Melt-rock interactions and melt-assisted deformation in the Lherz peridodite, with implications for the structural, chemical and isotopic evolution of the lithospheric mantle Véronique Le Roux To cite this version: Véronique Le Roux. Melt-rock interactions and melt-assisted deformation in the Lherz peridodite, with implications for the structural, chemical and isotopic evolution of the lithospheric mantle. Geo- chemistry. Université Montpellier 2, 2008. English. tel-00431325 HAL Id: tel-00431325 https://tel.archives-ouvertes.fr/tel-00431325 Submitted on 12 Nov 2009 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. UNIVERSITÉ MONTPELLIER II SCIENCES ET TECHNIQUES DU LANGUEDOC THÈSE Pour obtenir le grade de DOCTEUR DE L’UNIVERSITÉ MONTPELLIER II Discipline : Structure et évolution de la Terre et des autres planètes École Doctorale : SIBAGHE Présentée et soutenue publiquement Par VÉRONIQUE LE ROUX Le 7 novembre 2008 MELT-ROCK INTERACTIONS AND MELT-ASSISTED DEFORMATION IN THE LHERZ PERIDOTITE, WITH IMPLICATIONS FOR THE STRUCTURAL, CHEMICAL AND ISOTOPIC EVOLUTION OF THE LITHOSPHERIC MANTLE JURY M. Jean-Louis Bodinier Directeur de Recherche (CNRS UMII) Directeur de thèse M. Alain Vauchez Maître de Conférences (UM II) Co-directeur de thèse Mme Suzanne Y. O’Reilly Professeur (Macquarie University) Co-directrice de thèse M. Michel Grégoire Chargé de Recherche (Obs. Midi. Pyr.) Rapporteur M. Ben Holtzman Docteur (Columbia University) Rapporteur Mme Andréa Tommasi Directeur de Recherche (CNRS UMII) Examinateur M. Francis Albarède Professeur (ENS Lyon) Examinateur M. Jean-Pierre Lorand Directeur de Recherche (Muséum, Paris) Examinateur The most exciting phrase to hear in science, the one that heralds new discoveries, is not “Eureka!” (I've found it!), but “That's funny...” Isaac Asimov Table of contents TABLE OF CONTENTS ABSTRACT i DECLARATION ii ACKNOWLEDGMENTS iii CHAPTER I: INTRODUCTION 1 1. GENERAL INTRODUCTION 1 2. GEOLOGICAL SETTING OF THE LHERZ MASSIF 4 2.1 LITHOLOGIES 6 2.2 SAMPLING 9 3. PRESENTATION OF THE CHAPTERS 10 CHAPTER II: 13 THE LHERZ SPINEL LHERZOLITE: REFERTILIZED RATHER THAN PRISTINE MANTLE 13 1. INTRODUCTION 15 2. GEOLOGICAL BACKGROUND 17 3. STRUCTURAL AND MICROSTRUCTURAL DATA 19 4. GEOCHEMICAL DATA 21 5. DISCUSSION 24 5.1. STRUCTURAL RELATIONSHIPS BETWEEN HARZBURGITIC PROTOLITH AND SECONDARY LHERZOLITES 24 5.2. FORMATION OF THE LHERZOLITES BY REFERTILIZATION REACTION 25 5.3. REFERTILIZATION AND ISOTOPIC VARIATIONS 28 5.4. REFERTILIZATION FRONTS, A MOVING LITHOSPHERE-ASTHENOSPHERE BOUNDARY ? 29 6. CONCLUSIONS 30 CHAPTER III: 33 FEEDBACK BETWEEN MELT PERCOLATION AND DEFORMATION IN AN EXHUMED LITHOSPHERE-ASTHENOSPHERE BOUNDARY 33 1. INTRODUCTION 35 2. THE LHERZ PERIDOTITIC MASSIF (P YRENEES ,FRANCE ) 37 3. EVOLUTION OF CPO THROUGH THE LHERZ MASSIF 41 3.1. SAMPLING AND METHODS 41 3.2. WEAKENING OF CPO ASSOCIATED WITH THE REFERTILIZATION PROCESS 44 4. DISCUSSION 48 4.1. EVOLUTION OF PERCOLATION -DEFORMATION FEEDBACK MECHANISMS FROM HARZBURGITES TO LHERZOLITES 48 4.2. FORMATION OF A PERVASIVE WEBSTERITIC LAYERING IN REFERTILIZED LHERZOLITES 52 4.3. GEODYNAMIC ENVIRONMENT OF THE REFERTILIZATION PROCESS 53 5. CONCLUSIONS 54 CHAPTER IV: 57 ISOTOPIC DECOUPLING DURING POROUS MELT FLOW: A CASE-STUDY IN THE LHERZ PERIDOTITE 57 1. INTRODUCTION 59 2. BACKGROUND AND SAMPLING 60 3. SAMPLES PREPARATION AND ANALYTICAL METHODS 62 4. RESULTS 63 5. DISCUSSION 67 5.1. CONTRASTED ISOTOPIC COMPOSITIONS OF LHERZOLITES AND HARZBURGITES IN THE PYRENEES 67 5.2. MODELING OF ISOTOPIC REEQUILIBRATION DURING MELT PERCOLATION 68 5.3. RESULTS OF THE MODELING 71 6. CONCLUSIONS 75 CHAPTER V: CONCLUSION AND OUTLOOK 77 REFERENCES 81 SUPPLEMENTARY MATERIAL 93 Technical work 96 Article Mantle refertilisation, osmium isotopes and the “Primitive Upper Mantle” fallacy, Consequences for the nature of the late veneer 97 Table 1 - Crystallographic fabrics strengths of minerals 118 Figure 1 - Crystal-preferred orientations of minerals (CPO) 120 Table 2 - Major and minor elements compositions of minerals from Lherz peridotites 127 Table 3 - Whole-rock chemical compositions – major and minor elements (ppm) 138 Table 4 - Whole-rocks chemical compositions – trace elements (ppm) 142 Table 5 - Whole-rocks chemical compositions – trace elements normalized to primitive mantle 144 Figure 2 - Whole-rocks trace elements normalized to primitive mantle 146 Table 6 - Modal compositions of Lherz peridotites 147 Table 7 - REE clinopyroxenes concentrations (ppm) 148 Table 8 - REE clinopyroxenes concentrations normalized