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

tel-00344949, version 1 - 7 Dec 2008
4. Article soumis à G3
4. RESULTS
4.1 MAJOR ELEMENTS
All but two dredges �namely DR06‐1 and DR09‐1� have negative loss of ignition
reflecting the lack of alteration and the pristine nature of the samples. Nearly all samples
are typical basalts and form a homogeneous group in the total alkalis versus silica diagram
�figure 4.10�. Only the samples collected close to the Menard TF �DR09, DR20� or in the
transitional ridge segments S2 and N2 �DR06, DR27� are slightly different and present
higher alkali contents. DR09‐1 is andesitic. Major elements plot along similar
differentiation trends for all segments. This underlines the first‐order major element
homogeneity of the mantle source and the similarities in the mineral assemblage involved
in the crystallization process. It also suggests similarities in the physical conditions of the
magmatic plumbing system of the studied ridge section. At a smaller scale, it is noticeable
that samples from segment ends have slightly higher MgO, K2O/TiO2 and lower K2O
concentrations as compared to segment centers �figure 4.12�.
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Figure 4.10: Alkalis versus SiO2 diagram. The great majority of samples is
basaltic. The only exception is DR09, which is an andesitic basalt and is
located immediately south of Menard TF. DR20, located immediately to the
north of Menard TF, is also slightly more evolved thanthe majority of
samples. The other two samples that are slightly more differentiated than
the majority are DR06 and DR27.