Molybdenum Stable Isotope Composition of Meteorites: Constraints on Planetary Core Formation 155 VI–1
Total Page：16
File Type：pdf, Size：1020Kb
Research Collection Doctoral Thesis Stable isotope behaviour during planetary differentiation Implications of mass dependent Fe, Si and Mo isotope fractionation between metal and silicate liquids Author(s): Hin, Remco Christiaan Publication Date: 2012 Permanent Link: https://doi.org/10.3929/ethz-a-007621154 Rights / License: In Copyright - Non-Commercial Use Permitted This page was generated automatically upon download from the ETH Zurich Research Collection. For more information please consult the Terms of use. ETH Library DISS. ETH NO. 20825 STABLE ISOTOPE BEHAVIOUR DURING PLANETARY DIFFERENTIATION Implications of mass dependent Fe, Si and Mo isotope fractionation between metal and silicate liquids A dissertation submitted to ETH ZURICH for the degree of Doctor of Sciences presented by Remco Christiaan Hin Master of Science in Geosciences of Basins and Lithosphere, Vrije Universiteit Amsterdam born on December 1, 1983 citizen of The Netherlands accepted on the recommendation of Prof. Dr. Max W. Schmidt Prof. Dr. Bernard Bourdon Prof. Dr. Tim Elliott 2012 (Panta rhei, ouden menei) “And now that I have unfolded my sail at open sea and go with the wind – there is nothing in this whole world that remains. Everything flows, each thing shapes and passes by. Time too passes by in a continuous motion like a river that cannot stop its stream just as a running hour cannot stand still; as water pushes water forward while being pushed forward and pushing forward itself, as such time runs forward and chases itself and renews itself; what has been, is now past, and now is, what has not been; every moment changes.” Pythagoras explains Herakleitos views, as passed on to us through Ovidius’ Metamorphosae. (Personal translation of the Dutch translation by M. D’Hane-Scheltema.) Table of Contents Abstract/Zusammenfassung 3 Chapter I. Introduction 9 I–1. Planetary accretion and core formation 10 I–1.1 The formation of the Solar System 10 I–1.2. Differentiation of planetary objects 11 I–2. Fractionation of isotopes 15 I–2.1 Mass dependent fractionation of stable isotopes 15 I–2.2 Theoretical background of stable isotope fractionation 16 I–3. Objectives of this thesis 17 Chapter II. Methodology 19 II–1. Experimental 20 II–1.1. Starting mixtures 20 II–1.2. Piston cylinder experiments 20 II–2. From a quenched experiment towards chemical dissolution 32 II–3. Isotopic analyses 34 II–3.1. Ion exchange chemistry 35 II–3.2. MC-ICPMS 37 II–4. Failed Silicon isotope analyses by laser ablation 44 Chapter III. Experimental evidence for the absence of iron isotope fractionation betweenmetal and silicate liquids at 1 GPa and 1250-1300 °C and its cosmochemical consequences 47 III–1. Introduction 48 III–2. Methods 50 III–2.1. Experimental methods 50 III–2.2. Analytical methods 53 III–3. Results 56 III–3.1 Textures and elemental compositions of the experimental run products 56 III–3.2. Isotopic composition of the experimental liquids 60 III–4. Attainment of equilibrium 62 III–5. Discussion 67 III–5.1. Comparison to previous studies 67 III–5.2. Implications for Fe isotope variability in natural systems 69 III–6. Conclusions 74 III–7. Appendix 75 Chapter IV. Experimental determination of the Si isotope fractionation factor between liquid metal and liquid silicate 89 IV–1. Introduction 90 IV–2. Methods 92 IV–2.1. Experimental methods 92 IV–2.2. Analytical methods 94 IV–3. Results 97 IV–3.1 Textures and elemental compositions 97 IV–3.2. Isotopic compositions 100 IV–4. Equilibrium isotope fractionation 102 IV–5. Discussion 108 IV–5.1. Comparison with previous studies 108 IV–5.2. Implications for core formation 110 IV–6. Conclusion 112 Chapter V. Experimental evidence for Mo isotope fractionation between metal and silicate liquids 121 V–1. Introduction 122 V–2. Methods 124 V–2.1. Experimental methods 124 V–2.2. Analytical methods 125 V–3. Results 129 V–3.1 Textures and elemental compositions 130 V–3.2. Isotopic compositions 134 V–4. Discussion 135 V–4.1. Fractionation at 1400 °C 135 V–4.1. Fractionation at 1600 °C 142 V–5. Implications for core formation 142 V–6. Conclusions 145 V–7. Appendix 146 V–7.1. Mo concentration analyses by laser ablation ICPMS 150 V–7.1. Double spike calibration 151 Chapter VI. Molybdenum stable isotope composition of meteorites: Constraints on planetary core formation 155 VI–1. Introduction 156 VI–2. Analytical methods 158 VI–2.1. Sample preparation and Mo separation 158 VI–2.2. Mass spectrometry and data reduction 159 VI–3. Results 160 VI–3.1. Mo concentrations 160 VI–3.2. Mo isotope data and δ98/95Mo values 160 VI–4. Discussion 166 VI–4.1. Stable isotope composition of the inner solar system and bulk planetary bodies 166 VI–4.2. Comparison of experimental and observed Mo stable isotope fractionation during metal segregation 167 VI–5. Conclusions 172 Chapter VII. Conclusions 