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

tel-00344949, version 1 - 7 Dec 2008
4. Article soumis à G3
�high mantle Bouguer anomalies at the ridge axis discontinuities, figure 3 in Klingelhöfer et
al., 2006�. The model can also account for the non‐rigid character of the ridge discontinuity
marked by overlapping spreading centers at segment ends. OSC are mobile limits between
two segments where along‐strike ridge segment migration takes place at a rate of the same
order of magnitude as horizontal mantle flows �Macdonald et al., 1988�.
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5.3. MENARD TRANSFORM FAULT AND RIDGE SEGMENTATION
The Menard transform fault, first order discontinuity, is the most prominent geological
feature of the area. But does it also play a major geochemical role? Are there differences in
the geochemical characteristic s of samples on each side of the transform fault?
The first difference is the narrower range of geochemical variations along the ridge
north of Menard TF �excluding N2� comparatively to the southern area. This could be
attributed to the spreading rate increase towards the north. A faster spreading rate
indicates faster mantle movements favorable to homogenization processes �Allègre, 1981�.
The second difference lies in the degree of depletion on each side of the transform fault.
In the Nd‐Sr plot �figure 4.14�, the PAC2 data define an elongated field sub‐parallel to the
EPR field from 0.70235 to 0.70265. In this representation, the least radiogenic Sr �and
most radiogenic Nd� samples are found to the south of Menard TF whereas the most
radiogenic ones are from the north. Each side of the Menard TF can be represented as
individual data fields, which partially overlap each other around 0.70250 and 0.51308.
Although the average Sr isotopic composition of the north of Menard TF samples is slightly
higher than the one from the samples from the southern part of the ridge, the two fields are
not isotopically distinct. DR34, the most depleted sample of the whole ridge section and
located north of the TF is within the data field of samples located south of Menard TF. As
discussed in the previous paragraph, this sample, located at a segment extremity, might
represent a mantle residue, which would account for its extreme depletion characteristics.