UNIVERSITY OF CALIFORNIA Los Angeles A Tale of Two Earths: Reconciling the Lunar and Terrestrial Hadean Records A dissertation submitted in partial satisfaction of the requirements for the degree Doctor of Philosophy in Geochemistry by Patrick Boehnke 2016 © Copyright by Patrick Boehnke 2016 ABSTRACT OF THE DISSERTATION A Tale of Two Earths: Reconciling the Lunar and Terrestrial Hadean Records By Patrick Boehnke Doctor of Philosophy in Geochemistry University of California, Los Angeles, 2016 Professor Timothy Mark Harrison, Chair Studying early Earth history is complicated by the fact that the rock record doesn’t extend past 4 Ga and our only record for the Hadean (>4 Ga) comes to us from detrital zircons from the Jack Hills in Western Australia. The Hadean zircon record extends back to almost 4.4 Ga and has revealed that the early Earth may have had liquid water, a felsic crust, plate boundary interactions, and possibly a biosphere. On the other hand, analyses of lunar and meteoritic samples are used to argue for a hellish Hadean Earth where frequent, large impactors repeatedly destroyed the crust. Indeed, these two models stand in direct contradiction. The focus of this thesis is to examine the evidence for these two models and ultimately propose a reconciliation based on a new interpretation of the chronology of the lunar samples used to constrain the impact history into the early Earth-Moon system. ii In order to improve the understanding of zircon crystallization in igneous settings, we undertook experimental studies of zircon saturation which were analyzed using a novel ion imaging approach by a secondary ion mass spectrometer. This study confirmed the original model for zircon saturation, that it is a function of only temperature, melt composition, and Zr content. Indeed, the primary implication for the early Earth from this work is that zircons are much more likely to crystallize in a felsic rather than mafic magma and therefore simply the existence of Hadean zircons suggests a high likelihood for felsic Hadean magmatism. The majority of the thesis focuses on the interpretation of 40Ar/39Ar ages of lunar and meteorite samples, specifically with regards to impact histories derived from compilations of such ages. The primary complication with lunar and meteorite 40Ar/39Ar ages is that the vast majority show evidence for later disturbances due to diffusive loss of 40Ar. To try and extract meaningful thermal histories from these samples, we undertook investigations of samples from Apollo 16 and the Jilin chondrite. We then used an extension of the multi-domain diffusion model that can model samples containing multiple activation energies (i.e., whole rock samples with multiple K bearing minerals) to propose that the 40Ar/39Ar system can be used to recover shock heating temperatures and durations. Having shown the effects of diffusive 40Ar loss on the accuracy of 40Ar/39Ar dating, we then explored the question as to whether or not compilations of disturbed 40Ar/39Ar ages simply misestimate the timing of bombardment episodes or are fundamentally inaccurate. For this we created a simple numerical model that simulates a chosen impact history on a surface and then creates a histogram of 40Ar/39Ar plateau ages. Our results show that rather than simply iii misestimate timing, compilations of 40Ar/39Ar ages can lead to inferences of illusory bombardment episodes. Finally, we examine the 40Ar/39Ar ages of suite of geochemically related Apollo 16 rocks to examine the effects of mixing and brecciation on the accuracy of inferred ages. By analyzing multiple rocks from each soil sample, we show that three out of six samples are not compatible with a single thermal history. That is to say, despite their close proximity during sampling and geochemical similarities, analyzed rocks in the soil sample have unique chronologies. Based on these findings, we developed a simple numerical model which shows that internal isochrons of mixed samples can yield erroneous ages while retaining a statistically acceptable mean squared weighted deviation (MSWD). iv The dissertation of Patrick Boehnke is approved. Edward Donald Young Kevin D. McKeegan Ioanna Kakoulli David Shuster Timothy Mark Harrison, Committee Chair University of California, Los Angeles 2016 v For my family and friends vi Table of Contents Abstract of Dissertation…………………………………………………………………...………ii Committee Page…………………………………...………………………………………………v Dedication…………………………...……………………………………………………………vi Table of Contents………………………………………………………………………………...vii Acknowledgments………………………………………………………………………………viii Vita………………………………………………………………………………………………...x Chapter 1: Introduction – Impact Records and the Early Earth…...………………………………1 Chapter 2: Zircon Saturation Re-revisited……………………………………….