The Cenozoic volcanism in the FMC

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Table of contents

1. Introduction
1.1 The lithospheric mantle
1.2 Lithium
1.2.1 Geochemical features
1.2.2 The current research state of Li in the lithospheric mantle
1.3 water in the NAMs
1.3.1 Water in the earth mantle
1.3.2 The current research state of water in the NAMs
1.4 Objective of this thesis
2. Geological background
2.1 The pre-Cenozoic tectonic evolution of the FMC
2.2 The Cenozoic volcanism in the FMC
2.3 The lithospheric mantle beneath the FMC
2.4 The sampling localities
3. Analytical methods
3.1 Sample preparations
3.2 Electron micro-probe analysis (EPMA)
3.3 Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS)
3.4 Secondary ion mass spectrometry (SIMS)
3.4.1 Basic working principles
3.4.2 Standards for Li isotopes measuring by SIMS
3.4.3 Measuring procedure for Li isotopes by SIMS
3.4.4 Measuring procedure for water content by SIMS
3.5 Fourier transform infrared (FTIR) spectrometer
3.5.1 The rationale
3.5.2 Measuring procedure for water content by FTIR
3.5.3 Quantitative calculation procedure
4. Results
4.1 Petrological description
4.1.1 Mineral proportions and textures
4.1.2 Spongy textures in sample AL47P1
4.2 Mineral major element compositions
4.2.1 Ol
4.2.2 Opx
4.2.3 Cpx
4.2.4 Spl
4.2.5 Amp
4.2.6 AL47P1
4.3 Trace element compositions
4.3.1 Cpx in samples from Allègre
4.3.2 Cpx in samples from Mont Coupet
4.3.3 Amp in samples MC36 and MC53
4.3.4 Al, Ti, Cr and Ni in Ol from Allègre and Mont Coupet
4.4 Li concentrations and isotopic compositions in minerals
4.4.1 Samples from Allègre
4.4.2 Samples from Mont Coupet
4.5 Water content results measured by FTIR
4.5.1 Infrared spectra
4.5.2 Calculated water content from infrared spectra
4.6 Water content results from SIMS measurements
5. Thermobarometry
6. Discussion
6.1 Depletion by partial melting
6.2 Mantle metasomatism revealed by trace element compositions
6.2.1 Trace element variations in Cpx from Allègre
6.2.2 Trace element variations in Cpx from Mont Coupet
6.2.3 Trace element partition between Cpx and Amp in samples MC36 and MC53
6.2.4 Trace element compositions of metasomatic melts/fluids and melt-rock reaction kinetics
6.3 Li distribution and isotopic composition variations
6.3.1 intra- and inter-mineral Li and Li isotopes distribution in Allègre samples
6.3.1.1 Intra-granular and inter-mineral lithium concentration disequilibrium
6.3.1.2 Distinctive Li isotopic offsets among mineral phases induced by asynchronous metasomatic events
6.3.2 No obvious Li addition but large inter-sample Li isotopic composition variations in peridotite xenoliths from Mont Coupet
6.3.2.1 Nearly equilibrated Li partitioning among minerals phases and no concentration zoning in grains
6.3.2.2 Large isotopic composition variations caused by a recent metasoamtism .
6.3.3 Origin of melts with exceptionally light Li isotopic compositions
6.3.4 Precedence relationship of the carbannatitic mantle metasomatism and percolation of the negative δ7Li melt
6.4. Water content of minerals in peridotite xenoliths from the French Massif Central
6.4.1 Assignment of bands in the infrared spectra
6.4.2 Water partitioning among mineral phases
6.4.3 The low water content in minerals from Allègre samples – equilibrated with degassed host magmas
6.4.4 Mineral water concentration variations in xenoliths from Mont Coupet
6.4.4.1 Preservation of original water content during xenolith ascent
6.4.4.2 The processes controlling the water content in peridotite xenoliths from Mont Coupet
6.4.4.3 Relationship between water content and trace element concentrations in Ol .
6.4.5 Water content in the lithospheric mantle beneath the French Massif Central
References