3.10a� with β ranging from 0.3 to 0.5 and D0 � 0.5. Taking a
lower value for Ea, �Ea � 180 kJ� induces a reduction of D0 by a factor 1000 but the field of
, some Li isotopic profiles can be predicted for olivine 5‐20 : the predicted Li isotopic
zoning is in agreement with the observed profile C‐D‐E �Fig. 3.11�. A more rigorous
modeling of Li isotope effects due to diffusion would require the experimental
determinations of the values of Ea, D0 and β in olivine. However, it is clear that diffusion
upon cooling of the pillow is responsible for the various Li isotopic profiles observed in the
olivines
at different depths in the lava flow.
Figure 3.11 : Li isotopic zoning of Ol5‐20 compared to that predicted from
the diffusion model. The values of the parameters for Li diffusion were
taken as constrained in Fig. 3.10 : D0 varying from 3.8×10 ‐4 to 1.2×10 ‐3, β
� 0.35 and Ea � 180 kJ.