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Tungsten Isotope Constraints on Archean Geodynamics

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Tungsten Isotope Constraints on Archean Geodynamics Tungsten isotope constraints on Archean geodynamics Inaugural-Dissertation zur Erlangung des Doktorgrades der Mathematisch-Naturwissenschaftlichen Fakultät der Universität zu Köln vorgelegt von Jonas Tusch aus Meerbusch Köln, 2020 II Berichterstatter: Prof. Dr. Carsten Münker PD Dr. Daniel Herwartz Dr. Kristoffer Szilas Tag der mündlichen Prüfung: 10.09.2020 This thesis is dedicated to my wife Lena IV Table of contents I. Abstract ............................................................................................................... 1 II. Kurzzusammenfassung ..................................................................................... 3 III. Introduction ........................................................................................................ 6 Origin and evolution of the Earth .....................................................................................................6 Early Earth Processes (Moon-forming impact, accretion history, late veneer) .................................7 Tectonic Regimes, Archean rocks and their spatial distribution .............................................. 10 Application of short-lived isotope systems ................................................................................. 14 Mobility and secondary overprint of W and the implication for 182W anomalies ...................... 16 182W isotope systematics of the terrestrial mantle ...................................................................... 16 Significance of constraints on the dynamics of Early Earth processes ................................... 19 IV. Sythesis ............................................................................................................ 21 1 Uniform 182W isotope compositions in Eoarchean rocks from the Isua region, SW Greenland: the role of early silicate differentiation and missing late veneer ....................................................................................................................... 25 1.1 Abstract ................................................................................................................................ 25 1.2 Introduction .......................................................................................................................... 26 1.3 Methods ................................................................................................................................ 28 1.3.1 Whole rock major, trace element and isotope dilution measurements (HFSE, Th & U) 29 1.3.2 Ion exchange protocol for high precision W isotope measurements ............................ 30 1.3.3 Mass spectrometry for high precision 182W isotope measurements .............................. 34 1.4 Geological background and samples ............................................................................... 37 1.5 Results .................................................................................................................................. 43 1.6 Discussion ........................................................................................................................... 48 1.6.1 Methodology .................................................................................................................. 48 1.6.2 Elemental behavior of W in rocks from the Isua region ................................................. 53 1.6.3 Significance of uniform 182W isotope compositions ....................................................... 57 1.6.4 Significance of the 182W and W/Th signatures in mantle-like peridotites ...................... 59 1.6.5 Origin of 182W isotope anomalies in SW Greenland ...................................................... 60 1.7 Conclusion ........................................................................................................................... 64 V 2 Convective isolation of Hadean mantle reservoirs through Archean time . 66 2.1 Abstract ................................................................................................................................ 66 2.2 Introduction .......................................................................................................................... 67 2.3 Geological background and samples ................................................................................ 68 2.4 Results .................................................................................................................................. 71 2.5 Discussion ............................................................................................................................ 75 2.6 Conclusion ........................................................................................................................... 77 2.7 Method Section .................................................................................................................... 78 2.7.1 Analytical protocol .......................................................................................................... 78 2.7.2 Geological background .................................................................................................. 81 2.7.3 Sample selection ........................................................................................................... 83 2.7.4 Secular evolution of 182W isotope anomalies ................................................................. 86 3 Long-term preservation of Hadean protocrust in Earth´s mantle ............... 88 3.1 Abstract ................................................................................................................................ 88 3.2 Introduction .......................................................................................................................... 89 3.3 Geological and methodological overview ......................................................................... 90 3.4 Results .................................................................................................................................. 90 3.5 Discussion ............................................................................................................................ 91 3.6 Conclusions ....................................................................................................................... 101 3.7 Method section .................................................................................................................. 101 3.7.1 Geological background of our sample selection .......................................................... 101 3.7.2 Lower protocrust delamination model .......................................................................... 106 3.7.3 Analytical protocol ........................................................................................................ 110 References ............................................................................................................ 113 Appendix ............................................................................................................... 137 Appendix A..................................................................................................................................... 137 Appendix B..................................................................................................................................... 145 Appendix C..................................................................................................................................... 159 Acknowledgements .............................................................................................. 165 Erklärung ............................................................................................................... 167 VI Abstract I. Abstract Geodynamics on Earth are active since more than 4 billion years and have continuously lead to differentiation and homogenization of mantle and crust. However, evidence from short-lived nuclide decay systems (e.g. 129I – 129Xe and 146Sm – 142Nd) suggest that primordial heterogeneities, which formed in the early Hadean Eon (i.e. during the first ca. 100 million years after formation of the Earth), survived for a long period of time – in some cases for Eons and until present day. Studying rocks that display variations in the decay products of short-lived nuclide decay systems offer two intriguing perspectives: (1) The cause for their formation refers to processes that must have had operated at a time when no other witnesses preserved in the geological rock record. (2) Understanding the mechanisms that allowed for their long-term preservation provides important insights into the temporal evolution of the bulk silicate Earth (BSE) and into the processes that lead to the present-day state of the mantle, including the onset of plate tectonic processes on Earth. One of the short-lived decay series that has increasingly been applied in geochemistry is the 182Hf-182W decay system with a half-life of 8.9 million years (Vockenhuber et al., 2004)). Over the past decade, several studies investigated W isotope systematics in terrestrial rocks and found differences in the relative abundance of 182W. While Archean rocks were found to exhibit predominantly elevated 182W isotope compositions compared to the modern depleted mantle, modern oceanic island basalts (OIBs) and one Archean komatiites system (the ca. 3.55 Ga Schapenburg komatiite suite from the Kaapvaal Craton, southern Africa) were shown to display negative anomalies. These findings were
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