………………16 Ch. 2 Figures and Tables…………………………….…………………………………………..38 Chapter 3: A Model for Meteoritic and Lunar 40Ar/39Ar Age Spectra: Resolving the Effects of Multi-activation Energies…………………...……………………………………………………54 Ch. 3 Figures and Tables………………………………….……………………………………..73 Chapter 4: Illusory Late Heavy Bombardments…………………………………………………82 Ch. 4 Figures and Tables………………………………………………...………………………94 Chapter 5: Impact Mixing and Brecciation: The Silent Killer of Chronologic Information…….98 Ch. 5 Figures and Tables…………………………………………….…………………………110 Chapter 6: Conclusions…...…………………………………………………………………….117 Appendix A: A meta-analysis of geochronologically relevant half-lives: What’s the best decay constant?......................................................................................................................................122 Appendix B: Zircon U/Th Model Ages in the Presence of Melt Heterogeneity…...…………..145 Appendix C: EPMA and 40Ar/39Ar Data for Chapter 3………………………………………...167 Appendix D: Model Description for Chapter 4…………………………………….…………..173 Appendix E: 40Ar/39Ar Data for Chapter 5……………………………………………..………186 References…………………………..…………………………………………………………..203 vii Acknowledgements I am deeply grateful for discussions and exchange of ideas with Melanie Barboni, Beth Ann Bell, Rita Economos, Brenhin Keller, Robert Steele, Anke Watenphul, and Matthew Wielicki. Indeed, conversations with them greatly enriched my doctoral experience and I look forward to many future collaborations and scientific exchanges. Oscar Lovera was tremendously helpful in explaining and helping me to understand the Multi-Domain Diffusion model which was crucial to Ch. 3. All of the 40Ar/39Ar data was acquired in the New Mexico Geochronology Research Laboratory with the help of Matt Heizler. Matt Heizler’s detailed explanation of 40Ar/39Ar analyses and willingness to let me participate permitted me to take an active part in collecting much of the data for this thesis. Kevin McKeegan and Axel Schmitt are thanked for teaching me how to use an ion microprobe as well as explaining its inner workings. Their tutelage allowed me to become an independent user of the UCLA ion microprobe facility. I would like to thank Kevin McKeegan, Edward Young, Ioanna Kakoulli, and David Shuster for serving on my committee and providing insightful feedback that was invaluable for the writing of this dissertation and large parts of my doctoral research. Marc Caffee was the person who first introduced me to geochemistry while I was an undergraduate at Purdue University and I am grateful for our continued discussions and his advice. I would like to thank my advisor Mark Harrison for his wisdom, skepticism, wide ranging conversations, and tremendous insight into Earth science. His skill as a supervisor and continual support ensured that my PhD experience was a great one and prepared me for career success. Finally, I am deeply grateful for the love and support from my wife Elizabeth, my mother, my father, and my sister. viii The ion microprobe facility at UCLA is partly supported by a grant from the Instrumentation and Facilities Program, Division of Earth Sciences, National Science Foundation. This research was conducted with support from a grant to T. Mark Harrison from NSF-EAR’s Petrology/Geochemistry Program. Ch. 2 was published in Chemical Geology as Boehnke, P., Watson, E.B., Trail, D., Harrison, T.M. and Schmitt, A.K., 2013. Zircon saturation re-revisited. Chemical Geology, 351, 324-334. Ch. 3 was published in Earth and Planetary Science Letters as Boehnke, P., Harrison, T.M., Heizler, M.T. and Warren, P.H., 2016. A model for meteoritic and lunar 40Ar/39Ar age spectra: Addressing the conundrum of multi-activation energies. Earth Planet. Sci. Lett., 453, 267-275. Ch. 4 was published in Proceedings of the National Academy of Science as Boehnke, P. and Harrison, T. M., 2016. Illusory Late Heavy Bombardments. Proc. Nat. Acad. Sci. U. S. A. Ch. 5 is based on a manuscript in preparation for submission: Boehnke, P., Heizler, M. T., Warren, P. H. and Harrison, T. M., Impact Mixing and Brecciation: The Silent Killer of Chronologic Information. In prep. ix Vita Education University of California, Los Angeles, CA August 2014 Geochemistry Candidate of Philosophy University of California, Los Angeles, CA May 2013 Geochemistry Master of Science Purdue University, West Lafayette, IN May 2011 Physics Bachelor of Science Mathematics Bachelor of Science Awards and Invited Talks: 2014 Eugene Shoemaker Impact Cratering Award, Geological Society of America 2014 Eugene B. Waggoner Scholarship for outstanding academic performance and original research, Department of Earth, Planetary, and Space Sciences, UCLA 2013 Harold and Mayla Sullwold Scholarship for excellence in graduate student