Awarded to: Richard Carlson
Abstract: The Impact of Early Earth Differentiation on the Modern World
Medal lecture in:
Session 02c in Room 1, Goldschmidt2020 - Virtual Venue on Tuesday 23rd June 22:00 - 22:03

Citation: Rick Carlson was brought into the world of Nd isotope geochemistry by Günter Lugmair at Scripps, himself a Goldschmidt medalist. In many ways, neodymium isotopes have revolutionized planetary and especially terrestrial geochemistry because of the robustness and geological-memory-preserving properties of the Sm-Nd decay system, and Rick has been a leading neodymium isotopicist from the day one. As a Ph.D. student, he did some of the first lunar and terrestrial Nd isotope analyses back in 1978. This work established the age of the lunar crust at about 4.4 billion years, a value that has pretty much stood the test of time, though some people now claim that the lunar crust may be as old as 4.5 Ga. Also, quite early on, he did definitive work on the American type locality of continental flood basalts, the Columbia River basalt. He showed that crustal assimilation played a decisive role in the chemical evolution of these flood basalts, and they cannot simply be derived from a primitive mantle reservoir.
He came to the Department of Terrestrial Magnetism of the Carnegie Institution Washington in 1980, just after I left to go Germany, and he proceeded to build a world-leading isotope geochemistry group there that is now going as strong as ever. I will jump to the year 2005, when Carlson with his postdoc Maud Boyet dropped a veritable bombshell on the community: They showed that the atomic abundance of ¹⁴²Nd, daughter of the “short-lived” parent nuclide ¹⁴⁶Sm (T¹/₂ ≤ 100 Myr), in all terrestrial rocks differs from that found in chondritic meteorites. This completely unexpected discovery demonstrated that, if Earth has a chondritic Sm/Nd ratio and initial ¹⁴²Nd atomic abundance, then Earth must have been differentiated permanently into an early “enriched” (i.e. low-Sm/Nd) and an early “depleted” (high-Sm/Nd) reservoir, and this differentiation event must have occurred more than 4.53 Gyr ago. The past fifteen years have seen an enormous research effort directed at trying to understand this dilemma and to sort out the first 500 Myr of Earth-Moon history. Carlson has remained firmly at the forefront of this work. For example, Boyet et al. (2015) found that the ¹⁴²Nd systematics of the lunar crust is consistent either with a chondritic ¹⁴²Nd/¹⁴⁴Nd ratio combined with an increased Sm/Nd ratio, or alternatively, with the Moon having a chondritic Sm/Nd, but a lower-than-terrestrial ¹⁴²Nd/¹⁴⁴Nd ratio. We may still not know the ultimate solution to these puzzles, but whatever it is, it will have a profound impact on our understanding of the early history of our planet.
For the past several years, Rick and his postdoc Jonathan O’Neil have been leading an effort to unravel Earth’s Hadean history. Hades is the god of the Underworld; its entrance, the 4 billion year age barrier, is guarded by a three-headed dog called Cerberus. It’s hard to get in (unless you are dead!), and it’s even harder to get anything back out of it. You may know what happened to Orpheus! Anyway, rather than trying to decipher ancient zircons from younger rocks, Rick’s group looked at the Nd isotopic composition of the Nuvvuagittuq greenstone belt in eastern Canada, and found that these rocks yield an apparent ¹⁴²Nd/¹⁴⁴Nd vs. Sm/Nd isochron with an age of 4.27 Gyr (O’Neil et al., 2008). Although it is still being debated whether this age dates the actual emplacement of these rocks or the age of differentiation of their mantle sources, this is the first time that the 4 Gyr age barrier has been breached by anything other than the detrital or xenocrystic zircon grains preserved in much younger rocks. Subsequently, O’Neil and Carlson (2017) showed that a large block of Archean continental crust in northeastern Canada has inherited some of this much more ancient ¹⁴²Nd variability, and this led them to the conclusion that much of this younger Archean crust was generated by remelting of a >4.2 billion year old (“Hadean”) basaltic protocrust having the same isotopic and chemical characteristics as the Nuvvuagittuq greenstone belt they had previously analyzed. Thus, largely as a result of the research of Carlson and his collaborators, the Hadean (> 4 billion year) evolution of our planet is gradually being unravelled.
Rick Carlson has, in my opinion, one remarkable weakness: His fondness of fancy cars and car racing. Before I knew him better, I initially thought this was completely out of character. Why would Rick go out racing some vintage Corvette??? Now that he invited me to take a ride in his Tesla, I think I am beginning to understand. Particularly the part when you call your Tesla from your restaurant table and tell it: “Come and get me!”
Ladies and Gentlemen, it is my honor to present Rick Carlson, a scientist nearly at the pinnacle of his career, recipient of GSA’s Day Medal, AGU’s Bowen Award, member of the National Academy of Sciences, Fellow of the American Academy of Arts and Sciences, director of Carnegie’s Earth and Planets Laboratory, and Tesla driver, to receive the Victor Moritz Goldschmidt Award of the Geochemical Society.
Albrecht Hofmann,
Max Planck Institute for Chemistry
Mainz, Germany

Response: Instead of being able to offer my thanks in front of an assembled group of my geochemical colleagues, I am writing this from home. Our campus has been closed for 12 weeks in response to the coronavirus pandemic. I am grateful that the Carnegie Institution’s first concern when the pandemic arrived was the health and safety of their employees in this time when so many others have lost their livelihoods, if not their lives. Although I would have loved to celebrate this award with colleagues in person at the Goldschmidt meeting in Hawaii, I applaud the Geochemical Society’s bold move to continue with a virtual meeting. These responses to the pandemic well illustrate what drew me into a career in science in the first place. Science uses intellect and data to pursue what is right, whether to answer to a science question or decide on the proper treatment of employees or association membership.

