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

tel-00344949, version 1 - 7 Dec 2008
4. Article soumis à G3
which have been discussed in the previous paragraph 5.2. This distinction between
chemical signatures seen in rigid versus non‐rigid boundaries is the result of the difference
between a cold edge versus a mantle flow effect across a ridge boundary.
6. SUMMARY AND CONCLUSION
The data presented above, from a ridge section devoid of hotspot influence, emphasize the
commonly recognized depleted and first-order homogeneous character of MORB. In spite of this
regional homogeneity, the data reveal tight geochemical variations at the local segment scale. This is
interpreted as indicative of small-scale heterogeneity present in the mantle and sampled by different
melting processes. The data also illustrates the close relationship between the structural and the
chemical features of the ridge. Anomalous geochemical features as seen in S2 and N2 segments are
linked to the atypical morphology of these two transitional segments. The small-scale heterogeneity
is displayed within each segment, limited by non-rigid boundaries (OSC) where segment ends have
distinct signatures from segment centers. Interpretations invoke increasing mantle depletion as a
function of distance from the magmatic segment centers. The regional scale is illustrated within each
mantle domain, on each side of the Menard Transform Fault, a long-lived rigid discontinuity. The
thermal edge effect along the Menard TF creates an effect similar to the well-described TFE
(Langmuir and Bender, 1984). To summarize, the new data obtained from a ridge section sampling
the southern sub-Pacific mantle domain, far from any hotspot influence, shows clearly the different
scales of mantle heterogeneity within the southern Pacific mantle.
ACKNOWLEDGMENTS
This work was funded by CNRS/INSU and IFREMER. We wish to thank the Captain, Michel
Houmard, and the Crew of R/V L’Atalante for their efforts and expertise during the
PACANTARCTIC2 cruise. We also want to thank Jo Cotten (ICP-AES) and Joël Etoubleau (XRF) for
their careful analytical work.
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