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

tel-00344949, version 1 - 7 Dec 2008
4. Article soumis à G3
sectional area �figure 4.13�. In several cases the segment ends show a more depleted
character associated with lower cross‐sectional area, and with a deeper bathymetry than
the segment center. These observations suggest that less depleted basalts erupt
preferentially in the segment center where the magma supply is more substantial than at
segment ends. Other similar observations are reported in Batiza �1996� but trace element
ratios are often poorly correlated with axial depth and very rarely with isotope ratios
�Michael et al., 1994; Prinzhofer et al., 1989; Karsten et al., 1990�. As expressed by Detrick
�2000�, two different models are proposed for the melt supply beneath segments along a
fast spreading ridge. In the first model, mantle upwelling is “sheet‐like” and the melt is fed
into crustal magma chambers along the entire length of each spreading segment
�Parmentier and Phipps Morgan, 1990�. In the second model, mantle upwelling is “plume‐
like” and the melt is focused at the segment center and then undergo a lateral transport
toward segments ends �Macdonald et al., 1988; Wang and Cochran, 1993�. Because of the
regular along segment isotopic variations, none of these two models is consistent with the
intra‐segment variability reported here.
Alternative models have to take into account the isotopic heterogeneity of the mantle
source. If the source were heterogeneous at the segment scale, a depletion gradient from
the segment center towards each extremity would lead to the observed trace element and
isotopic variations. But the origin of such a gradient would be dicey. If the source were
homogeneous at the segment scale but heterogeneous at a small scale, a low degree of
melting would preferentially sample the enriched component at the segment center. This is
inconsistent with the higher cross section at the segment center �figure 4.13�, which is
generally interpreted as indicative of a higher magma supply at segment centers compared
to their extremities �Sheirer and Macdonald, 1993�. Rather, a model where the mantle
source is heterogeneous due to the melting processes can be invoked. Melting is initiated
and enhanced beneath the segment center. Then, horizontal, along axis, mantle flow allows
transport of residual mantle from segment center towards their ends. This interpretation
can explain that samples collected at segment ends are more depleted than those found in
the center. Lower MgO contents in basalts from the segment center �figure 4.12� are
consistent with the model if the residence time of the liquids is longer where the crust is
thicker. It also fits the interpretations of geophysical observations that indicate a more
substantial magma supply at segment center and anomalously thin crust at segment ends
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