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

tel-00344949, version 1 - 7 Dec 2008
4. Article soumis à G3
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4.2.1 South Menard TF
The rare earth element �REE� patterns for all 3 segments vary from depleted with
�La/Sm�N � 0.57 to almost flat with �La/Sm�N � 1.0. The Sr‐Nd isotopic range is rather
large, with Sr compositions varying from 0.70235 to 0.70252.
‐ Samples from segment S1 present the most depleted REE patterns of all 3 segments
with 4 dredge samples exhibiting very uniform compositions �figure 4.11�. Looking further,
there is a trend of increasing depletion in �La/Sm�N, 88Sr/ 86Sr and 143Nd/ 144Nd from south to
north �figure 4.12�. The most depleted sample, DR4‐2, is located at the northern end of the
segment.
‐ Segment S2 includes 2 dredges, namely DR5 and DR6. Three types of basalt can be
distinguished petrographically and geochemically. Dredge 6 contains two rock types. One
sample is depleted �La/Sm�N � 0.69, REE concentrations 20 fold chondritic, 88Sr/ 86Sr �
0.7024 and the other is slightly more enriched �La/Sm�N � 0.91, REE concentrations 70
fold chondritic, 88Sr/ 86Sr � 0.7025. The most enriched sample displays a negative Eu
anomaly characteristic of plagioclase fractionation.
‐ S3 presents two groups of patterns with different REE concentrations of 25 to 70 times
the chondrites. Again, the most enriched samples display a negative Eu anomaly
characteristic of plagioclase fractionation. They are from the dredge closest to Menard TF
and have the least radiogenic Nd isotopic ratio.
4.2.2 North Menard TF
The great majority of samples from all 6 segments shows a very narrow range of rare
earth element patterns, comparatively to the south of Menard TF, with a mean �La/Sm�N of
0.62 � 0.05 �n�17�. Two samples are outside this range �1� DR27 which is the only
enriched sample of the studied ridge axis section with �La/Sm�N � 1.12 �2� DR34 which is
the most depleted sample of the ridge section with �La/Sm�N � 0.44. This homogeneity
with one exception concerning DR27 is also observed for Sr isotopic ratios, which average
0.702480 � 15 �n�16�. It is worthwhile noting that this range is smaller than the
estimated external reproducibility and suggests that this estimate is rather pessimistic.
‐ N1 presents sub‐parallel REE patterns with concentrations ranging from 20 to 50 times
the chondrites. The most enriched sample, which is also the closest to Menard TF, displays
a negative Eu anomaly characteristic of plagioclase fractionation.