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

tel-00344949, version 1 - 7 Dec 2008
3. Article soumis à GCA en révision
relative �1σ�. Because we never observed significant and systematic matrix effects on the
Li ion yield at low Li contents �μg/g range�, all the analytical spots �on olivine and glass�
were corrected with the ion yield determined on the silicate glass standard. The variations
introduced by matrix effects on the Li concentration measurement, if existing, are
considered negligible compared to the variations observed in the present sample. For the
two melt inclusions in olivine, the Li concentrations were calculated from the ratio of the
count rate on 7 Li �measured during the isotopic analysis� to the primary beam current
relative
to that of standards during the same analytical session.
2.2. Li isotopic composition measurements
96
The Li isotopic compositions were measured with the two ion microprobes �ims 3f
and ims1270� using a set of olivine and glass standards �Beck et al., 2004�. The isotopic
ratios are given in Table 3.2 in δ‐units using the δ 7 Li notation �δ 7 Li � 1000 �� 7 Li/ 6 Li�sample /
� 7 Li/ 6 Li�LSVEC ‐1�� relative to the LSVEC international standard �Flesch et al., 1973�. The
two instruments were used to cross‐check the large range of δ 7 Li observed. Because of the
large radius of the magnet of the ims 1270, the Li isotopic ratio can be measured without
cutting the cross‐over of the secondary beam by the entrance slit, i.e. with a maximum
transmission. This is at variance with the ims 3f for which the cross‐over must be cut with
the entrance slit by more than 50% to reach the mass resolution M/ΔM of 1200 required to
remove the interfering 6 LiH � at mass 7. Variations of instrumental mass fractionation in the
secondary optic are thus less likely on the ims 1270 than on the ims 3f. The instrumental
mass fractionation for Li �αinstLi � � 7 Li/ 6 Li� measured / � 7 Li/ 6 Li�true� determined on our
standards range from ~1.020 to ~1.035 during the different analytical sessions. The major
reasons for the variation of αinstLi between the different analytical sessions on the ims 3f
and the ims 1270 are �i� variations in the efficiency of the electron multipliers, isotopic
fractionation on the first dynode changing with ageing of the multiplier, and �ii� differences
in the secondary settings. We also observed variations of αinstLi during a given session
before and after the cleaning of the duoplasmatron source: this is most probably due to a
change in the shape or/and in the density of the primary beam. All these changes were
monitored by running our standards and by duplicating some spots. Duplicates made at
different times at the same spot in the present sample give reproducibilities ranging from
0.8 to 1.9‰. A similar reproducibility �given below at �1σ� was obtained for αinstLi on our
standards: for sessions with the ims 3f we obtained � 1.53‰ �n � 5� for olivine BZ29 and