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

tel-00344949, version 1 - 7 Dec 2008
4. Article soumis à G3
interaction. They are therefore a good choice to study processes that take place along a
fast‐spreading ridge. In this context, 3 to 4 regularly spaced dredges were collected in each
segment.
Figure 4.12: Along‐axis variations of the ridge physical �cross sectional area,
depth� and chemical �MgO, K2O, �La/Sm�N, Sr‐Nd isotopic compositions�
parameters. Note the generally good coherence in the variations of all parameters.
Dashed lines indicate the location of the overlapping spreading centers with a grey
shade used for atypical segments S2 and N2. For further description, see text §4.
A first order observation is that intra‐segment geochemical variations are very limited.
For example, within segment S1, �La/Sm�N varies from 0.743 to 0.643 from south to north.
These values are theoretically all within error, as given by the external reproducibility of
this ratio, � 2%. In the same way, for the same segment S1, Sr isotopic compositions vary
from 0.702422 � 9 to 0.702367 � 9. The two extreme values are significantly different. The
existence of coherent variations between trace element �La/Sm�N and isotope ratios
� 87 Sr/ 86 Sr� suggests further that the small geochemical variations with latitude are real and
related to source heterogeneity. Another clear example of trace element‐isotope co‐
variation within narrow ranges of Sr isotope compositions, is found in the segment N5,
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