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

tel-00344949, version 1 - 7 Dec 2008
4.2. Constraints on Li isotope compositions of crustal reservoirs beneath the FMC
2. Article soumis à EPSL
Samples from the FMC volcanic suite show a very good correlation between Li
concentration and Li isotope composition, ranging from low Li concentration �5.5 μg/g�
and relative heavy isotopic compositions �δ 7 Li � �7‰� in basalts, to high concentrations
�20 μg/g� and lighter δ 7 Li values �δ 7 Li � �0.5‰� in highly evolved melts. This trend is
consistent with the contamination of mantle‐derived melts with low δ 7 Li crustal
components.
4.2.1. Li isotopic composition of the mantle end‐member.
Because the least evolved lavas of the Chaîne des Puys are expected to have been
preserved from crustal contamination effects, basaltic samples �Puy16, Puy21� are likely to
represent the isotopic composition of their mantle reservoirs. Based on Sr, Nd, and Pb
radiogenic isotopic data, Wilson and Downes �1991� proposed that the FMC Cenozoic
basaltic lavas have strong HIMU affinities �low 87 Sr/ 86 Sr �� 0.7038� and high 206 Pb/ 204 Pb
�� 19.5��. However, the origin of this geochemical signature is still a matter of debate.
Figure 2.11 : Li isotopic composition of Chaîne des Puys samples plotted vs 87Sr/ 86Sr.
Source for mantle end‐members: MORB: Elliot et al., 2006; Tomascak et al., 2008 and
reference therein; HIMU: Nishio et al. 2005; Ryan and Kyle, 2004; Hawaii: Chan and
Frey, 2003; Polynesia: Nishio et al., 2005; Carbonatites: Halama et al., 2008.
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