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

tel-00344949, version 1 - 7 Dec 2008
2. Article soumis à EPSL
Due to the lack of constraints on the process that occurs in the lower crust, we are not
able to propose an unambiguous solution to explain the extreme low δ 7 Li values invoked
by our modeling. But the fact, not to be disputed, is that at least a portion of the lower crust
beneath the Chaîne
des Puys displays a light Li signature.
5.
Conclusion
The intraplate volcanic series of the Chaîne des Puys shows large variations of Sr, Nd, Pb
and Li isotopic ratios associated to progressive differentiation. These data suggest a
magmatic evolution related to AFC processes which occurred within magma chambers
located in the lower crust. Using Sr, Nd and Pb isotopic composition of granulitic xenoliths,
we have calculated parameters for the AFC modeling. We then applied these parameters to
the calculation of Li characteristics in the reservoirs involved in the AFC process:
�i� The mantle end‐member: Expressed in the least evolved lavas, the isotopic
composition of the mantle component is clearly related to an HIMU affinity. The Li isotopic
composition in this reservoir shows high δ 7 Li �� �7‰�. This result is in good agreement
with other δ 7 Li measurements in OIB lavas of HIMU affinity �Ryan and Kyle, 2004; Nishio et
al., 2005� �Fig. 2.12�.
�ii� The lower crust end‐member: a first approximation of the in situ δ 7 Li value in the
lower crust beneath the Chaîne des Puys, is calculated using an AFC model. If we assume a
DLi ranging from 0.2 to 0.3 �Ryan and Langmuir, 1987�, the Li abundances in the crustal
component are estimated at 35 μg/g in order to account for the evolution of Li
concentration. Therefore, an extremely low value of δ 7 Li �� ‐9‰� is needed to reproduce
the Li isotopic variation of the Chaîne des Puys magmatic series.
Because low δ 7 Li values found in exhumed samples could be generated by diffusion‐
induced fractionation during the exhumation process, in situ δ 7 Li of the lower crust was
still unknown �Marshall et al., 2007�. Mixing relationships throughout the AFC process
along the Chaîne des Puys volcanic suite allow us to place constraints on the Li signature of
the lower crustal end‐member. Hence, this calculation gives an indirect method to assess
the in situ value of δ 7 Li in a portion of the lower crust beneath the French Massif Central.
These results indicate that the isotopic composition of the lower and upper continental
crust can be, at least locally, significantly different.
57