A Portrait of Terrestrial Volatile Evolution from Mantle Noble Gases

A Portrait of Terrestrial Volatile Evolution From Mantle Noble Gases The Harvard community has made this article openly available. Please share how this access benefits you. Your story matters Citable link http://nrs.harvard.edu/urn-3:HUL.InstRepos:37944946 Terms of Use This article was downloaded from Harvard University’s DASH repository, and is made available under the terms and conditions applicable to Other Posted Material, as set forth at http:// nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of- use#LAA A Portrait of Terrestrial Volatile Evolution from Mantle Noble Gases a dissertation presented by Jonathan M. Tucker to The Department of Earth and Planetary Sciences in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the subject of Earth and Planetary Sciences Harvard University Cambridge, Massachusetts October 2016 ©2016 – Jonathan M. Tucker all rights reserved. Dissertation advisors: Jonathan M. Tucker Professor Sujoy Mukhopadhyay Professor Charles H. Langmuir A Portrait of Terrestrial Volatile Evolution from Mantle Noble Gases Abstract Volatile elements and compounds provide important insights into large-scale planetary and tectonic processes. The chapters in this thesis exploit the wealth of information provided by noble gas measurements to address a variety of questions regarding the origins and distribu- tions of volatiles in the Earth. Chapter 1 presents new He-Ne-Ar-Xe data from basalts from the equatorial Mid-Atlantic Ridge, demonstrating a large degree of heavy noble gas heterogeneity in mid-ocean ridge basalts (MORBs). The He and Ne data can be explained primarily by a mixture of a depleted mantle component and a HIMU-like component, with the constraint that the HIMU component is a combination of recycled and primitive material. While most mantle-derived Xe is recycled atmospheric Xe, the compositions of depleted MORBs, HIMU-type MORBs, and the Iceland plume cannot be related solely by different amounts of recycled air. Rather, HIMU-type MORBs and the Iceland plume sample a less degassed reservoir. Furthermore, differences in the amount of 129I-derived 129Xe between the depleted and HIMU-type MORBs suggests that HIMU-type MORBs are sampling a reservoir that formed within the first 100 Myr of solar system history and has not extensively mixed with the depleted mantle since. Chapter 2 explores the hypothesis that high 3He/22Ne ratios intrinsic to the depleted mantle were generated by multiple episodes of magma ocean outgassing and atmospheric loss during Earth’s accretion. We argue that magma ocean outgassing in the aftermath of giant impacts during accretion can raise the mantle 3He/22Ne ratio, but multiple episodes of outgassing and atmospheric loss are required to achieve the ratio observed in the depleted mantle. The preservation of low 3He/22Ne ratios in primitive plumes suggests that later giant impacts such as the Moon-forming giant impact did not homogenize the whole mantle. The iii requirement for episodes of atmospheric loss to achieve high 3He/22Ne ratios during accretion may also provide an explanation for Earth’s nonchondritic volatile element ratios such as N/H as N would be more susceptible to loss processes than H. Chapter 3 uses the combined constraints of the radiogenic noble gases 4He*, 21Ne*, 40Ar*, 136 and Xe*U to examine degassing processes at mid-ocean ridges. We show that ratios of radiogenic noble gases cannot be simultaneously explained by any equilibrium degassing model, questioning the use of such models to reconstruct pre-degassing magmatic contents and hence mantle fluxes of elements like C. We argue that kinetic fractionation prevents slowly diffusing volatiles from achieving their equilibrium partitioning between vesicles and melt and present 4 a new simple model of disequilibrium degassing that self-consistently explains CO2- He*- 21Ne*-40Ar* compositions in MORBs. Application of this model suggests that the average MORB mantle C/3He ratio and C flux may be a factor of 2 higher than that inferred from equilibrium degassing-based estimates. Chapter 4 presents new He data on depleted MORBs from the subtropical north Mid- Atlantic Ridge. Correlations between He and Pb isotopic compositions in this region as well as others globally suggest that, absent plume influence, He isotopes in MORBs can beex- plained by a mixture of an intrinsic, relatively unradiogenic depleted component and highly radiogenic recycled oceanic crust, a manifestation of the “marble cake” mantle. With a simple mixing model, we estimate that the mantle source of average MORBs has ~1-5% recycled oceanic crust. Chapter 5 presents new He-Ne-Ar-Xe data on a subset of the depleted MORB samples de- scribed in Chapter 4. Ne isotopic compositions are less nucleogenic