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

tel-00344949, version 1 - 7 Dec 2008
4. Article soumis à G3
segment variations discussed in the previous paragraph 5.2. Each trend reflects individual
mantle domains each with their own history, on each side of Menard TF.
As seen in figure 4.12, the change in the depletion character of the samples is not
gradual along the studied ridge section. Four segments from S2 to N2 across Menard TF,
show data that is scattered and this has to be attributed to the effect of this rigid
discontinuity and the presence of off‐axis volcanic alignments. Looking into more detail at
the samples from the dredges closest to the Menard TF, namely DR09 and DR20, their
major element contents indicate evolved characteristics �figure 4.10�. This is coherent with
their higher rare earth concentrations, with patterns that display europium anomalies
indicative of plagioclase fractionation. Their �La/Sm�N is also higher than further away
from the TF without a significant change in their isotopic ratios. These chemical features
are likely to be caused by crystal fractionation close to the transform fault. Lower
upwelling rates and mantle temperatures near the rigid boundary create perturbations in
melt production, which explain the evolved character of the erupted magmas �Morgan and
Forsyth, 1988�. Similar chemical features associated to the Transform Fault Effect
�Langmuir and Bender, 1984� are described along the southern part of the PAR and shown
in figure 3 of Vlastélic et al. �2000�. This is in contrast with the effects observed at second
order non‐rigid boundaries such as the OSC separating the different ridge segments and
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Figure 4.15: Sr isotopic composition versus �La/Sm�N. Regression lines are
calculated through the two groups of samples located south �dashed line� and
north �solid line� of Menard TF.