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 unraveled.
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.