to chondrites 152 Figure 3 - REE clinopyroxenes concentrations normalized to chondrites 154 Table 9 - Hf, Nd and Sr isotopic ratios of the Lherz peridotites 155 Summary of chemical techniques and methods 156 Abstract ABSTRACT Differentiation of the lithospheric mantle occurred principally through partial melting and extraction of basaltic melt. In this scheme, harzburgites are widely considered as refractory mantle residues left after extraction of a basaltic component, whereas fertile lherzolites are regarded as pristine mantle only weakly affected by partial melting. However, a part of tectonically-emplaced mantle rocks displays local evidence of melt percolation and have been ascribed to igneous refertilization of refractory peridotites. In this context, the present work re-investigates the nature of the Lherz spinel lherzolites (French Pyrenees), from the type- locality of lherzolites, described as a piece of preserved fertile mantle. Structural and geochemical data presented in this study show that the Lherz lherzolites are not pristine but formed through a refertilization reaction between MORB-like ascending partial melts and a depleted lithosphere from which the Lherz harzburgites are the remnants. As the type locality of lherzolites, the Lherz peridotites were partly used to infer the composition of the primitive upper mantle and these results may have important implications for the nature of the late veneer. Additionally, the analysis of crystal-preferred orientations (CPO) of minerals from Lherz peridotites highlight a strong feedback between percolation of basaltic melts and deformation under near-solidus conditions at the lithosphere-asthenosphere boundary, suggesting that refertilization may be a major process involved in the rejuvenation of the lithosphere. Variations in CPO patterns and intensity record the relative contribution of dislocation glide and diffusion processes, ruled by a balance between instantaneous melt fraction and local strain rate. These results also suggest that the pervasive websteritic layering that formed synchronous to the refertilized lherzolites may result from deformation-assisted melt segregation due to decreasing permeability in the system. Finally, this work investigated the effect of melt percolation on Hf, Nd and Sr isotopic variations. Isotope systematics of the Lherz peridotites shows that a strong isotopic decoupling may arise at the melt percolation front between depleted harzburgite and refertilized lherzolite. The one-dimensional modelling used shows that these decoupled signatures are generated by porous flow and governed by critical parameters such as the concentrations of the elements in the melt to the matrix, chemical diffusivities or efficiency of isotopic homogenization. We observe that, under certain circumstances, melt-rock interactions can generate enriched, “intraplate-like” isotopic signatures in the transition zone between upwelling partial melts and lithospheric mantle. These results suggest that a part of isotopic signatures of mantle-related rocks could be generated by diffusional processes associated with melt transport. i Declaration DECLARATION This thesis is a cotutelle work carried out in University of Montpellier II, France, and in Macquarie University, Australia, over the period October 2005 to October 2008. I hereby declare that all data and interpretations presented in this thesis are my own work except for information cited from published or unpublished work of others that has been fully acknowledged. No part of this thesis has been submitted at any other university than Macquarie University. Veronique Le Roux 1st of December 2008 ii Acknowledgments Acknowledgments This thesis results from the collaboration between Geosciences Montpellier (France) and GEMOC ARC National Key Centre (Australia). This work has been supported by INSU DYETI Program, University of Montpellier II, Macquarie University, ARC Discovery project grants, the French Ministry of Research and ‘Bourses Lavoisier cotutelle’. I want to thank all the people who have been implicated in my work during these three years, and particularly my official supervisors Jean-Louis Bodinier, Alain Vauchez and Suzanne O’Reilly. I am also grateful to Andrea Tommasi and Olivier Alard who were involved in some parts of my project. Thanks a lot to the staff in Geosciences Montpellier (particularly Claude Merlet, Olivier Bruguier and Simone Pourtales) and in GEMOC ARC National Key Centre (particularly Peter Wieland and Norman Pearson) for their help