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

tel-00344949, version 1 - 7 Dec 2008
3. Article soumis à GCA en révision
found in the pillow rim is perfect for in situ measurement with electron or ion microprobe.
It appears homogeneous for major elements �Bougault and Hekinian, 1974; Kamenetsky,
1996 and this study, Table 3.1� and Li abundances are uniform ��Li� � 3.4 � 0.1 μg/g, n �
10�. For δ 7 Li, two analyses of glass were performed and almost identical values were
obtained: �6.4 �1.0‰ and �6.7 �1.2‰. These δ 7 Li values are in the upper range of the
oceanic basalts array �Chan and Frey, 2003; Elliott et al., 2006�.
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4.2. Olivines and their melt inclusions
The Li abundances are rather constant in the three olivine crystals �Figs. 3.7 and 3.8�
and range between 1.1 and 1.4 μg/g. The apparent partition coefficient calculated between
the olivine cores and the glass is close to 0.35, a value indistinguishable from the
experimentally determined partition coefficients �Brenan et al., 1998; Herd et al., 2004;
Zanetti et al., 2004 and references therein�.
In contrast, the Li isotopic systematics of olivine vary from core to rim depending on
whether they are in contact with �i� glass, �ii� fine‐grained groundmass or �iii� olivine.
Olivine in pillow rim �Fig. 3.7� has a uniform Li concentration ��Li� � 1.2 � 0.1 μg/g, n � 8�
and Li isotopic composition (δ 7 Li � �6.2 �1.2 ‰) at the exception of a point with a low
δ 7 Li near the olivine/matrix interface �δ 7 Li � �2.6 �1.4 ‰�. Olivines in pillow interior
�Ol5‐20, Fig. 3.8� show either a strong isotopic zoning from core �δ 7 Li � �6 ‰� to rim
�δ 7 Li � ‐11 ‰� when in contact with the fine‐grained groundmass �profile C‐D‐E in Fig.
3.8�, or a rather constant Li isotopic composition when in direct contact with another
olivine crystal �profile F‐G in Fig. 3.8�. The two olivines in contact show however a slight
difference in isotopic composition, δ 7 Li � �6 ‰ for the large one and δ 7 Li � �8.6 ‰ for
the small one, but this difference is not significant taking into account analytical accuracy.
No
clear change in Li concentration is associated to the Li isotopic variations �Fig. 3.8�.