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

tel-00344949, version 1 - 7 Dec 2008
2. Article soumis à EPSL
any assumption made for one of these parameters allows calculating the other two
parameters
by fitting the Li concentrations and isotopic compositions.
Minimum values for the Li characteristics of the crustal component can be evaluated
by considering the unrealistic case where Li is seen as a perfectly incompatible element
�DLi � 0�. In this particular instance, it is necessary to assume that �Li�c � 10 μg/g in order
to account for the evolution of Li concentration and also an extremely low value of δ 7 Lic �‐
50 ‰� is needed to reproduce the Li isotopic variation observed within the magmatic suite
�Fig. 2.11�. A more realistic �i.e. less incompatible� behavior of Li during the fractional
crystallization process requires a higher Li content in the contaminant and therefore a less
drastically low δ 7 Lic value. During low pressure crystallization, DLi values are close to 0.2–
0.3 and are broadly independent of the proportions of mineral phases involved �Ryan and
Langmuir, 1987�. Using DLi � 0.3 as a maximal value, our calculations require �Li�c less than
40 μg/g and δ 7 Lic lower than ‐9 ‰ �Fig. 2.11�. These low δ 7 Li values are significantly lower
than of any measured in sedimentary rocks so far and are therefore unlikely to be simply
inherited from their protolith compositions. Li abundance in the corresponding crustal
component is much higher than its average content in the lower continental crust, which
has been reported by several authors as ranging from 5 to 14 μg/g �Taylor and McLennan,
1985; Rudnick and Presper, 1990; Shaw et al., 1994; Rudnick and Fountain, 1995;
Wedepohl, 1995; Gao et al. 1998�. This high Li concentration is likely related to the
sedimentary
origin of the lower crust beneath the Chaîne des Puys.
Table 2.2a: Bulk distribution coef ficients for Sr and Nd as ca lculated fr om log‐log diagrams.
D �1st D �2nd
step� Step�
Sr 0.57 0.9
Nd 0.25 0.35
Pb 0. 23
0.5
Th
0
0
D �3rd
Step�
2.9
1.1
0.5
0,2
53