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

tel-00344949, version 1 - 7 Dec 2008
3. Article soumis à GCA en révision
the δ 7 Li value of the melt will progressively increase implying that phenocrysts recording a
degassing event should present an increase of δ 7 Li values from core to rim �Beck et al.,
2004�.
This is exactly at the opposite of what is observed in the present samples.
6. DIFFUSION INDUCED LI ISOTOPIC FRACTIONATION DURING PILLOW
COOLING
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6.1. The rationale for Li isotopic fractionation during chemical diffusion
Because Li is incompatible in olivine �D Li Ol‐melt � 0.3, Zanetti et al., 2004� a large
difference in lithium concentration exists in the present picrite between the Li‐poor
olivines ��Li� � 1.2 � 0.1 μg/g, n � 38� and the surrounding Li‐rich glass or matrix ��Li� �
3.4 � 0.1 μg/g, n � 10�. This gradient in Li concentration can induce chemical diffusion of
lithium from the matrix toward the crystals at a late magmatic stage �Jambon and Semet,
1978; Lowry et al., 1981; Giletti and Shanahan, 1997; Richter et al., 2003; Barrat et al.,
2005;
Beck et al., 2006; Jeffcoate et al., 2006; Parkinson et al., 2007�.
Because of their large relative mass difference, 6 Li and 7 Li diffuse at different rates
and can thus be fractionated by several tens of per mil during this process �Jambon, 1980;
Richter et al., 1999, 2003�. Recently, diffusion‐induced Li isotopic fractionation has been
invoked to explain the large isotopic zoning found in some Martian and lunar phenocrysts
�Barrat et al., 2005; Beck et al., 2006�. Rather few terrestrial samples have already been
examined to constrain the behavior of lithium isotopes during diffusion processes
�Lundstrom et al., 2005; Teng et al., 2006; Jeffcoate et al., 2006; Halama et al., 2007;
Parkinson et al., 2007�. From all these studies it is clear that the extent to which lithium
isotopes will be fractionated during chemical diffusion will directly depend on the cooling
rate of the sample : the volume of olivine which will acquire lower δ 7 Li values will increase
with duration of diffusion. Thus, the strong difference in lithium isotopic profiles observed
between Ol4‐11 �near the pillow surface� and Ol5‐20 �deeper in the pillow� can most likely
be ascribed to different cooling paths. Finally, the fact that diffusion controls the Li isotopic
profiles in the present olivines is demonstrated by the lack of isotopic zoning when no