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

tel-00344949, version 1 - 7 Dec 2008
3. Article soumis à GCA en révision
might be significantly higher in the olivine phenocrysts �0.3�β�0.5� than values reported
for silicate melts �β � 0.215� �Richter et al., 2003�. Additional experimental data for β, D0
and Ea are clearly needed to quantify the process, and to take into account the anisotropy of
the
orthorhombic crystal structure of olivine.
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The crystals investigated during the course of this study are exceptionally large, and
allowed a correct estimation of the size of the diffusion front. Hence, our results suggest
that the pristine δ 7 Li values can be completely erased in the most common phenocrysts
that are largely smaller, even in the case of fast cooled volcanics. The use of Li isotopes in
crystals for tracking mantle heterogeneities should be seen with extreme caution.
Furthermore, it could be proposed that the Li isotopic systematic in crystals can be used to
evaluate the cooling rates of rocks, but unfortunately, the extremely fast diffusion of Li in
crystals
reduces considerably the interest of this tool for the solid Earth Geochemistry.
Acknowledgments:
We thank X for the editorial handling and Xi to Xn f or constructive reviews of this work.
The valuable picrite collected during the FAMOUS expedition has been kindly provided by
the Lithothèque of IFREMER. We gratefully acknowledge the Programme Dyeti and the
Programme National de Planétologie �CNRS‐INSU� for financial support. Denis Mangin,
Michel Champenois and Claire Rollion‐Bard are thanked for their help with SIMS analyses.
This paper is dedicated to the memory of Daniel Bideau, for his friendship and discussions
on the FAMOUS area.