173 VII–1. Summary of the main conclusions 174 VII–2. General conclusions and implications 175 VII–2.1. Stable isotope fractionation 175 VII–2.2. Conditions of core formation 177 Bibliography 179 Appendix AI. 190 Appendix AII. 194 Appendix AIII. 196 Acknowledgements 204 Abstract/Zusammenfassung — Page 3 ABSTRACT Stable isotope fractionation may inform on the conditions and chemical consequences of core-mantle differentiation in planetary objects. Equilibrium fractionation of stable isotopes, however, decreases strongly with increas- ing temperature and at very high temperatures becomes difficult to observe. With the advent of mass spectrometry, though, stable isotope fractionation has in many cases become observable during high temperature processes such as core formation. Nevertheless, the magnitude and direction of equilibrium stable isotope fractionation during such processes still remain largely unknown. In this study, I have experimentally determined the Fe, Si and Mo isotope fractionation factors between liquid metal and liquid silicate. I used these factors to interpret variations in stable isotope compositions in natural rocks and uncover their implications for planetary differentiation processes. Experiments were performed in a centrifuging piston cylinder in talc/pyrex assemblies at a pressure of 1 GPa. Elemental tin was used to lower the melting temperatures of iron-based alloys to below 1500 °C. Silicate compositions were adapted to reach an appropriate liquidus. The centrifugation segregated the liquid metal and the liquid silicate, enabling analyses of bulk pieces of metal and silicate that were free of cross contamination. The bulk pieces of metal and silicate were cleaned and crushed prior to ion exchange chemistry to separate the element of interest from its matrix. Analyses were then performed on a multi-collector inductively coupled plasma mass spectrometer. Experiments for Fe isotope fractionation were run at temperatures of 1250-1300 °C. The analyses demonstrate that 8 of the 10 experiments equilibrated in a closed isotopic system. Statistically significant iron isotope fractionation between the quenched metals and silicates was absent in 9 of the 10 experiments and all 10 experiments yield an average fractionation factor of 0.01 ± 0.04‰. The presence or absence of carbon or sulphur did not affect this result. At low pressures, Fe isotopes thus do not fractionate during metal-silicate segregation under magmatic conditions. This implies that the 0.07 ± 0.02‰ heavier composicomposi-- tion of bulk magmatic iron meteorites relative to the average of bulk ordinary/ carbonaceous chondrites cannot result from equilibrium Fe isotope fractionation during core segregation. The up to 0.5‰ lighter sulphide than metal fraction in iron meteorites and in one ordinary chondrite can only be explained by fractiona- tion during subsolidus processes. Silicon isotope fractionation between liquid metal and liquid silicate was studied at 1450 °C and 1750 °C. Metal is consistently enriched in light isotopes relative to the silicate, yielding average metal-silicate fractionation factors of -1.48 ± 0.08‰ and -1.11 ± 0.14‰ at 1450 and 1750 °C, respectively. These results are unaffected by the presence or absence of carbon. The temperature Page 4 — Abstract/Zusammenfassung dependence of equilibrium Si isotope fractionation between metal and silicate 30 × 6 2 liquids can therefore be described as Δ SiMetal-Silicate = -4.47(±0.31) 10 /T . Using a bulk silicate Earth δ30Si value of -0.29 ± 0.01‰, this temperature dependence can be used to calculate δ30Si values for Earth’s core. For metal-silicate equili- bration temperatures of 2500-3500 K, the Si isotope composition of Earth’ core is -1.01 ± 0.05‰ to -0.66 ± 0.03‰. Mass balance calculations indicate that for those core compositions, the Bulk Earth has a δ30Si of -0.38 ± 0.02‰ or -0.33 ± 0.01‰, respectively, if Earth’s core is assumed to contain 6 wt% Si, or -0.40 ± 0.02‰ or -0.35 ± 0.02‰, respectively, for 8 wt% Si in the Earth’s core. The reported Si isotope compositions of enstatite chondrites are significantly lighter (-0.62 ± 0.05‰) and they are therefore unlikely to represent the Bulk Earth Si isotope composition. A comparison with the reported Si isotope composition of other chondrites is unfortunately hindered by the dispute about these Si isotope compositions. Molybdenum isotope compositions in the silicate were 0.193 ± 0.030‰ and 0.120 ± 0.020‰ heavier than in the metal at 1400 and 1600 °C, respectively. This fractionation is independent of the presence or absence of carbon and tin as well as of oxygen fugacity in the range ΔIW-1.79 to ΔIW+0.47. Equilibrium Mo isotope fractionation between liquid metal and liquid silicate can therefore 98/95 × 5 2 be described as Δ MoMetal-Silicate = -4.80(±0.55) 10 /T . The experiments performed at 1600 °C furthermore demonstrate that non-equilibrium isotope fractionation results from highly reactive capsule material.