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

tel-00344949, version 1 - 7 Dec 2008
3. Article soumis à GCA en révision
Abstract
90
In order to constrain the post‐magmatic behavior of lithium isotopes in crystals
during the cooling of volcanics, the study of a visibly unaltered pillow‐lava that contains
large olivine crystal has been undertaken. This sample was chosen because it contains
olivine crystals which are in principle identical, but which underwent different cooling
histories depending on whether they are in the glassy rim or in the interior of the pillow.
Ion microprobe was used to measure lithium abundances and isotopic compositions in
olivine phenocrysts, in some of their melt inclusions and in the pillow rim glass. Profiles in
δ 7 Li through olivines show very different patterns which are correlate with the expected
cooling history of the pillow lobe. Olivines embedded in the pillow rim glass display no
variation in Li concentration ��Li� � 1.2 μg.g ‐1 � and isotopic composition �δ 7 Li � �6.2 �1.2
‰, n � 29). At variance, a huge Li isotopic zoning is found from crystals core �δ 7 Li � �6
‰, �Li� � 1.1 μg.g ‐1 � to rim �δ 7 Li � ‐11 ‰, �Li� � 1.4 μg.g ‐1 � for olivines set in
microcrystalline groundmass. The core of the large olivine phenocrysts, both in rim glass
and microcrystallized matrix, have the same δ 7 Li value as those in the pillow rim glass �δ 7 Li
�6 ‰, �Li� � 3.4 μg.g ‐1 �
�.
A fractionation of lithium isotopes during late stage chemical diffusion is the only
process able to explain the diverse zoning in olivines from the same pillow lava. Though the
6 Li and 7 Li diffusion rates are not known precisely for olivines, the present data allow to
better constrain them. These results suggest that the pristine magmatic δ 7 Li values can be
completely erased in small phenocrysts, even in the case of fast cooled lavas. The use of Li
isotopes in crystals for tracking mantle heterogeneities should be thus seen with extreme
caution. The same is true for melt inclusions which might lose part of their original Li in the
surrounding phenocrysts by diffusion and acquire by this process higher δ 7Li values. The Li
isotopic heterogeneity produced by diffusion upon cooling must result in artificial
variations of bulk isotopic analyses if the mass analyzed is too small.