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Soutenance de thèse de Xiaoyan Gu

Vendredi 3 juin 2016 à 14h, amphi du CRPG

Melt-rock interaction signatures in peridotite from sub-continental mantle (French Massif Central) : A trace element, H, Li and δ7Li approach

Abstract

Peridotite xenoliths from Allègre and Mont Coupet, in the southern French Massif Central (FMC), have been investigated to constrain the evolution of the sub-continental lithospheric mantle beneath the FMC and the behaviors of Li and H during melt-rock reactions. In-situ measurements were performed for major and trace elements in Ol, Px and Amp when existing via EPMA and LA-ICP-MS respectively, for Li concentrations and isotopic compositions in Px and Ol via SIMS, and water concentrations in minerals via FTIR.

Negative HFSE anomalies and markedly high LREE/HREE ratios reflect a metasomatism following an earlier partial melting process in the lithospheric mantle for Allègre and Mont Coupet samples. The Lithium systematic of Allègre xenoliths indicates that they experienced at least two stages of metasomatism by different melts. Exceptionally high Li contents in Cpx (up to 50 ppm) and slightly Li enriched Ol rims are ascribed to a diffusive Li uptake from the xenolith host magma. On the other hand, extremely light Li isotopic compositions preserved in Ol cores with δ7Li as low as -25‰ suggest another metasomatic event prior to xenolith entrainment by their host magma. In contrast, xenoliths from Mont Coupet have Li contents in the range of normal mantle and display homogeneous intragranular Li distributions and nearly equilibrated inter-mineral Li partitioning. The negative δ7Li values of Cpx and Opx in some Mont Coupet samples were brought by exchange with a small amount of melt. The preservation of large inter-mineral Li isotopic fractionation observed in these samples indicates that the melt percolation should occur shortly before the entrainment to the surface. The negative δ7Li signature of the metasomatic agents may be related to a subduction environment, produced by the Variscan orogeny.

Water content in Allègre xenoliths ranges from 10.6 to 12.4 ppm in weight, much lower than the water content in the MORB source mantle. It implies that water were lost during the degassing of the host magma during slow cooling of the lava lake. No core-rim variations from profile analysis suggest that xenoliths have achieved water diffusive equilibrium with the host magmas. Peridotite xenoliths from Mont Coupet have retained their original water content from the mantle depths. Partial melting has controlled the water content in most samples from Mont Coupet. However, the subsequent carbonatite-related metasomatism has affected the sample MC34, which had the highest water content among the Mont Coupet xenoliths. And the aqueous agent responsible for presence of Amp in samples MC36 and MC53 has not lead to the considerable increase of water content.

Jury

Laurie REISBERG, CNRS CRPG, Nancy, examinateur
Christiane WAGNER, UPMC, Paris, examinateur
Julien BERGER, Université Paul Sabatier, Toulouse, examinateur
Sylvie DEMOUCHY, Université Montpellier 2, Montpellier, rapporteur
Qunke XIA , Université de Zhejiang, Chine, rapporteur
Etienne DELOULE, CNRS CRPG, Nancy, directeur
Jannick INGRIN, Université Lille 1, Lille, co-directeur
Lydéric FRANCE, Université de Lorraine, Nancy, co-encadrant




publié vendredi 20 mai 2016