I have already been honored to have a close association with the Geochemical Society throughout my career, so being awarded their Goldschmidt Medal is truly icing on the cake. My nominators, Al Hofmann, Stan Hart, Alex Halliday, and Bernie Wood have been my geochemical heroes for quite some time. I am humbled that they consider me worthy of this award and grateful that they took the time from their busy schedules to nominate me.

The introductory Earth science courses I took at UCSD, taught by Jeff Bada, Jim Hawkins, Miriam Kastner and Doug Macdougall, opened my eyes to the fact that modern Earth science involves the application of cutting-edge chemical and physical approaches to understand the Earth. I was hooked. This was a time when the relatively new theory of plate tectonics was transforming investigations of local geology into a globally connected system of processes driving the dynamics of the whole planet. The Apollo program extended this planetary perspective beyond Earth and opened the opportunity for me to work in Günter Lugmair’s laboratory, an experience I’ll always treasure. I still remember listening to Günter, Kurt Marti, Harold Urey and Hannes Alfven discussing over lunch how the Solar System came to be. I didn’t understand how the analysis of rocks could address topics of this magnitude, but I absolutely wanted to give it a try.

A discussion with George Wetherill convinced me to visit Carnegie’s DTM for a postdoc interview. As job offers were not exactly clogging up my mailbox at the time, I welcomed the invitation. The visit and the encouragement of long-time colleague Bob Stern, who was then a postdoc at DTM, led me to accept the postdoc. This was a time of transition at DTM after Stan Hart and Al Hofmann had left, leaving Louis Brown, Typhoon Lee, Fouad Tera, and eventually me as the geochemists in this department. George Wetherill and then Sean Solomon built up the geochemistry/cosmochemistry group by adding Conel Alexander, Erik Hauri, Julie Morris, Larry Nittler, and Steve Shirey. In my wildest dreams, I could not have wished for a more intelligent, creative, and pleasant group to work with.

After a frustrating but not entirely futile attempt to distinguish crustal contamination from compositional variability in the mantle sources of continental flood basalts, the opportunity presented itself to apply the Re-Os system to study the continental mantle lithosphere, a topic that had intrigued me since taking classes from Tom Jordan at Scripps. Rich Walker, a postdoc at NBS, as it was called then, provided the technique – resonance ionization. Our experience with this technique provides a good example of why geochemistry is such a great field, and why Carnegie is such a great place to work. After having built our own resonance ionization system at DTM, and hiring Rich Walker as a postdoc, before our instrument was productively used, I received a review request from GCA regarding Rob Creaser’s 1991 paper on negative ion analysis for Re and Os. The first read of the paper showed the path forward. After asking permission from Dimitri Papanastassiou, one of the coauthors, the next morning Steve Shirey, Lou Brown, and I converted our home built mass spectrometer to negative ion operation and saw an Os signal orders of magnitude bigger than we would ever achieve by resonance ionization. We, shaking in our shoes, went to Director Wetherill to tell him that we were going to abandon the very expensive laser system that he had just paid for and switch to negative ionization. George’s only response was along the lines “We tried negative ionization in the 1950’s. It creates lots of interferences.” A more prophetic statement was never made. Nevertheless, this technique enabled a variety of studies of the history of the continental lithospheric mantle, many involving Graham Pearson first as a postdoc and then valued colleague, that culminated in a large Continental Dynamics sponsored project in southern Africa. I am most proud of the Kaapvaal Project in the way that it brought together a diverse set of techniques, including the seismology component led by David James, and involved a great group of people from two continents.

My career then took a most rewarding turn thanks to Maud Boyet who was struggling to find variability in ¹⁴²Nd in terrestrial rocks. After measuring a large number of samples that reproduced the standard very well, we decided that we could make a useful contribution by determining the bulk Earth value of ¹⁴²Nd/¹⁴⁴Nd to high precision by measuring chondritic meteorites. This failed miserably but provided much more interesting evidence for differentiation events that accompanied Earth formation and showed that one could measure nucleosynthetic variability at the planetary scale in elements other than the noble gases. This latter pursuit was continued with Cr by Liping Qin and provided the first glimpse that primitive carbonaceous chondrites likely are not a major component of Earth’s building blocks. The techniques advanced by Maud Boyet were then used by Jonathan O’Neil to discover the first terrane on Earth that displays a range in ¹⁴²Nd/¹⁴⁴Nd likely because it was formed almost 4.3 billion years ago.

My career has been a not entirely random walk through a number of subjects that took advantage of new technique developments to pursue new problems. This to me is the most enjoyable aspect of geochemistry, but a close second is the openness within the geochemical community I’ve worked with. I am extremely grateful that the Carnegie Institution allowed us to keep our laboratory open to visitors so we could share in their creativity and they could benefit from access to our instrumentation and techniques. I have always enjoyed the open discussions about science and techniques with colleagues that include all my nominators along with Francis Albarède, Lars Borg, Catherine Chauvel, Bill Hart, Dmitri Ionov, Günter Lugmair, Graham Pearson, Roberta Rudnick, and Dominique Weis, among a host of others.

In closing, I would like to thank all the people and organizations that made it possible for me to pursue the science that led me to this award. First among these are the Geochemical Society, my nominators, and the many colleagues I’ve had the privilege to interact with through my career, but especially the colleagues and postdocs at DTM who have made it such a great place to work. None of my achievements would have been possible without the financial support of the Carnegie Institution, NSF, and NASA. Finally, I want to thank Sonia Esperança for all she has done to contribute to my life in a positive way, both at work and at home. Being able to share my life with Sonia, whose wisdom and calm demeanor have helped me through the tough times, is something for which I will always be grateful.