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

tel-00344949, version 1 - 7 Dec 2008
4.2 TRACE ELEMENTS AND SR‐ND ISOTOPES
4. Article soumis à G3
The data are presented in rare earth diagrams �figure 4.11� using normalization values
from chondrite �Evensen et al., 1978�, as a function of latitude in figure 4.12 and in a Nd
versus Sr isotopic composition plot in figure 4.14. The great majority of samples is
geochemically homogeneous and present typical MORB patterns with a clear depletion in
most light rare earth elements and in most cases a slight negative anomaly in Eu due to
plagioclase fractionation. All samples from the PACANTARCTIC2 cruise, named PAC2 and
shown in figure 4.14, are located in the global MORB field and below the Pacific Reference
Line �PRL� defining the two sub‐Pacific mantle domains limited by the Easter microplate
�Vlastélic et al., 1999�. PAC2 data extend the PAR 66‐53°S data field towards more
radiogenic Sr �0.70264� and less radiogenic Nd �0.51306� values.
Figure 4.11: Chondrite‐normalized rare earth element patterns of samples from
each segment. Atypical segments S2 and N2 show atypical patterns with higher
concentrations and europium anomalies. Other atypical patterns as compared to
typical N‐MORB light rare earth depleted patterns are seen in segments S3 and N1
on both sides of Menard TF.
The description below reveals characteristics specific to individual segments. For each
segment, the variations are within analytical error but the coherence in the variation of the
different geochemical parameters suggests that these variations are real and that the
analytical errors are most likely over evaluated. The Menard transform fault �figures 4.9,
4.8� is a major geological feature, a first order discontinuity cutting across our study area.
Thus, it is natural to present the data from each side successively.
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