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Approches pétrogéochimique et géomathématique de la cyclicité magmatique en contexte d'accrétion océanique : de la fusion partielle aux séquences de réalimentation/vidange des réservoirs crustaux.

Abstract : The present studies, based on geochemical and petrographic features of mid-ocean ridge lavas, permitted constraining the temporal evolution of a magma reservoir and the partial melting of the mantle source beneath the spreading ridge axes. In lack of constraints about absolute ages, the relative chronology between samples has been constrained from their geographic and stratigraphic relationships.

In a first step, a geomathematic model has been elaborated to track the evolution of a periodically replenished magma chamber. The flux of replenishment magma evolves sinusoidally through time. The application of the model to a natural site requires the constraint of the amplitude of the chemical variations in lavas recovered by a high quality sampling. Besides, geological input parameters have to be approached through hypotheses on magma reservoir geometry and physical-thermal state. The geochemical resolution approach of this model has been applied to three accretion sites: the East Pacific Rise (EPR), the North Fiji Basin propagator, and the Juan de Fuca Ridge (JdF). For the North Fiji propagator and the JdF, the incompatible trace element compositions calculated for the replenishment liquid perfectly fit those determined from experimental constraints. For the EPR and the JdF, the periodicity of the magma chamber evolution has been determined (750 and 1000 years, respectively). The geomathematic model is thus useful to characterize the instantaneous flux of magma through the reservoir and is a complement to geomorphological and geophysical studies that estimate the mean magma budget periodicity.

The second part of the dissertation applies to dikes and lavas that outcrop at the southern tip of the JdF, along both flanks of the West Blanco Depression. The Sr-Nd isotope compositions have been used to characterize a heterogeneous mantle beneath the Blanco fracture zone. The mantle is composed of highly depleted peridotites, isotope compositions of which are close to a pure mantle end-member, veined with a more enriched component. The kinematic regime at the southern tip of the JdF, evolving from propagation to accretion, might have play a role in the remelting at the JdF axis of a residual mantle source, previously melted beneath a propagating segment. Besides, precise analyzes of zoning patterns in plagioclase phenocrysts and xenocrysts have highlighted the mixing in the magma chamber between H2O-bearing dacitic melts and anhydrous mafic melts. For cogenetic lavas, based on the depth fluctuations of the magma differentiation proxy Mg#, such mixings have been interpreted as evidence for episodic replenishment of the magma chamber. The results of the geomathematic model, but also of the more experimental interpretations, suggest that the replenishment liquid is a basaltic N-MORB (Mg#~68.5).

Then, in a third study, the examined lavas have been collected transversally to the Central Indian Ridge axis around 19°S, a complex ridge portion, potentially interacting with the Réunion hotspot. The Sr-Nd isotope compositions have highlighted a significant fluctuation of the mantle source through time. Some compositions tend toward those of the Gasitao off-axis volcanic ridge, suggesting a ridge-hotspot interaction. However, most of the compositions require the consideration of an additional enriched mantle component, also involved in the genesis of the volcanic Rodrigues Island. Besides, to reproduce the incompatible trace element enrichment of these lavas, both variations of the melting degree and of the mantle source are required. The repartition of the enriched lavas along the profile then suggests that the melting rate fluctuates periodically, over a period of 200-250 Ky.
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Contributor : Carole Cordier <>
Submitted on : Thursday, June 28, 2007 - 7:35:49 PM
Last modification on : Tuesday, February 2, 2021 - 9:52:02 AM
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Carole Cordier. Approches pétrogéochimique et géomathématique de la cyclicité magmatique en contexte d'accrétion océanique : de la fusion partielle aux séquences de réalimentation/vidange des réservoirs crustaux.. Géochimie. Université de Bretagne occidentale - Brest, 2006. Français. ⟨tel-00158336⟩

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