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

tel-00344949, version 1 - 7 Dec 2008
4. Article soumis à G3
‐ N2 is represented by a single sample, DR27, the most enriched of the studied ridge
section with a small negative Eu anomaly. It has the most radiogenic Sr isotopic ratio
�0.702643� of the studied ridge section associated with the most unradiogenic Nd isotopic
ratio �0.513061�.
‐ 4 dredges, DR28‐31, have sampled N3. Three samples have sub‐parallel REE patterns.
The lowest in concentration and the most depleted patterns belong to the samples from the
segment ends, namely DR28 and DR31 respectively. All 4 samples have similar Sr and Nd
isotopic compositions, within error.
‐ N4 is represented by 3 samples, 2 of which have identical REE patterns. The third one,
DR34, is the most depleted of the studied ridge section. It is located at the northern
segment end. This sample has also the most radiogenic Nd isotopic composition north of
Menard TF, 0.513108.
‐ N5 has been sampled by 3 dredges that have sub‐parallel depleted REE patterns, with a
slight negative Eu anomaly for samples with La concentration above 4.7 ppm. Sr isotopic
compositions show a trend of depletion with the most unradiogenic value associated with
the most radiogenic Nd towards the northern segment end.
‐ N6 has been sampled by only one dredge represented by one sample, DR38. It has a
depleted REE pattern and Sr‐Nd isotopic compositions within the calculated averages of
the ridge section to the north of Menard TF.
5. DISCUSSION
It is well known that the physical and chemical properties of the mantle are the
controlling factors of the segmentation styles, influencing mantle upwelling and melting
�Batiza, 1996�. The hierarchy of magmatic and tectonic ridge segmentation has been well
described �see review in Macdonald et al., 1988�. First‐order segmentation is defined by
transform faults, which are rigid axial discontinuities whereas second‐order segmentation
is defined by non‐rigid discontinuities such as OSC. Such a segmentation hierarchy can be
disturbed by local phenomena like the presence of nearby seamounts, as seen in S2 and N2.
Therefore, after discussing the geochemical signatures specific to S2 and N2, the link
between the tectonic segmentation, the magmatic processes and mantle upwelling will be
explored.
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