than average MORBs, in- dicating the influence of a relatively undegassed component, which may be especially strongly sampled near 29◦N. Ar, and Xe isotopic compositions extend from moderately radiogenic values to highly unradiogenic values, demonstrating extreme variability absent significant variability in lithophile chemistry. Globally, Ar and Xe isotopic compositions correlate in oceanic basalts, interpreted to result from mixtures of degassed material having radiogenic Ar and Xe with undegassed material and recycled air having unradiogenic Ar and Xe. iv Contents 0 Introduction 1 1 The heavy noble gas composition of the depleted MORB mantle (DMM) and its implications for the preservation of heterogeneities in the mantle 4 1.1 Introduction ...................................... 7 1.2 Methods ........................................ 9 1.3 Results ......................................... 13 1.4 Discussion ....................................... 20 1.5 Conclusions ....................................... 29 2 Evidence for multiple magma ocean outgassing and atmospheric loss episodes from mantle noble gases 36 2.1 Introduction ...................................... 38 2.2 Determining MORB mantle source 3He/22Ne ratios . 40 2.3 The 3He/22Ne ratio of the depleted mantle ..................... 42 2.4 3He/22Ne ratios in the present-day mantle and in planetary materials . 44 2.5 Can plate tectonics generate reservoirs with high 3He/22Ne ratios? . 46 2.6 Implications for Earth’s accretional history ..................... 57 v 2.7 Conclusions ....................................... 61 3 Kinetic controls on volatile fluxes at mid-ocean ridges and implications for the mantle carbon flux 71 3.1 Introduction ...................................... 73 3.2 Equilibrium degassing models do not explain MORB data . 76 3.3 Kinetic fractionation during degassing ........................ 81 3.4 Reconstructing initial noble gas contents in individual samples . 89 3.5 Implications for the mantle carbon content and flux . 94 3.6 Conclusions .......................................103 4 Using helium isotopes to constrain the amount of recycled crust in the average MORB source 112 4.1 Introduction ......................................114 4.2 Coupled He-Pb variations in MORBs imply a ubiquitous marble cake mantle . 116 4.3 Geodynamic consequences ..............................125 4.4 Where does subducted oceanic crust reside? . 133 4.5 Conclusions .......................................135 5 Extreme Ne, Ar, and Xe heterogeneity in depleted MORBs from the subtropical north Mid-Atlantic Ridge 144 5.1 Introduction ......................................146 5.2 Methods ........................................148 5.3 Results .........................................150 5.4 Discussion .......................................155 5.5 Global variations in heavy noble gases . 170 5.6 Summary and conclusions ...............................173 Appendix A Supplemental material for Chapter 1 180 A.1 Data tables .......................................181 vi A.2 Supplementary figures .................................186 Appendix B Supplemental material for Chapter 2 188 3 22 B.1 Calculation of He/ Nem by correcting for air contamination and degassing (Method 2) ............................................189 B.2 Three-component mixing model . 190 B.3 Calculation of 3He/22Ne ratios in primordial and modern terrestrial reservoirs . 191 B.4 Coupled hydrodynamic escape/magma ocean outgassing model . 198 B.5 Terrestrial noble gas abundances . 200 B.6 21Ne*/40Ar* vs. 4He*/40Ar* .............................202 Appendix C Supplemental material for Chapter 3 205 C.1 Calculation of radiogenic noble gas abundances . 206 C.2 Derivation of Rayleigh-like degassing formulation . 207 C.3 Data tables .......................................211 Appendix D Supplemental material for Chapter 4 212 D.1 The effect of intermediate reactions on mixed lithology mixing calculation . 213 D.2 Data tables .......................................215 Appendix E Supplemental material for Chapter 5 216 E.1 Data tables .......................................217 vii List of figures 1.1 Map of sample locations ................................ 10 1.2 Ne-Ne plots ....................................... 12 1.3 4He/3He vs. 206Pb/204Pb and 143Nd/144Nd ..................... 15 21 22 206 204 143 144 1.4 Ne/ NeE vs. Pb/ Pb and Nd/ Nd ................... 16 21 22 4 3 1.5 Ne/ NeE vs. He/ He ............................... 17 1.6 Ne-Ar hyperbolas ................................... 17 1.7 Ar-Xe hyperbolas ................................... 18 1.8 Xe-Xe plots ....................................... 24 3 22 2.1 Correlation diagrams to determine He/ Nem ................... 41 3 22 2.2

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