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

tel-00344949, version 1 - 7 Dec 2008
4. Article soumis à G3
ABSTRACT
Major, trace element and Nd‐Sr isotope data in basalts collected along the Pacific
Antarctic ridge axis between 53 and 41°S, far from any hotspot influence, reveal tight
coherent geochemical variations within the depleted mantle. All samples are located within
the global MORB field and below the Pacific reference line defining the two mantle domains
limited by the Easter microplate �Vlastélic et al., Nature 399, 1999, 345‐350�. The data
extend the PAR 66‐53°S data field towards more radiogenic Sr �0.70264� and less
radiogenic Nd �0.51306� values. Anomalous geochemical features along this ridge section
are linked to the atypical morphology of two segments. Excluding these two segments, the
along ridge geochemical variability illustrates the mantle heterogeneity at different scales.
It is closely related to the morphological segmentation of the ridge. Two contrasting types
of ridge discontinuities are superimposed: �i� the first scale, defined by the Menard
transform fault and �ii� the second‐order segmentation, limited by overlapping spreading
centers. The geochemical data are interpreted in terms of mantle heterogeneities related to
the rigid or non‐rigid character of the ridge discontinuities.
Keywords: Pacific Antarctic ridge segmentation, Sr­Nd isotopes, mantle heterogeneity.
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